Fluids doi: 10.3390/fluids11080189

Authors: Jingmu Bai Jian Sun Yunfei Li Hailun Ren Qingzhao Liu Wenhui Shen Lu Wang Weihong Li Bingxin Zhu Zheng Wang Yang Liu

Agitator reactors are widely used in chemical production processes, and their structural design has a significant impact on power consumption. Therefore, this study performs numerical simulations of multi-stage cup-shaped paddle agitators with different geometric parameters, and discusses in detail the effects of the paddle spacing (S/H), the ratio of the upper paddle length to the reactor radius (Ls/R), the ratio of the lower paddle length to the reactor radius (Lx/R), and the ratio of the upper to lower paddle lengths (Ls/Lx) on the reactor’s power characteristics and internal flow field. Equations were derived to relate the power number (Np) to parameters such as Re, Ls/R, and Lx/R. The study found that, at the same Reynolds number, torque exhibits a slight upward trend as the paddle spacing increases; the best mixing effect is achieved when S/H is 0.333. Based on this pitch, when Ls/R, and Lx/R exceed 0.67, the mixing process fails to form a stable and complete radial circulation; when Ls/R and Lx/R are less than 0.53, the high-velocity zone in the flow field decreases, leading to the formation of dead zones. Therefore, selecting a multi-stage cup-shaped impeller with an Ls/R value of 0.53, an Lx/R value of 0.53, and Ls/Lx of 1 can achieve better mixing results with lower power consumption. These findings provide a reference for the energy-efficient optimization design of multi-stage cup-shaped impeller mixers in industrial applications.

Fluids doi: 10.3390/fluids11080188

Authors: Reza Arefi Ioan Nistor Abdolmajid Mohammadian

Tsunamis, though infrequent, pose catastrophic threats to coastal communities and infrastructure, underscoring the urgent need for effective mitigation strategies. This review provides a comprehensive evaluation of current engineered countermeasures, specifically seawalls, breakwaters, and water-filled canals, by synthesizing findings from post-tsunami field surveys. Evidence from major events, such as the 2011 Tohoku Tsunami, indicates that well-designed structural measures can substantially reduce tsunami wave energy and delay inland inundation, thereby improving the resilience of coastal infrastructure. However, their effectiveness depends heavily on proper geometric configuration, construction quality, regular maintenance, and integration with non-structural measures such as evacuation planning. Recent experimental and numerical investigations have also highlighted the potential of innovative countermeasures, in attenuating bore forces and protecting critical infrastructure. Despite advancements, significant knowledge gaps remain, particularly concerning the long-term reliability, cost-effectiveness, and adaptability of these systems under diverse tsunami conditions. This critical review aims to identify these gaps and outline key priorities for future research and development. The findings contribute to a deeper understanding of structural tsunami mitigation and support the advancement of integrated, resilient strategies for reducing disaster risk in tsunami-prone regions worldwide.

Fluids doi: 10.3390/fluids11080187

Authors: Olga A. Azarova Tatiana A. Lapushkina Ekaterina V. Reshetova Oleg V. Kravchenko

The main objective of this study is to obtain the average parameters of gas-discharge plasma when controlling the steady position of the bow shock wave (BSW) using the combined action of a gas discharge initiated by a current from an external source and a magnetic field near the frontal surface of the model. The studies were carried out using both experimental and numerical methods in xenon and air. A comparison of the numerical and experimental dependences of the relative distance of the steady BSW from the model on the discharge power showed good agreement. Based on the conducted flow modeling, taking into account the dependence of the adiabatic index on the degree of ionization and the degree of nonequilibrium, and using the theory of Burm et al., gas-discharge plasma characteristics were obtained, such as the degree of ionization and the degree of nonequilibrium, the electron density and the electron temperature in the absence and presence of a magnetic field. By this way an integrated experimental–computational system was formed in which the measured characteristics of the discharge and BSW, as well as the numerically obtained averaged plasma parameters in the impact zone, are combined with the theory of Burm et al. to clarify the thermodynamic state of the medium and determine the corresponding characteristics of the gas-discharge plasma. The obtained results can be used for assessing the characteristics of plasma gas dynamic and magnetohydrodynamic phenomena in high-speed flows; for example, in the development of control systems that take into account the influence of plasma parameters and the electric and magnetic fields.

Fluids doi: 10.3390/fluids11080186

Authors: Leonardo Di G. Sigalotti

This collection features eleven excellent papers that are included in the second edition of the Special Issue, “Pipe Flow: Research and Applications” [...]

Fluids doi: 10.3390/fluids11080185

Authors: Yang Zhang Yikun Wang

An adaptive immersed boundary method (IBM) for simulating non-isothermal incompressible flows with convective heat transfer involving a rotating circular cylinder is developed. Both Dirichlet- (isothermal) and Neumann (zero heat flux)-type temperature boundary conditions are implemented. In addition to the discrete momentum forcing and energy forcing adopted to effectively satisfy the prescribed velocity and temperature boundary conditions, a mass source/sink term is introduced into the continuity equation to meet the mass conservation at the immersed boundary. The Navier–Stokes equations are solved using the fractional step method implemented on a staggered Cartesian grid system. Time stepping is performed using a second-order Adams–Bashforth/backward-differentiation method, while spatial derivatives are approximated with a second-order centered scheme. Testing of the flow induced by a rotating disk demonstrates that the spatial accuracy of the presented algorithm is second-order. Furthermore, the proposed method is validated by forced convective flow past a rotating isothermal circular cylinder. Finally, mixed Rayleigh–Bénard convection in a square cavity with an embedded adiabatic rotating circular cylinder is simulated, showing that heat transport can be greatly enhanced by increasing the rotating rate and radius of the cylinder at larger Prandtl numbers in the laminar regime.

Fluids doi: 10.3390/fluids11070184

Authors: Evgeniia V. Mishchenko Xuelin Guan

We study the unsteady mechanical response of an incompressible viscoelastic polymeric fluid in a flat channel, governed by the Vinogradov–Pokrovskii rheological model. The motion arises from an electrohydrodynamic reduction of Poiseuille type, after which the mechanical subsystem decouples from the electric field; the velocity then depends on time and on the transverse coordinate only. Treating the rheological parameter as small, we reduce the governing system in the leading-order approximation to a non-autonomous second-order evolution equation whose stiffness coefficient relaxes exponentially in time, so that the nonstationarity is driven by the internal relaxation of the normal stress rather than by an external force. For spatially homogeneous initial normal stress, we diagonalize the Galerkin system in the sine basis and obtain an explicit modal representation in which each mode satisfies a Bessel equation whose order depends on the mode number. This yields a critical index that splits the modes into three regimes—real order, zero order, and purely imaginary order—a structure absent from the classical UCM and Oldroyd-B solutions. Using the explicit representation, we prove convergence of the modal series and show that the solution decays in the long-time limit, so that the rest state is asymptotically stable in the natural energy phase space. The analytical solution is confirmed numerically.

Fluids doi: 10.3390/fluids11070183

Authors: Zhuhai Zhong Xiaodan Zhang Kunlun Bai Meng Wang Xiaodong Lu

The main reason for limiting the working flow range of the compressor is the unstable flow phenomenon of the compressor working at a small flow rate, including stall, surge, and rotational instability. Among them, the rotating stall phenomenon is particularly prone to occur during the operation of centrifugal compressors. In this paper, a three-stage nitrogen centrifugal compressor is taken as the research object, and the dynamic development process of rotating stall in the diffuser is captured by full-channel numerical calculation. After research, the leading-edge vortex at the diffuser inlet is the cause of rotating stall. In the throttling process, the backflow in the diffuser causes the channel blockage and the stall phenomenon triggered by the leading-edge overflow. There are six stall channels in the first-stage diffuser and nine stall channels in the second-stage diffuser. The propagation direction is the same as the rotation direction of the blade, and the propagation speeds are 4.348% and 5.26%, respectively.

Fluids doi: 10.3390/fluids11070182

Authors: Jonathan Rodolfo Guereca-Ibarra Guillermo Urriolagoitia-Sosa Beatriz Romero-Ángeles Jacobo Martínez-Reyes Jorge Alberto Gomez-Niebla Edder Jair Rodríguez-Granados Jonatan Mireles-Hernández Miguel Martinez-Mondragon Aldo Saul Laguna-Canales David Sánchez-Hernández Santiago Barrañón-Salmón

The analysis of blood flow behavior has been studied in various aspects. Recently, these studies took a turn when they were developed using Computational Fluid Dynamics (CFD), thereby advancing understanding of fluid behavior. Artery geometries and the properties of flow offer a field of interest for developing studies in which the behavior of liquid and structures can be observed. Biomodel development has evolved to allow for greater ease and precision. With a geometry similar to the real one, mechanical properties can be used to conduct studies with greater precision, yielding greater results on the behavior of a fluid and a structure. This research focuses on developing a biomodel of the right carotid artery. CFD and structural analyses are performed using the generated arterial geometry and a control volume that simulates blood flow. The structural analysis was performed using the Finite Element Method (FEM) to obtain results that provide data on the effects on the arterial walls. The results presented focus on the velocity, pressure, and wall shear stress generated by the flow. In addition, a section on displacement and Von Mises stress results is presented, demonstrating how the structure of the arterial walls is affected by these effects.

Fluids doi: 10.3390/fluids11070181

Authors: Wenxing Sun Duanjiao Li Junwen Yao Yun Chen Yanjun Ma Yongfei Ma Xutao Chen Yupeng Zou

To address the cleaning requirements for marine growth on offshore platform jackets, an underwater cleaning robot featuring a combined “chassis + thruster-assisted adhesion + magnetic adhesion” mode is designed. The robot is equipped with four thrusters and a magnetic-adhesion wheeled chassis, enabling stable attachment and movement on varying-diameter pipes. Kinematic models in both inertial and body-fixed coordinate systems are established, and six-degree-of-freedom (6-DOF) dynamic equations are derived. These equations systematically incorporate key factors including added-mass forces, damping forces, hydrostatic restoring forces, ocean current disturbances, and thruster torques. Based on CFD simulations employing overset grids, moving reference frames, and simple harmonic motion techniques, the damping, added-mass, and thruster thrust and torque coefficients for each degree of freedom are identified. The obtained parameters demonstrate reasonable consistency with the CFD internal validation and preliminary external verification, providing a complete theoretical model and simulation data to support the motion control and operational stability analysis of the underwater cleaning robot. The established dynamic model addresses the free-navigation condition of the robot without cleaning operation. The additional hydrodynamic effects during cleaning operations will be considered in future work.

Fluids doi: 10.3390/fluids11070180

Authors: Jiang Li Aqiang Chen Yifan Bu Hang Xiao Baisong Hu Haidong Zhang

A Laval nozzle is a pivotal component of supersonic separation technology for dehydration, but its performance is constrained by low separation efficiency due to intractable conflicts between droplet nucleation and growth rates. To address this issue, this work for the first time applied the Foelsch-MOC method to design an expansion section with a first-order differentiable and second-order continuous profile. A numerical model for pure water vapor condensation based on the ideal gas equation of state (EOS) was then established to optimize nozzle structures for droplet growth. Major findings revealed that the droplet radius of the Foelsch-MOC nozzle was 30.5% higher than that of the conical nozzle, while its outlet liquid mass fraction was 5.8% lower. The liquefaction capacity was found to increase first and then decrease with the expansion coefficient, peaking at 0.5. Moreover, increasing the axis shift from 9.3 mm to 25 mm promoted the outlet liquid mass fraction by 1.2% and decreased the droplet radius by 9.3%, thereby improving liquefaction capacity. These results highlight the droplet growth superiority of the novel expansion profile, providing a valuable guideline for designing nozzles that simultaneously ensure liquefaction efficiency and facilitate droplet development.

Fluids doi: 10.3390/fluids11070179

Authors: Filiberto Hueyotl-Zahuantitla Mario A. Aguirre-López

Environmental engineering is increasingly relying on advanced numerical tools to address the complex challenges associated with energy efficiency, air quality, pollutant transport, and sustainable infrastructure design [...]

Fluids doi: 10.3390/fluids11070178

Authors: Bobur Bakhtiyorov Khusniddin Mamadaliev Khayot Aminov Shakhzod Khojiqulov Oybek Begimov Nilufar Turopova Javokhir Shodmonov

This study presents a computationally efficient quasi-one-dimensional mathematical model based on the traveling wave method to investigate hydraulic shock attenuation using a gas-hydraulic damper in pipeline systems. Unlike conventional models, this formulation accounts for fluid compressibility and incorporates a non-linear boundary condition strictly satisfying gas mass conservation within the damper. The model was successfully validated against a MATLAB 2024 Simulink benchmark, demonstrating a maximum pressure amplitude discrepancy of only 5–8%. A parametric analysis evaluated the effects of damper volume, initial gas pressure, and pipe diameter on surge suppression. Results show that insufficient damper volume causes extreme negative pressure drops, risking severe cavitation and fluid column separation. Conversely, excessive volume induces “over-damping,” undesirably increasing system inertia and delaying steady-state recovery. Crucially, scaling analysis reveals that a damper optimized for a specific pipe diameter loses efficacy in larger pipes, as the flow’s kinetic energy scales with the diameter’s square. This model provides a robust, precise computational tool for the optimal and safe design of pipeline networks.

Fluids doi: 10.3390/fluids11070177

Authors: Damian Craiem Mariano E. Casciaro Ezequiel López Sofía Sarraf Sebastián Graf Edmundo Cabrera Fischer Alejandro Valda Eduardo E. Rodríguez

Abdominal aortic aneurysm (AAA) is a critical condition with high rupture risk, and the maximum diameter alone is insufficient for prediction. Pulse wave velocity (PWV), a surrogate of aortic stiffness, can be estimated using 4D-Flow magnetic resonance imaging (MRI), but requires validation under dilated conditions. This in vitro study examined the relationship between PWV and stiffness by comparing healthy and aneurysmal compliant aortic models. Two latex phantoms were fabricated to represent normal and AAA geometries. A circulatory MRI-compatible system simulated physiological inlet flow, with flow rates measured across perpendicular planes using 4D- and 2D-Flow MRI. PWV was derived from transit times of the systolic upstroke and interplane distances. Complementary 1D numerical simulations and laboratory flowmeter measurements were performed. Although the wall elasticity and thickness were identical, PWV in the healthy model ranged from 6.2 to 7.7 m/s and in the aneurysmal model it ranged from 14.2 to 15 m/s. This increase was confirmed by temporal overlap of thoracic flow curves and reduced slope in the transit time–distance regression. Results were consistent across simulations, 2D-Flow, and flowmeter data. Findings highlight that indirect 4D-Flow assessment of thoracic stiffness in the presence of AAA must account for wave reflections introduced by dilation, which significantly alter PWV estimation.

Fluids doi: 10.3390/fluids11070176

Authors: Paerhati Abudukelimu Junde Liu Genying Gao Jianping Zhou Yan Qiao Zhongshan Ma Xinwei Wang

Heat loss in steam injection pipelines is one of the primary causes of low thermal efficiency in heavy oil steam injection. To investigate the heat loss and heat transfer variation patterns in steam injection pipelines, a three-dimensional numerical model was established using Fluent software. The effects of overhead pipeline insulation parameters and environmental factors on heat loss and radial temperature distribution were analyzed. This study systematically quantifies the effects of sunlight radiation for the first time and proposes a heat loss correction equation for sunlight conditions. Additionally, overhead and buried pipelines were compared within the same modeling framework. The results show that selecting insulation materials with low thermal conductivity and increasing the insulation thickness can effectively reduce steam pipeline heat loss. For overhead pipelines, as sunlight radiation intensity increased from 0 W/m2 to 800 W/m2, actual heat loss decreased from 68.43 W/m2 to 58.03 W/m2. A sunlight-dependent effective heat transfer coefficient, hSunlight (E, T), was fitted to extend surface temperature heat loss calculations under sunlightt conditions, in which determination (R2) is 0.9951 and the average error is 5.57%. Under the same operating conditions, the heat loss of the buried pipelines was 62.59 W/m2, which was 8.5% lower than that of the overhead pipelines. The findings provide a theoretical basis for the insulation design, thermal efficiency assessment, and operational optimization under sunlight radiation conditions of steam injection pipelines.

Fluids doi: 10.3390/fluids11070175

Authors: Muhammad Idrees Khan Hua-Dong Yao

Quantum algorithms for flow simulation are advancing rapidly, but reproducible wall-bounded benchmarks with classical reference data are still needed to evaluate future quantum and hybrid quantum-classical solvers. This work presents a forced D3Q19 multiple-relaxation-time (MRT) lattice-Boltzmann method (LBM) benchmark for body-force-driven Poiseuille flow in a three-dimensional channel. The solver combines periodic streamwise and spanwise boundaries, halfway bounce-back walls, moment-space relaxation, and body-force forcing with the half-force velocity correction. The solution is verified against the analytical parabolic profile using relative L2 and maximum profile errors, mass conservation, extrapolated wall slip, and wall-normal leakage. A verification study over grid resolution, relaxation time, forcing strength, and initialization demonstrates second-order grid convergence and robust conservation behavior. The verified timestep is then decomposed into quantum-relevant primitives, including streaming, wall reflection, moment transformation, MRT relaxation, equilibrium evaluation, forcing, macroscopic recovery, and measurement. The resulting benchmark connects flow-solver accuracy metrics with operator-level requirements for quantum implementation, providing a compact reference problem for future quantum processing unit (QPU)-assisted, hybrid quantum-classical, and quantum-linear-solver-based computational fluid dynamics (CFD) studies. Performance gains over classical LBM execution are not assessed here.

Fluids doi: 10.3390/fluids11070174

Authors: Francesco De Vanna

This third edition of the Special Issue “Industrial CFD and Fluid Modeling in Engineering” continues a line of inquiry begun in two earlier collections, in which computational fluid dynamics (CFD) is examined not as an abstract numerical exercise but as a working instrument of engineering practice [...]

Fluids doi: 10.3390/fluids11070173

Authors: Xiaohui Ye Lingke Zhu Jianzhong Fu Junfei Zheng Baikang Zhu Xiuqing Shi Shouxin Zhang

The rotary vane vacuum pump plays a key role in the vapor recovery process at gasoline stations, where its geometric configuration directly influences system efficiency and stability. To address the limitations of conventional designs, this study develops a multi-objective optimization framework using particle swarm optimization (PSO). Under fixed chamber dimensions and specified constraints, the optimal design parameters are identified. The optimized pump achieves a 12.8% increase in average theoretical flow rate over the reference pump. Correlation analysis reveals that the exhaust port start angle has the strongest influence on performance, exhibiting a trade-off between compressed chamber pressure and exhaust pulsation. The rotor radius shows strong negative correlations with all objectives, while the intake port end angle is found to be a redundant parameter. Constraint sensitivity analysis indicates that the rotor radius and exhaust port start angle respond stably to variations in key constraints. Numerical simulations confirm a well-matched exhaust process, with the exhaust pulsation ratio minimized at an exhaust port start angle of 235°. This systematic approach provides an effective methodology for enhancing pump performance while respecting geometric constraints.

Fluids doi: 10.3390/fluids11070172

Authors: Riccardo Martino Fabiana Gargiulo Luca Sarno Kamil Urbanowicz

Unsteady laminar pipe flows, prevalent in oscillating liquid columns, physiological pulsatile systems, and hydraulic transients, exhibit instantaneous velocity distributions that deviate significantly from the classical Hagen–Poiseuille parabolic profile. This deviation is governed by the interaction of inertial effects, viscous diffusion, phase lags, and localized flow reversal. Despite extensive documentation of these phenomena, a systematic framework for quantifying such morphological deviations remains largely undeveloped. This study proposes a suite of physically grounded indicators to quantify velocity profile distortion. These metrics characterize complementary aspects of non-Poiseuillean behavior, specifically: centerline curvature variation, profile flattening, energetic divergence, radial peak migration, reverse-flow fraction, and the emergence of inflection points. The methodology employs an analytical reconstruction of the local velocity field through a generalized Fourier–Bessel solution of the linearized Navier–Stokes equations, calibrated against experimental free-surface motion in a vertical U-tube undergoing damped oscillations. The application of these indicators demonstrates consistent correlations between geometric, energetic, and topological features during acceleration, deceleration, and flow-reversal phases. The findings indicate that no single parameter adequately captures the complexity of unsteady fields, but the integrated framework provides a robust classification of instantaneous flow regimes. This approach delineates the limitations of Poiseuille-based assumptions and establishes a quantitative foundation for advanced unsteady friction models.

Fluids doi: 10.3390/fluids11070171

Authors: Filippo Bittoni Francesca Dell’Agnello Francesco Duronio Joris Degroote Andrea Di Mascio Michele Battistoni

Currently, Computational Solid Mechanics (CSM) and Computational Fluid Dynamics (CFD) simulations are not enough to correctly estimate the different physical characteristics found in the human cardiovascular system. As an alternative to individual simulations, Fluid Structure Interaction (FSI) simulations can yield more accurate physical quantities. In this study a comparison between rigid-wall CFD of a thoracic aorta affected by an aneurysm and the FSI of the Descending Thoracic Aortic Aneurysm (DTAA) itself was performed. The 18-year-old patient-specific geometry of the aorta and its branches was based on the National Institutes of Health public database. A patient-specific pulsatile blood flow waveform and a pressure three-element Windkessel model were set for boundary conditions. Parameters such as wall pressure, velocity distribution, Wall Shear Stress (WSS), Time-averaged Wall Shear Stress (TAWSS), Oscillatory Shear Index (OSI), wall displacement and Von Mises Stress (VMS) were investigated. The research shown that blood flow in the aorta is strongly affected by the onset of the aneurysm, which causes recirculation and uneven flow within the aneurysmal bulge. The results highlight that rigid-wall CFD, which cannot capture wall deformation and aneurysm compliance, leads to an overestimation of velocity, WSS, and TAWSS by 15, 21, and 32% respectively, compared to FSI during the systolic peak; furthermore, a key novelty is represented by the slight underestimation of pressure during the systolic peak, an aspect not previously detailed in the DTAA literature.

Fluids doi: 10.3390/fluids11070170

Authors: Haibin Wang Shifa Zhan Lei Geng Jixin Wang Xiaosong Zhang Tong Li Zhenneng Lu Cantao Ye

A rapid prediction framework for multi-physics field distributions in coal mine airways of variable lengths is presented. The framework integrates a Computational Fluid Dynamics model, a Proper Orthogonal Decomposition model, and machine learning techniques. The study first obtains multi-physics field distributions of temperature, velocity, species mass fraction, etc., in mining airways using CFD simulations under various operating parameters. It then constructs a POD model to decompose the high-dimensional raw snapshot data into mean field and pulsation field components, performing singular value decomposition on the pulsation field to obtain POD spatial modes and corresponding POD coefficients. Machine learning algorithms, including GA-BPNN and Bayes-XGBoost, are employed to construct predictive models of the POD coefficients. The results show that after fitting the relationship between operating parameters and POD coefficients, the multi-physics field distribution within the training parameter range can be rapidly predicted. When the cumulative energy contribution of POD modes exceeds 0.99 of the total energy, the Bayes-XGBoost model achieves minimum R2 values of 0.9448, 0.9999, and 0.9996 for velocity, temperature, and oxygen mass fraction predictions, respectively. This work provides a practical engineering solution for real-time prediction of multi-physical fields in variable-length mine airways, and achieves fast and accurate prediction within the training parameter range.

Fluids doi: 10.3390/fluids11070169

Authors: Darsh Nathawani Matthew Knepley

We present a one-dimensional shear-force-driven droplet formation model with a flux-based error estimator. The model is derived using asymptotic expansion and a front-tracking method to simulate the droplet interface. The model is then discretized using the Galerkin finite element method in the mixed form. However, the solution gradients exhibit large jumps across element boundaries and can grow rapidly due to the highly convective pinch-off process. This leads to an erroneous droplet interface and incorrect curvature. Therefore, the mesh must be sufficiently refined to capture the interface accurately. The mixed form of the governing equation naturally provides smooth interface gradients that can be used to compute the error estimate. The computed error estimate is then used to drive the adaptive mesh refinement algorithm. The efficacy of the error estimator is illustrated by comparing the droplet profiles obtained with adaptive refinement to those obtained with regular refinement. The adaptive mesh refinement approach reduces the computational cost significantly without compromising accuracy. For an 85% glycerol droplet in co-flowing air, AMR reproduces pinch-off location, surface area, volume, and pinch-off time with only ≈1% accuracy loss compared to the highly refined reference while reducing wall-clock time from 638 s to 153 s (4.17× speedup) and reducing the maximum element count from 800 to 146 (81.75% reduction).

Fluids doi: 10.3390/fluids11070168

Authors: Emilia Georgiana Prisăcariu Oana Dumitrescu

Honeybee (Apis mellifera) flight and vertical locomotion were investigated using high-speed imaging and schlieren flow visualization. Free-flight recordings were analyzed to extract wingbeat frequency, projected stroke amplitude, wingtip trajectories, and membrane deformation. The wingtip trajectory exhibited a pronounced asymmetry between upstroke and downstroke, suggesting a dominant role of the downstroke in thrust production. Significant membrane deformation was observed near stroke reversal, indicating strong wing flexibility and dynamic modulation of wing shape during flapping. A novel support-based framework was introduced to characterize vertical locomotion through the support polygon formed by leg contact points and the displacement of its centroid relative to the body. This movement function quantified changes in support distribution and revealed adaptive leg-contact strategies during wall climbing. Schlieren visualization provided qualitative evidence of wingtip vortex formation, although finer wake structures remained difficult to resolve. These findings provide new experimental observations of honeybee flight kinematics and introduce a quantitative framework for analyzing vertical locomotion using support redistribution metrics.

Fluids doi: 10.3390/fluids11070167

Authors: C.Q. Ru

Explicit expressions for bubbly Taylor–Couette flow fields are rarely available in the literature. The present work aims to derive explicit expressions for bubble velocity fields in laminar gas–liquid Taylor–Couette flow between two rotating coaxial cylinders with the buoyancy effect on gas bubbles. It is assumed that the angular velocity of the rotating cylinder(s) is moderately low and the bubble radius is relatively small so that the Stokes number of bubbles is small enough and, consequently, the radial bubble migration is ignorable and the bubble volume fraction can be treated as being constant in a limited period of time. Explicit leading-order solutions are derived for the spiral rising bubble velocity field in the dilute limit. Unlike the heavy particles dominated by the Stokes drag, the added mass and lift forces are shown to be relevant for the bubbly flows. The radial bubble velocity field is discussed in detail for several cases of major interest under the condition that the added mass coefficient is equal to the lift force coefficient, as assumed by some authors in the literature. Our results show that the radial-to-azimuthal velocity ratio of bubbles is linearly proportional to the Stokes number of bubbles and can be controlled by the angular velocity of the rotating cylinder(s) and the bubble radius so that the assumption of ignorable radial bubble migration can be reasonably justified within a limited period of time (for example, in the first few tens of revolutions of the rotating cylinder(s)).

Fluids doi: 10.3390/fluids11070166

Authors: Kahinan Pastro Amine Benmoussa Ricardo Awazu Frederico Rodrigues Mohammadmahdi Abdollahzadehsangroudi

The Tesla turbine operates on viscous shear between parallel discs and, despite its mechanical simplicity, is typically characterized by low efficiency. In the present study, three-dimensional computational fluid dynamics (CFD) simulations performed using ANSYS Fluent are used to examine a hybrid Tesla turbine design in which 0.25 mm thick partial height blades are fitted on the disc faces, with 1 mm distance between them, thereby creating a 0.5 mm flow passage. Simulations employing the k-ω Shear Stress Transport (SST) turbulence model were performed for three blade counts (3, 6, and 9) and three blade geometries (curved, straight, and inverted curve) at rotational speeds from 1000 to 19,000 rpm and inlet pressures of 2 to 4 bar. Comparative analyses with standard 1 mm plane-disc rotors and reduced-gap 0.5 mm plane-disc rotors show that the hybrid arrangement consistently provides better torque and efficiency; this enhancement is not only due to the reduced gap but also to increased pressure-induced momentum and improved flow guidance provided by the blades. The curved blade was found to be the most favourable configuration, and the efficiency was positively related to the number of blades, with a maximum efficiency of 57.5% at 13,000 rpm using nine blades. The analyses sustain the conclusion that adding blades to the rotor discs positions the Tesla turbine model as a hybrid apparatus, combining viscous and pressure mechanisms to significantly enhance turbine performance.

Fluids doi: 10.3390/fluids11070165

Authors: Montri Maleewong Roger Grimshaw

The Korteweg–de Vries (KdV) equation is widely known as a canonical model for weakly nonlinear and weakly dispersive waves, notably and historically for water waves in shallow depths. Being integrable, it has a rich solution set of interacting solitary and periodic waves. Recently, we extended it with several forcing/friction terms to describe the evolution of wind-driven water wave packets in shallow water. The outcome is a modified KdV–Burgers equation, whose relevant solutions are principally solitary wave trains forming a soliton gas. In this article that is extended further by allowing the water depth to be slowly spatially varying, and introducing a basic horizontal current, also slowly spatially varying. The outcome is a modified KdV–Burgers equation with spatially slowly varying coefficients. We adapt the Whitham modulation theory for a slowly varying solitary wave train, allowing for the prediction of wave amplitude growth/decay due to a combination of the slowly varying background and the forcing/friction terms. Numerical simulations using a Fourier spectral method are performed to exhibit and validate the modulation theory.

Fluids doi: 10.3390/fluids11070164

Authors: Duc Hau Nguyen Sylvain S. Guillou Kim Dan Nguyen

Kelvin–Helmholtz (K–H) instabilities play an important role in mixing and entrainment processes in stratified and sediment-laden flows, while their development can be affected by particle properties and rheological contrasts. In this study, a two-phase numerical framework is used to investigate the onset and nonlinear evolution of K–H billows in sediment-laden shear layers. The governing equations are solved using a finite-volume approach with second-order TVD discretization, and the effects of the Richardson number (Ri), sediment particle size, and viscosity ratio (W) are examined systematically. In addition to concentration, vorticity, and interphase slip-velocity fields, a box-counting dimension (BCD) diagnostic is introduced as a complementary measure to quantify the instability onset time, the effective growth-start time, and the maximum geometrical growth rate of the sediment–water interface. The results show that, within the tested parameter range, coherent instabilities develop for Ri≲0.25. Decreasing Ri leads to earlier BCD-based onset, shorter effective growth intervals, larger BCD growth rates, and stronger vortical structures. Sediment granulometry also affects the instability evolution: fine silt-like particles delay the onset and produce broader, more diffuse vortical structures, whereas fine-sand-like particles promote earlier destabilization and more compact billows. The viscosity ratio W modifies the vortex morphology, with high viscosity contrast reducing peripheral deformation and concentrating vorticity within the core region. These findings complement classical descriptions of stratified shear instabilities by highlighting the additional effects of sediment granulometry and rheological contrast. The proposed BCD-based diagnostic provides a practical quantitative tool for comparing interfacial growth in sediment-laden shear flows relevant to estuarine, coastal, and hydraulic engineering applications.

Fluids doi: 10.3390/fluids11070163

Authors: Serhii Tarasov Dmytro Redchyts Koldo Portal-Porras Unai Fernandez-Gamiz Ihor Kostyukov Andrii Tarasov Svitlana Moiseienko Volodymyr Zaika Jesus María Blanco Ilzarbe

Small-scale vertical-axis wind turbines (VAWTs) are increasingly essential for the “blue economy,” providing autonomous power to remote coastal communities, offshore platforms, and marine industries. However, the design of efficient Darrieus-type rotors is complicated by complex unsteady aerodynamics, particularly the phenomenon of dynamic stall. This study aims to establish and validate a cost-effective yet accurate mathematical modeling approach for simulating unsteady turbulent flow around a Darrieus H-rotor to support practical engineering applications. The research methodology integrates computational fluid dynamics (CFD) with physical experiments in a hydrodynamic channel. The numerical model utilizes the unsteady Reynolds-averaged Navier–Stokes (URANS) equations closed with the Strain-Adaptive Linear Spalart–Allmaras (SALSA) turbulence model, chosen for its efficiency in capturing flow separation. The system of initial equations was being devised relatively to an arbitrary curvilinear coordinate system. The pressure and velocity fields have been coordinated using the artificial compressibility method adapted to calculate non-stationary problems. Experimental verification was conducted in the GT-400 hydrodynamic tube using a three-bladed H-rotor model, where flow structures were visualized via the colored jet method at tip speed ratios λ ranging from 2 to 5 and Reynolds number 1470. The findings reveal that dynamic stall occurs over a significant portion of the blade trajectory, characterized by vortex generation at the leading edge and subsequent advection along the chord. Qualitative comparison demonstrates a high degree of correlation between the calculated vortex dynamics and physical flow spectra. These results confirm that the URANS-SALSA approach provides a rational compromise between computational cost and physical accuracy.

Fluids doi: 10.3390/fluids11070162

Authors: Yisen Guo Yang Liu Mark Sussman Hui Hu Yongsheng Lian

Supercooled large droplets (SLDs), typically defined as droplets with diameters exceeding 100 μm, represent a significant meteorological hazard to aviation safety. Unlike conventional cloud-sized droplets, SLDs have higher inertia and can follow more ballistic trajectories, leading to impingement well aft of leading-edge ice protection systems. SLD icing is further complicated by high-speed splashing, secondary-droplet re-impingement, delayed solidification, and surface water runback. This paper reviews recent progress in understanding SLD impact, splashing, surface water transport, and ice accretion. The review discusses droplet impact on dry and wet surfaces, oblique impingement, ambient-air effects, non-instantaneous solidification, runback dynamics, and downstream ice growth. Emerging ice protection technologies, including superhydrophobic, lubricant-infused, and compliant surfaces, are also evaluated. By synthesizing these developments, this review connects fundamental droplet-impact physics with practical aviation icing challenges and mitigation strategies.

Fluids doi: 10.3390/fluids11060161

Authors: Marco Robert Herberg Guglielmo Luca Bambino Stefano De Pinto Giuseppe Pascazio Marco Donato de Tullio

This study investigates the application of the Co-Flow Jet (CFJ) active flow-control methodology to an automotive rear wing through a combined CFD and experimental campaign conducted on a modified McLaren 765LT. The work evaluates the aerodynamic response, energy performance, and practical integration of embedded Co-Flow systems under representative on-track conditions. An extensive CFD design campaign assessed multiple Co-Flow architectures, from which three representative configurations incorporating embedded ducted axial fans were selected for experimental testing. The results indicate that aerodynamic performance is strongly influenced by the interaction between momentum injection, vehicle conditions, and duct architecture. The most effective configuration achieved drag reductions of up to 9% together with downforce increases of approximately 15% under highly loaded conditions, significantly exceeding the repeatability levels of the measurements. The efficiency analysis further showed that, under selected operating conditions, the aerodynamic benefits obtained from the Co-Flow system can exceed the electrical power required by the actuation system. However, increased mass-flow capability alone was not found to guarantee improved aerodynamic performance or efficiency. The results demonstrate the successful integration and operation of a fan-driven Co-Flow system on a production-based vehicle and highlight the importance of momentum injection level and duct design. The findings should be interpreted within the scope of the investigated vehicle and operating envelope. Due to confidentiality constraints, part of the absolute aerodynamic data could not be disclosed, and the results are therefore presented primarily as relative variations.

Fluids doi: 10.3390/fluids11060160

Authors: Khaled Elagamy

This research analyzes the wavy, axisymmetric flow of a Ree–Eyring non-Newtonian nanofluid, infused with motile microorganisms, within a porous, tapered cylindrical channel under a transverse magnetic field. This investigation presents a theoretical framework that may inform the improvement of energy efficiency and thermal management in biomedical engineering applications, such as drug delivery systems and microfluidic biosensors. The work provides an extended insight by a contribution to the evaluation of entropy generation, explicitly considering the influence of motile microorganisms, thereby bridging a gap in the existing literature. The comprehensive physical model further incorporates the combined effects of Joule heating, viscous dissipation, nonlinear thermal radiation, and chemical reactions. Methodologically, the governing nonlinear equations of the system were rendered tractable under long-wavelength and low-Reynolds-number assumptions and subsequently solved using the numerical Runge–Kutta–Fehlberg technique. The key conclusion is that, based on the present numerical model, careful selection of magnetic field strength and microorganism motility parameters may reduce irreversible energy losses, potentially improving the net usable work in advanced nanofluid transport systems for biomedical applications, subject to experimental validation. The most significant finding reveals that the magnetic field serves as a dual-purpose control parameter: increasing its strength boosts total entropy generation by 20–30% while simultaneously raising the Bejan number, confirming heat transfer as the dominant irreversibility mechanism in the system. Additionally, nanoparticle concentration diminishes substantially with elevated chemical reaction rates and Schmidt numbers, while microorganism density is highly sensitive to the Péclet number, which causes flow disruptions.

Fluids doi: 10.3390/fluids11060159

Authors: Xiangquan Li Renwei Ji Ho-Seong Yang Yuquan Zhang Ratthakrit Reabroy Peng Dou Linfeng Chen Lixin Xu

As core structural components for deep-sea oil and gas exploitation, deep-sea risers are continuously subjected to wind, wave, and current loads, which readily induce vortex-induced vibration (VIV) and further trigger structural fatigue damage. Furthermore, the progressive exploitation of deepwater and ultra-deepwater oil and gas resources has exacerbated the complexity and risk of riser VIV, rendering it a critical engineering problem that urgently requires effective solutions. This paper presents a comprehensive review of numerical studies on deep-sea riser VIV, systematically elaborating the fundamental principles, research advances, and application scenarios of three mainstream numerical approaches: semi-empirical models, computational fluid dynamics (CFD) models, and computational structural dynamics (CSD) models. The respective accuracy advantages and inherent limitations of each numerical method are thoroughly analyzed. Additionally, this review focuses on key research hotspots and challenging issues, including VIV responses of flexible risers, dynamic fluid–structure boundary coupling, internal–external flow coupling effects, wake interference of multi-riser systems, efficient VIV prediction, and vibration suppression optimization. The current technical bottlenecks in existing research are clarified. This study aims to provide a systematic theoretical framework and methodological reference for subsequent numerical investigations and engineering applications of riser VIV, and offer technical support for the optimal structural design and safety risk prevention of deep-sea riser systems.

Fluids doi: 10.3390/fluids11060158

Authors: Luis Américo Carrasco-Venegas Juan Taumaturgo Medina-Collana Luz Genara Castañeda-Pérez Aurelio Carrasco-Venegas Daril Giovanni Martínez-Hilario José Vulfrano González-Fernández César Gutiérrez-Cuba Héctor Ricardo Cuba-Torre Lia Elis Concepción-Gamarra Rodolfo Paz-Salazar Salvador Apolinar Trujillo-Pérez

Accurate prediction of contaminant transport and self-purification processes in rivers remains challenging because pollutant dispersion, biochemical reactions, and hydrodynamic conditions interact across multiple spatial scales. This study aims to develop and compare mathematical models for soluble contaminant transport and biodegradable organic matter removal in channels and rivers. Unsteady advection–diffusion–reaction equations were formulated for one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) transport scenarios and solved through numerical techniques based on the transformation of partial differential equations into systems of ordinary differential or algebraic equations. In parallel, the classical Streeter–Phelps model and an extended formulation incorporating turbulent diffusion were implemented to evaluate organic load degradation and oxygen deficit dynamics. Simulations were performed using a Matlab R2019a-based computational framework under representative hydraulic and reaction conditions obtained from literature data and empirical correlations. The results showed that, under specific conditions, the 3D model reproduced trends comparable to those predicted by the 2D model, while the latter approached the behavior of the 1D formulation. The Streeter–Phelps model predicted an organic load removal efficiency of 97.74%, a purification index of 1.9564, a critical time of 18.43 h, and a critical distance of 6.93 km. These findings provide a useful framework for river water-quality assessment and support future applications involving complex hydrodynamic and pollutant-loading scenarios.

Fluids doi: 10.3390/fluids11060157

Authors: Michele Ferlauto

Flow control has assumed a key role in many applied aspects of fluid dynamics and propulsion in efforts toward CO2-neutral growth in air, ground and maritime transportation [...]

Fluids doi: 10.3390/fluids11060156

Authors: Siyu Zou Chang’e Wu Yangyang Tang Qifeng Lv Yue Ma Jinghua Tang

Research on the effect of channel bending on control rod drop is crucial; this paper employs FLUENT dynamic mesh technology to study the drop behavior of a control rod in a simplified control rod channel after bending deformation. It compares the differences in rod drop under different fluid media and various bent channel geometries (straight channel, C-shaped bent channel, S-shaped bent channel), and analyzes the variation patterns of rod drop time, velocity, static pressure, dynamic pressure, and flow field. The results show that under C-shaped and S-shaped bends, the changes in the flow field when the control rod descends without contacting the channel have no effect on the rod drop time.

Fluids doi: 10.3390/fluids11060155

Authors: Nils Tångefjord Basse

Dixit et al. proposed an asymptotic drag scaling method for zero-pressure-gradient flat-plate turbulent boundary layers based on the approximation M∼Uτ2δ, where M is the kinematic momentum rate through the boundary layer, Uτ is the friction velocity, and δ is the boundary-layer thickness. In the present paper, an explicit Reynolds-number-dependent correction to this approximation is derived from the logarithmic mean-velocity profile. Integration of the log law across the layer yields M∼Uτ2δf(Reτ), where Reτ=δUτ/ν is the friction Reynolds number and f(Reτ) is given analytically. Application of the correction to the dataset compiled by Dixit et al. shows that the corrected scaling gives an exponent consistent with the asymptotic value −1/2 within bootstrap confidence intervals, whereas the uncorrected formulation does not. The correction should be viewed as a leading-order amendment, since the derivation uses the logarithmic law outside its strict range of validity.

Fluids doi: 10.3390/fluids11060154

Authors: Sinan Erucar Taylan Bagci V. S. Ozgur Kirca

Particle Image Velocimetry (PIV) has recently evolved from a costly, specialized technique into an accessible method thanks to affordable hardware and open-source software. This work introduces a time calibration validation method tailored for low-cost or Do-It-Yourself (DIY) PIV systems. By utilizing inexpensive components such as light-dependent resistors (LDRs), basic resistors, and data acquisition devices or microcontrollers, the study enables accurate timing analysis of light pulses from synchronized lasers or LEDs. Experimental data obtained in real time using a National Instruments USB-6003 DAQ device confirm the system’s ability to detect light pulses with high temporal resolution. Through voltage signal interpretation, the synchronization accuracy of light sources is validated across different sampling rates. Moreover, the study demonstrates how the internal frequency settings of PIVlab, an open-source PIV software package, can be customized to enhance acquisition flexibility. Timing deviations of up to 20% were identified across selected default frequency settings. The proposed method ensures that low-cost systems maintain sufficient accuracy for phase-sensitive flow measurements, such as oscillatory flow or wave action, contributing to the broader adoption of PIV in resource-limited environments. It presents a low-cost method for validating timing accuracy in PIV systems, employs widely available components and is adaptable to multiple platforms, and enables precise synchronization checks critical for flow visualization.

Fluids doi: 10.3390/fluids11060153

Authors: Bruce Butler Joe Alexandersen Sameer Rao

We present a material distribution topology optimization (TO) framework that directly generates capacity-specific radial trims for severe-service control valves. The method uses an out-of-plane resistance modified two-dimensional turbulence model and objective functions that maximize directional change to create tortuous pressure-staging geometries at predefined channel depths. Four trims targeting non-dimensional capacities (CV) of 0.672, 0.96 (two objectives), and 1.248 were optimized, MSLA-printed, and tested in a globe valve using IEC 60534 procedures. The measured capacities ranged from −13.7% to +4.8% of the targets for a fully 2D optimization process, dropping to a maximum of 7.8% when coupled with a hybrid 3D tuning step. Acoustic detection indicated incipient cavitation at a pressure drop ratios greater than 0.87 for the most highly staged design and 0.73 for the highest capacity design, which is consistent with our simulations of the flow field before fabrication. These results demonstrate that TO can deliver fit-to-service, capacity-tuned trims with excellent cavitation suppression, reducing reliance on large parametric design libraries.

Fluids doi: 10.3390/fluids11060152

Authors: Lingquan Li Jianglan Li Zhouteng Ye Jia Yan Linchuan Tian Xiaoquan Yang

A numerical study of shock wave propagation through multiple raindrops is presented using a density-based compressible two-phase flow solver coupled with a sharp-interface volume-of-fluid (VoF) method. The piecewise linear interface calculation (PLIC) approach is employed to reconstruct gas–liquid interfaces and capture droplet deformation during shock interaction. The numerical framework is first validated using a one-dimensional gas–liquid shock tube problem and a shock–helium bubble interaction benchmark. The method is then applied to investigate shock interactions with single, double, and multiple raindrops under compressible flow conditions. Numerical results show that complex wave structures, including shock reflection, diffraction, and wave interference, develop during shock propagation through raindrop fields. Interactions between neighboring droplets lead to local pressure amplification and non-uniform flow structures.

Fluids doi: 10.3390/fluids11060151

Authors: Andi Cahya Ichi Sri Poernomo Sari Gunawan Yanuar

This study presents a combined experimental and numerical investigation of xanthan gum solutions at 100 and 300 ppm in turbulent smooth pipe flow under temperatures of 30–50 °C and Reynolds numbers of 8000–12,000. Water was used as the Newtonian reference fluid, while xanthan gum was modelled using temperature- and concentration-dependent shear-thinning properties. Experimental pressure-drop data were used to evaluate drag-reduction behaviour, whereas numerical simulations were employed to analyse the associated flow and heat-transfer responses. The results show that XG 100 ppm produced a relatively stable drag-reduction response of approximately 31–39%, while XG 300 ppm showed a wider and more condition-dependent range of about 25–45%. Water exhibited higher Nusselt numbers of approximately 68–106. In contrast, XG 100 ppm produced Nusselt numbers of approximately 45–69, while XG 300 ppm showed lower values of about 35–61. The corresponding heat-transfer reduction ranged from approximately 26–48% for XG 100 ppm and 23–46% for XG 300 ppm. These findings confirm a clear hydraulic–thermal trade-off, indicating that the xanthan gum concentration should be optimised according to both pressure-loss reduction and heat-transfer requirements.

Fluids doi: 10.3390/fluids11060150

Authors: Marwan Khaled Martin Sommerfeld Laurin Mächtig Kai Alexander Schulz Alexander Woitalka Bernhard Weigand

In the context of aircraft engine technologies, sprays are used to inject water into the engine cycle to enhance efficiency and reduce emissions. Accurate specification of droplet injection boundary conditions is therefore essential for reliable numerical predictions. This study presents a numerical validation of a water spray configuration previously characterized using phase Doppler anemometry. An Euler/Lagrange approach is applied to simulate the spray using two distinct injection strategies: an array of injector points (Case 1) and a solid-cone injector (Case 2). Numerical results are compared with experimental data to assess droplet size and velocity distributions. Both approaches capture the main spray characteristics, while Case 1 provides improved agreement due to a more accurate representation of the injection conditions. In addition, the influence of droplet–droplet collisions is investigated using different collision-regime maps. While the collision models lead to significantly different collision outcomes, only minor differences are observed in spray characteristics, with noticeable deviations occurring in the downstream region. Overall, the results demonstrate the importance of accurate injection modeling for reliable spray predictions, while simpler injection approaches remain viable with reduced accuracy. The influence of collision modeling is limited under the present conditions and for the investigated spray metrics, providing insight into its role and limitations in polydisperse sprays.

Fluids doi: 10.3390/fluids11060149

Authors: Maria Adele Cecchini Giulio Soldati Peter Jordan Sergio Pirozzoli

The present work investigates fluid–structure instabilities and flow-induced oscillations of a pitching NACA0012 airfoil through numerical simulations. The flow is modeled using the compressible Navier–Stokes equations in a non-inertial rotating reference frame, while the structural dynamics are represented by a torsional spring–mass–damper system. The analysis focuses on the effects of reduced velocity, equilibrium angle of attack, and elastic axis position on the aeroelastic behavior at low Reynolds number (Re=1000). Particular attention is devoted to characterizing the transition from vortex-shedding-dominated oscillations to fully developed limit-cycle oscillations and to assessing its sensitivity to aerodynamic and structural parameters. The results show a transition from steady flow to vortex shedding and, at higher reduced velocities, to limit-cycle oscillations. Increasing the equilibrium angle of attack promotes an earlier onset of instability and stronger aerodynamic forcing, while moving the elastic axis downstream has a similar destabilizing effect due to the larger aerodynamic moment arm (up to approximately 20% reduction of the critical reduced velocity). The nature of the transition is found to depend strongly on the equilibrium angle of attack, with distinct behaviors observed at low and high incidence. Frequency analysis highlights the progressive coupling between fluid and structural dynamics: vortex shedding dominates in the weakly coupled regime, whereas the structural frequency governs the response in the limit-cycle regime. The study provides a consistent description of the mechanisms driving flow-induced oscillations and of the parameters controlling aeroelastic stability.

Fluids doi: 10.3390/fluids11060148

Authors: Mohamed El Abbassi Cornelis Vuik

This article reviews the linear solvers available in OpenFOAM and assesses their impact on the convergence behaviour of the SIMPLE algorithm. The discretisation of transport equations in CFD results in large and sparse linear systems, for which the choice of linear solver strongly influences the computational time. Although the solver does not change the final discrete solution, the difference in speed and robustness between the solvers can be more than one order of magnitude. A brief overview is given concerning how the velocity and pressure fields are decoupled in OpenFOAM, followed by a detailed review of the main linear solver families, including direct methods, basic iterative methods, multigrid methods and Krylov subspace methods, with attention to their practical strengths and weaknesses. The performance of the most advanced solvers is evaluated on a full-scale non-reacting kiln case consisting of 2.3 million cells. The pressure-corrector equation is identified as the main bottleneck in the SIMPLE algorithm. The conjugate gradient (CG) solver with a multigrid (MG) preconditioner is found to be the fastest and most stable method, achieving speed-ups of up to a factor of 7 compared to the slower advanced methods. Using MG as a preconditioner also improves the robustness of the Bi-CGStab method.

Fluids doi: 10.3390/fluids11060147

Authors: Runze Gao Yurong Li Yu Lv

A reinforcement-learning-based wall-modeled large-eddy simulation (RL-WMLES) framework is proposed to improve the physical consistency of near-wall turbulence predictions. In this approach, a reinforcement learning agent is coupled with the WMLES solver to dynamically adjust a compensating stress term, with the objective of enforcing agreement between the LES solution and the law of the wall. The agent is trained using the proximal policy optimization (PPO) algorithm, where the state is defined as the discrepancy between the near-wall LES velocity and the wall-model prediction, and the action corresponds to modifying a parameterized support viscosity distribution. The proposed method is implemented within a high-performance CFD solver and trained on turbulent channel flow. Numerical results demonstrate that the trained agent effectively reduces the log-layer mismatch and significantly improves the accuracy of near-wall velocity predictions. Furthermore, the RL-WMLES framework exhibits a degree of generalization capability: the trained agent performs robustly with varying levels of numerical dissipation and Reynolds numbers. By introducing a simple interpolation strategy, the same agent can be successfully applied to configurations with different matching locations. Overall, the RL-WMLES framework provides a flexible and data-driven approach for enforcing physical constraints in turbulence modeling. The method shows strong potential for extension to more complex flows.

Fluids doi: 10.3390/fluids11060146

Authors: Lei Wang Zhiqiang Hu Lilin Li Zhenxiang Zhang Liang Tao

This study utilizes a large-scale numerical simulation model to investigate the hydrodynamic behavior and particle transport characteristics of gas–liquid–solid three-phase flow in vertical wellbores featuring multi-source confluence and curved geometries. Simulation results indicate that increasing flow velocity shifts the dominant control mechanism from surface tension to inertial forces, transitioning the flow pattern from slug flow to churn flow. In curved pipe sections, centrifugal phase separation and geometric shielding effects cause significant flow asymmetry and maintain large bubble stability at the inner wall. Additionally, the multi-inlet structure induces shear rate gradients that result in the spatial coexistence of two distinct bubble scales. Furthermore, localized gas concentrations exceeding 70% at the upper inlet can trigger severe gas-locking phenomena and intense pressure pulsations.

Fluids doi: 10.3390/fluids11060145

Authors: H. Pichard A. Maurel P. A. Martin P. Petitjeans V. Pagneux

An optical lens focuses light and a similar device can be developed to focus surface water waves. A detailed description of such hydrodynamic lenses is given, for which the focusing is induced by shaping the bathymetry of the bottom. Classically, the Luneburg lens uses a specific radial variation of the refractive index. The modified Luneburg lens (MLL) introduces an extra degree of freedom, permitting the focal point to be tuned. It is shown how to design the MLL for water waves, and then its performance is evaluated. Compared with a simple parabolic-shaped mount, the MLL is shown to be free of spherical aberration, resulting in a focus with larger intensity and smaller size of the focal point. Moreover, the focusing properties can be tuned and enhanced thanks to the possibility of changing the position of the focal point. The focusing quality of the MLL is described in all water-depth regimes (covering dispersive and non-dispersive waves) and the focusing of linear and nonlinear waves is revealed experimentally. The option of moving the focal point outside the lens, where the water depth is constant, may be useful when locating devices for harvesting wave energy.

Fluids doi: 10.3390/fluids11060144

Authors: Lixia Chen Chao Yuan Junlong Zhao

Optimal sensor placement is a crucial issue in scientific and engineering research. This study proposes an autoencoder-based deep learning framework for automated optimal sensor layout and flow field reconstruction. A dataset is established based on transient Computational Fluid Dynamics (CFD) simulation results of a three-dimensional finite-span airfoil under various Reynolds numbers and angles of attack, enabling high-precision reconstruction of airfoil surface pressure distribution using sparse pressure coefficient data. The multilayer perceptron of both the encoder and decoder adopts an optimal five-layer structure with 200 nodes per layer, and the ReLU activation function delivers superior performance with a training loss reduction of over 45%. When using 50 sensors, the proposed architecture determined detailed placement and obtained a reconstruction error of 0.0604, which outperforms traditional manual sensor placement. The reconstruction accuracy of aerodynamic loads improves with increasing sensor count, but exhibits diminishing returns beyond the optimal threshold 50, necessitating a balanced selection that optimizes performance-to-cost ratio. The proposed method adaptively captures critical flow regions with high gradients, cutting sensor quantity by 60–80% versus grid-based placement. This method can flexibly use either CFD or experimental data in practical applications, offering an efficient solution for aerodynamic field reconstruction.

Fluids doi: 10.3390/fluids11060143

Authors: Giulio Croce Nicola Suzzi

A probability-based model (PBM) is developed to predict the evolution of a population of impinging droplets on a solid substrate and the eventual transition between dropwise and filmwise regimes. A dedicated heat transfer model is designed, in order to estimate the evaporating mass flux when the solid substrate is heated. Statistical information such as the droplet size distribution and the influence of surface wettability, required by the PBM, are derived using a previously developed high-fidelity individual-based model (IBM). The PBM is verified with the high-fidelity model for a small patch of solid substrate. Then, validation with experimental evidence from the literature is carried out in the case of in-flight ice on the NACA0012 airfoil. Results show that the present PBM is capable of investigating in-flight ice problems and can be integrated with a CFD analysis of the air flow past an airfoil flying through a cloud of supercooled droplets to predict the possible onset of ice accretion on the airfoil surface. Compared to Messinger-like models, the influence of surface morphology on runback water flow is incorporated in the PBM through the modeling of a discontinuous wetting layer, contributing to the design of passive and active anti-icing systems.

Fluids doi: 10.3390/fluids11060142

Authors: Lam Lam Yifan Yang Jiahang Chen Lap Mou Tam Afshin J. Ghajar

Additive manufacturing (AM) introduces surface roughness that is much larger than that in chemically etched printed circuit heat exchanger (PCHE) channels, limiting the applicability of established design correlation. In this study, four selective laser melting (SLM) 3D-printed stainless steel test sections were tested, namely two semicircular and two rounded-edge semicircular channels, at hydraulic diameters of 2 mm and 4 mm. Water was used as the test fluid in the experiment, with a Reynolds number ranging from 500 to 7000 and wall heat flux ranging from 20 to 90 kW/m2. Scanning electron microscopy image characterization shows significant material accumulation concentrated at the rounded edges of the as-built channels. The experimental results show that for the entire flow regime, the printed rounded edge increases the friction factor by approximately 9% for 2 mm and 4 mm channels. The filleting design would increase the effective hydraulic roughness in small-diameter AM channels. The SLM 3D-printed rougher channel has a lower transition Reynolds number and higher turbulent friction factors compared to the etching channel. The data were compared with existing smooth PCHE channel data and rough AM mini-channel correlation, and two empirical correlations were developed for SLM 3D-printed mini-channels for transition and turbulent regimes.

Fluids doi: 10.3390/fluids11060141

Authors: Yixiang Zou Yue Ma Jingwen Yan Chang’e Wu Qifeng Lv Jianqiang Shan

As a high-performance innovative fuel rod design, helical cruciform fuel (HCF) exhibits significant advantages over conventional circular fuel rods, such as a larger heat transfer area per unit volume, enhanced fluid flow and heat transfer characteristics due to its helical geometry, and a periodic self-supporting configuration. These attributes make it a highly promising option for future advanced reactor applications. Using the SST k-ω turbulence model, this study numerically investigates single-phase flow and heat transfer in a triangularly arranged 7-rod compact HCF fuel bundle, focusing on the effects of cross-sectional geometry and helical pitch on its three-dimensional flow and heat transfer behavior. Numerical results indicate that reducing the concave arc radius R increases the heat transfer surface area of the rod bundle, effectively enhancing heat transfer performance and reducing wall temperature; decreasing the helical pitch substantially strengthens fluid mixing. However, when the concave arc radius R becomes excessively small, the cross-flow intensity exhibits a local minimum in the concave region, resulting in a significant degradation of convective heat transfer capability in this area. These findings provide valuable insights for the structural optimization and design selection of HCF.

Fluids doi: 10.3390/fluids11060140

Authors: Ismatulla Khujaev Khusniddin Mamadaliev Muzaffar Hamdamov Shohjaxon Ravshanov Makhbuba Boborakhimova Oybek Begimov Shokhrukh Chulliyev

This study conducted a numerical simulation of laminar flow within a cylindrical pipe using a semi-implicit method. The full Navier–Stokes equations in cylindrical coordinates were solved, with modifications to the SIMPLE algorithm to handle pressure-linked equations. We evaluated three key thermophysical parameters—dynamic viscosity, specific heat capacity, and thermal conductivity—under both constant and variable conditions in the entrance region. Due to the process’s two-dimensional, time-dependent nature, third-kind boundary conditions were used to accurately model the effects of ambient temperature, external wind, and the pipe’s geometric and physical features. From the numerical results, we analyzed the velocity field, pressure distribution, surface friction coefficient, and temperature distribution at various pipe cross-sections. These findings are of practical and scientific importance: they offer insights into the hydrodynamics and thermal behavior of the internal flow and enhance understanding of fluid flow and heat transfer, improving predictive models. This advancement supports better design and operational control in pipeline systems.

Fluids doi: 10.3390/fluids11060139

Authors: Gonzalo López-Villacís Pablo Valle-Velasco Martha Sevilla-Abarca Diana Peralta-Zurita Segundo Espín-Lagos

This study presents the hydraulic characterization of a direct-acting pressure-reducing valve (PRV) using a combined experimental and numerical approach. An experimental test bench was implemented to measure inlet, control port, and outlet pressures over a flow rate range from 0 to 4.0 m3/h, under a constant inlet pressure of 8 bar and a set pressure of 3 bar. In parallel, a three-dimensional steady-state CFD model was developed using a sequential force balance analysis between hydraulic and spring restoring forces. The results show good agreement between numerical predictions and experimental data, with a maximum error below 10% in outlet pressure. The pressure drop exhibited a nonlinear increasing trend with flow rate, reaching values close to 1.8 bar at 4.0 m3/h. The flow coefficient Kv remained within a range of 2.2–3.0, while the pressure regulation coefficient S remained below 0.05, indicating stable regulation performance. Additional simulations at 25 bar provided improved agreement with manufacturer data, suggesting that catalog curves may be based on nominal pressure conditions. The proposed methodology demonstrates that steady-state CFD coupled with force balance analysis is an effective and computationally efficient approach for predicting the hydraulic behavior of direct-acting PRVs.

Fluids doi: 10.3390/fluids11060138

Authors: Andrey Kozelkov Andrey Kurkin Kseniya Plygunova Vadim Kurulin Vitaliy Gerasimov

Two methods of accounting for the motion of the bodies—the deforming mesh method and the method of overlapping meshes (or overset mesh method)—are compared using problems with floating bodies, which are typical for the shipbuilding industry. Three problems are considered: oscillation of the cylinder on the water surface, movement of the box under the influence of waves, and heaving and pitching of the ship model in head waves. Numerical computations are carried out in the LOGOS software package, the simulation methodology used is based on the solution of a system of Reynolds-averaged Navier-Stokes equations, and the Volume of fluid (VOF) method to take into account the free surface. In all problems, the characteristics of the movement of bodies are evaluated; the resistance force of the ship model is also determined in the third problem; control values obtained using two methods of accounting for moving bodies are compared with the available experimental data. The results of numerical simulation have shown that both methods predict body movement parameters well; the accuracy in determining the resistance force in the task of streamlining the ship’s hull is also comparable: the difference between the maximum deviations of the resistance coefficient in the computations with deformation and overlapping computation meshes is 0.5%. In the case of computations of the three-dimensional problem, the time spent when using the mesh-deformation method turned out to be 10% more; therefore, the method of overlapping meshes can be considered more optimal when solving such shipbuilding tasks as self-propelled tests and streamlining the ship’s hull with and without wind and wave loads.

Fluids doi: 10.3390/fluids11060137

Authors: Hongfu Mi Runmei Zhou Sixu Chen Nanfang Li Aijie Huang Yu Feng Peng Shao Shuo Wang Yihui Niu Wenhe Wang Geng Tang Hang Yi

Leakage incidents at natural gas distribution stations (NGDSs) present severe fire and explosion risks, demanding immediate, data-driven emergency responses. While crucial for minimizing hazard impacts, real-time prediction of gas dispersion ranges remains a significant operational challenge. To partially address this critical safety need, this study introduces a rapid-response prediction framework prototype integrating computational fluid dynamics (CFD) with machine learning (ML). Specifically, a comprehensive database of 500 experimentally validated CFD leakage scenarios at 60 s was developed first, specifically focusing on mapping gas concentration contours within the critical 5–15% flammability range. To identify the most effective real-time predictive tool, three ML algorithms, including a backpropagation neural network (BPNN), long short-term memory (LSTM), and gated recurrent unit (GRU), were evaluated. The BPNN initially outperformed the sequence models, with a coefficient of determination (R2) of 0.96, a mean squared error (MSE) of 1.35, a mean absolute error (MAE) of 0.77, a maximum absolute error (MaxAE) of 4.94 and an average training time of 4.23 s per epoch. To further meet the stringent speed and precision demands of emergency scenarios, the model was enhanced via particle swarm optimization (PSO-BPNN). This optimized framework achieved exceptional accuracy (R2 = 0.99, MSE = 0.34, and MAE = 0.38) while reducing the training time to just 1.42 s per epoch under the current computational configuration. The developed CFD-ML prototype provides a practical, highly efficient tool for NGDS operators and emergency responders, enabling them to instantly visualize hazard zones, optimize evacuation protocols, and safely mitigate leakage incidents before ignition occurs.

Fluids doi: 10.3390/fluids11060136

Authors: Mattia Pelosin Gianluca D’Errico Tommaso Lucchini Paolo Albertelli

Spray impingement cooling is a well-established heat removal technique employed across a wide range of industrial processes. A particularly significant cooling regime arises when the temperature of the cooled surface surpasses the Leidenfrost temperature of the spray. Developing an accurate numerical framework for this regime holds considerable potential for optimising industrial applications such as cryogenic machining and spray quenching. This paper presents a Eulerian–Lagrangian Conjugate Heat Transfer (CHT) model tailored for spray impingement under Leidenfrost conditions. Two heat transfer sub-models are incorporated to characterise droplet–solid thermal interaction: the first, developed by Breitenbach, is grounded in a theoretical analysis of the droplet impingement process, while the second, proposed by Deb, relies on a semi-empirical correlation. Both models were validated against an experimental correlation obtained from a literature study on orthogonal water spray impingement, yielding mean relative errors of 3.54% for the Deb model and 5.2% for the Breitenbach model across a broad range of operating conditions and surface temperatures.

Fluids doi: 10.3390/fluids11060135

Authors: Pavel Bulat Pavel Chernyshov

Ducted fans are widely used in vehicles with a high engine power per unit swept area, including hovercraft propulsors and vertical take-off aircraft. Computational fluid dynamics (CFD) is a powerful tool for selecting the aerodynamic configuration of new aircraft and engines and for determining their optimal operating conditions. The full Navier–Stokes equations, closed by the Shear Stress Transport (SST) and Spalart-Allmaras (SA) turbulence models, are used to simulate airflow induced by the rotating blades of quadcopter ducted-fan propulsors. Thrust characteristics of the ducted fan are analyzed based on numerical simulations in different flight modes, such as hovering and oblique inflow. Tip clearance and inner-wall effects on thrust and power are reported. For the studied four-blade ducted fan, varying the blade angle of attack from 16∘ to 32∘ raises the thrust coefficient from 0.27 to 0.84 and the power coefficient from 0.18 to 0.50. At a constant shaft power of 3750 W, the optimal relative tip clearance for moderately loaded blades is 1.5% (16∘ angle). For heavily loaded blades (32∘ angle), maximum thrust occurs at zero clearance. However, even at 0.8% clearance, losses are less than 0.1% compared to the closed-tip configuration. For technological reasons, a small clearance is generally preferred.

Fluids doi: 10.3390/fluids11060134

Authors: Qingmin Pan Yongguang Hu

The impact of warmer droplets on cold leaves in sprinkler anti-frost is a case of agricultural engineering involving multiphysics. This study models the leaf as an elastic body of finite thickness, incorporates the temperature field, and establishes a fluid–solid–thermal multiphysics coupling model. The effects of droplet velocity, droplet diameter, and initial temperature are analyzed accordingly. The results show that the higher the Weber number (We) of the droplet, the higher the droplet spreading coefficient and the leaf stress. The maximum spreading coefficient and maximum leaf strain at We of 1583.1 are 1.58 and 4.75 times those at We of 1055.4, respectively. There is a gradual decrease in the leaf deformation, a very rapid process, a cycle of about 10% of the spreading time. The temperature at the impact point on the leaf surface increased with the droplet’s initial temperature but could be influenced by an air bubble trapped at the droplet’s bottom. The modeling and analysis of the dynamics of droplet impact on plant leaves enabled a better understanding of the mechanisms of sprinkler frost protection.

Fluids doi: 10.3390/fluids11060133

Authors: Tomé A. F. da Silva Brendon O. Pontes Elias S. Lima Rodrigo C. V. Coelho

Particles moving in a fluid interact through the flow field they generate, which can lead to complex nonlinear dynamics. One important example is the synchronization of oscillatory motion in biological systems, such as the coordinated beating of cilia or flagella. In this work, we investigate the synchronization of two oscillators interacting through a Newtonian fluid using numerical simulations based on the lattice Boltzmann method. The oscillators are modeled as solid particles undergoing periodic motion, while hydrodynamic interactions are resolved explicitly through the surrounding flow. We analyze how synchronization depends on key physical parameters, including the fluid viscosity, the distance between the oscillators, the natural oscillation frequency, and the initial phase difference. The results are compared with predictions from the Kuramoto model in order to relate the hydrodynamic interaction to an effective phase coupling. We find that the coupling strength required for synchronization increases with both the oscillation frequency and the fluid viscosity, while it decreases with the distance between the oscillators. These results provide insight into the mechanisms underlying fluid-mediated synchronization and help bridge microscopic hydrodynamic models with reduced phase-oscillator descriptions.

Fluids doi: 10.3390/fluids11060132

Authors: Zeeshan Qadir Memon James Liburdy

Flow over permeable beds is important in sediment transport and mixing processes, yet detailed velocity and stress measurements remain difficult to obtain, particularly close to the sediment–water interface (SWI). In this work, we use refractive-index-matched PIV to study turbulent open-channel flow over and within a permeable bed composed of monodisperse borosilicate glass beads. Measurements are reported for three low-ReK cases, ReK=0.224, ReK=0.335, and ReK=0.360, to resolve the mean velocity structure and the associated viscous, turbulent, Reynolds, and dispersive stress distributions. The results show that both the mean velocity and the turbulence intensity decrease rapidly below the SWI, indicating strong damping within the porous bed. Above the bed, the flow retains a boundary-layer structure, and increasing ReK enhances the turbulence intensity without changing the overall regime. The results indicate a shift from turbulent transport above the bed to viscous control within the porous layer, while dispersive stresses peak near the interface. Overall, the SWI controls momentum exchange within a thin region and the porous bed suppresses turbulence penetration into the subsurface.

Fluids doi: 10.3390/fluids11060131

Authors: Belén Reverte-Badillo Clara Trillo-Yagüe Andrés Cremades Ricardo Vinuesa Sergio Hoyas

Fluid-dynamic drag accounts for a substantial fraction of energy consumption across air, ground, and maritime transport systems, making its reduction a critical lever for decarbonizing mobility. While active flow control (AFC) strategies have demonstrated significant drag reduction potential, their design remains constrained by heuristic physical assumptions about dominant flow structures. Recent developments in deep reinforcement learning (DRL) have emerged as a transformative paradigm, capable of autonomously discovering control strategies in high-dimensional turbulent environments. This perspective traces the evolution of drag reduction approaches from classical passive and active control approaches toward data-driven methods based on DRL. A particularly promising direction is the integration of explainable artificial intelligence (XAI) with DRL, which provides physically interpretable information about flow regions associated with drag generation and guides the learning process toward physically meaningful actuation schemes. As a result, XAI-guided DRL controllers have been shown in canonical configurations to achieve comparable or improved drag reduction with substantially lower actuation power than controllers trained directly for drag minimization. This transition from opaque optimization toward flow control informed by dynamical causal relationships represents a key step for the development of energy-efficient and sustainable flow-control solutions for transport systems.

Fluids doi: 10.3390/fluids11060130

Authors: Yonghua Yan Yong Yang

Turbulence remains one of the most persistent challenges in fluid dynamics and engineering [...]

Fluids doi: 10.3390/fluids11060129

Authors: Jesus Gonzalez-Trejo Cesar A. Real-Ramirez Ruslan Gabbasov Fernando Aragon-Rivera Carlos E. Alvarado-Rodriguez

In slab continuous casting, the internal hydrodynamics of the submerged entry nozzle (SEN) play a determining role in mold flow stability and product quality, particularly when external electromagnetic flow-control technologies are not employed. This study analyzes a novel bifurcated SEN design intended to promote stable, highly symmetric outlet jets under asymmetric inlet flow conditions produced by typical flow-control devices. The proposed configuration combines three geometric modifications: a square-section bore, a flow-divider bottom wall derived from a rotated mountain-type geometry, and two bell-shaped protrusions that act as flow modulators positioned immediately above the outlet ports. The hydrodynamic behavior inside the nozzle was investigated using complementary experimental and numerical approaches. Physical modeling was conducted in a scaled water model using particle image velocimetry (PIV) to characterize time-averaged velocity fields and flow fluctuations. In parallel, three-dimensional large-eddy simulations (LESs) were performed to resolve transient flow structures and quantify jet characteristics at the nozzle exits. Both approaches show consistent results. The combined action of the flow modulators and the flow-divider bottom wall robustly induces the formation of two nearly identical counter-rotating vortices in the lower region of the SEN. This flow structure suppresses stagnation and recirculation zones near the outlet ports, mitigates inlet-induced asymmetries, and enhances flow evacuation efficiency. Quantitative analysis of the outlet jets indicates a significant reduction in angular dispersion and a flow-rate imbalance below 0.2%, markedly lower than that observed in conventional SEN configurations. The results demonstrate that appropriate internal geometric design can effectively stabilize SEN hydrodynamics without active control systems, offering a feasible and scalable strategy for improving mold flow stability in industrial continuous casting operations.

Fluids doi: 10.3390/fluids11050127

Authors: Josefina Janeth Miranda-Blancas José Martínez-Trinidad Abraham Medina-Ovando Luis Alfonso Moreno-Pacheco Fernando Alonso-Cruz Osvaldo Quintana-Hernández Ricardo Andrés García-León

Capillary imbibition is the process by which liquids are absorbed into porous materials as a result of capillary pressure differences at the pore scale. Accurate characterization of imbibition dynamics, particularly in the presence of gravitational potential, is essential for understanding fluid transport in diverse systems such as soil, fractured rocks, filtration media, and plant roots. This study presents systematic imbibition experiments using filter papers with pore sizes of 2.5 µm, 11 µm, and 20 µm, each inclined at 80° to quantify the influence of gravitational potential on imbibition behavior. For horizontally positioned samples, the imbibition front propagated radially and symmetrically, exhibiting a power law dependence on time. The measured temporal exponents ranged from 0.386 to 0.403, consistently lower than the theoretical value of 1/2 predicted by the Lucas–Washburn law. With increasing permeability, the temporal exponent approached the Washburn limit, indicating a marked dependence of imbibition dynamics on pore structure. For the inclined configuration at an 80° angle, the imbibition fronts remained nearly circular but exhibited a pronounced displacement of the front center toward gravity. This displacement increased with permeability, from approximately 0.497 cm for the 11 µm filter paper to 3545 cm for the 20 µm filter paper, highlighting the combined effects of permeability and gravitational potential on fluid movement. Furthermore, the advance of the imbibition front was significantly slower in the smallest pores (2.5 µm) compared to the larger ones. Experimental results were evaluated against a theoretical model proposed by Medina, demonstrating moderate quantitative agreement at early times, when gravitational potential effects are less significant. These findings confirm that both the temporal scaling exponent and the spatial evolution of the imbibition front are governed by the porous medium’s permeability and inclination angle, providing experimental evidence of deviations from ideal Washburn behavior in real porous systems.

Fluids doi: 10.3390/fluids11050128

Authors: Sungchan Hong Takeshi Asai

Surface orientation can influence the aerodynamic response of modern soccer balls, particularly in the drag crisis regime. This study quantified the orientation-dependent aerodynamic characteristics of the FIFA World Cup match ball Trionda using a single specimen and examined how these differences affect simulated flight at sea level and 1500 m altitude. Two reproducible reference orientations were defined: a red-panel-centered orientation (Series A) and a seam-junction-centered orientation (Series B). Each reference orientation was rotated by 0°, 90°, and 180°, resulting in six fixed-orientation conditions. Wind tunnel measurements were repeated three times per condition to obtain drag, lift, and side-force coefficients, and two-dimensional non-spinning flight simulations were performed for representative long-kick and free-kick conditions. All six orientations exhibited drag crisis behavior, but the transition response magnitude, subcritical drag level, and supercritical drag state differed among conditions. The representative transition region occurred at approximately Re = 2.0 × 105 to 2.5 × 105. Among the tested conditions, B-90 showed the lowest full-range mean drag coefficient (0.231), whereas A-90 showed the highest (0.266). In the simulations, lower drag orientations consistently produced longer flight ranges, and the B-90 > A-90 ordering was preserved across representative launch conditions and the expanded parametric comparison. These findings indicate that the aerodynamic response of Trionda cannot be represented adequately by a single mean drag coefficient and that surface orientation should be considered in aerodynamic characterization and flight prediction.

Fluids doi: 10.3390/fluids11050126

Authors: Kai Wu Lixia Hu Zhanghao Wan Fupeng Liu Tao Jiang Qiang Zhou Li Luo

This study utilizes computational fluid dynamics (CFD), numerical simulation of particle separation characteristics enhanced by synergistic extraction–shearing is performed, and the two-phase flow in a liquid–solid stirred tank is simulated using the Eulerian–Eulerian two-fluid model and the standard k−ε model. The effects of impeller speed, the hole arrangement pattern of the annular shroud, and the hole area on the multiphase fluid dynamics behavior and stirring power inside the tank are systematically studied. The results show that stirring speed is a key operating parameter affecting turbulence intensity and particle mixing uniformity. When the stirring speed increases from 2000 r/min to 4000 r/min, the overall tank turbulence increases significantly, but the stirring power increases from 4.69 kW to 36.57 kW. The annular cover at the bottom is arranged with vertical openings, which enables full energy transfer within the tank and effectively enhances the turbulence intensity in the middle and lower sections of the flow field; the horizontal opening form is more conducive to the radial diffusion of particles in the middle layer. Reducing the hole area by half increases the fluid jet velocity and local shear stress, effectively improving particle distribution uniformity, while the stirring power decreases by 43.75%, thereby achieving the collaborative optimization of mixing efficiency and energy consumption.

Fluids doi: 10.3390/fluids11050125

Authors: Yu-Wei Wang Bin-Bin Wu Wen-Qing Li Shuai Xu Zhe-Hui Ma Tian-Xiao Zhang Chuang Liu Jin-Yuan Qian

Dry gas seals (DGSs) are currently the preferred sealing method for high-speed rotating machinery, widely used in the fields of petrochemicals and energy and power. This study analyzes the effect of groove structure and operating parameters (rotary ring speed and inlet pressure) on the performance of the sealing system. The results show that a swallowtail-like groove demonstrates a dual effect of improving film stability and reducing leakage under specific working conditions. Specifically, under the inlet pressure of 4.5852 MPa and rotational speed of 10,380 rpm, the swallowtail-like groove achieves a 1.84% reduction in leakage and a 0.32% increase in opening force compared with a conventional spiral groove. Rotational speed has the greatest impact on the gas film stability of the cluster spiral groove. Increasing inlet pressure enhances the dynamic stabilization of gas film. Dynamic analysis indicates that the opening force demonstrates a linear proportionality with inlet pressure, whereas leakage follows an exponential growth. This work can provide guidance for optimizing the groove structure in dry gas sealing systems.

Fluids doi: 10.3390/fluids11050124

Authors: Assel Begimova Zhanna Nadirova Kazim Nadirov Gulmira Bimbetova Berik Sakybayev

Pour point depressants (PPDs) based on functionalized polyolefins were obtained and evaluated for their efficiency in pour point reducing of Kumkol waxy crude oil (Kazakhstan), which contains 15.2 wt.% paraffin and has a pour point of +17 °C. An ethylene–propylene copolymer (EPR-505A) was treated through grafting of maleic anhydride (MA-g-PO) and then converted into three different derivatives that had an identical polymer backbone: an ester-functionalized, an amide-functionalized, and a combined ester–amide additive. The obtained products were tested at 500 g/t through kinematic viscosity measurements, equilibrium and kinetic interfacial tension analysis, pour point determination, cooling curve analysis, and optical microscopy. The ester derivative reduced the pour point by 7 °C, the amide derivative did so by 5 °C, and the combined additive achieved a 10 °C pour point reduction and a more than twofold decrease in kinematic viscosity at 0 °C. Interfacial tension measurements and adsorption kinetics allowed us to assume that ester groups govern macromolecular solubility and diffusion mobility, while amide groups enhance adsorption affinity at paraffin crystal surfaces. Their combined action shifts crystallization from a collective to a dispersed regime. These findings establish structure–activity relationships between polar group architecture and PPD efficiency.

Fluids doi: 10.3390/fluids11050123

Authors: Nguyen The Doan Ngo Minh Tuan Vu Lai Hoang Tran The Long

This paper presents a study on evaluating the effectiveness of nanofluid Minimum Quantity Lubrication (NF MQL) in machining Hastelloy C276 alloy—a difficult-to-cut material. The study compares NF MQL using different types of nanoparticles (Al2O3, MoS2, SiC, and GrP) with dry and pure MQL conditions in terms of surface roughness, cutting force components, and especially the variation of cutting forces over time. Experimental results indicate that the graphene-containing nanofluid MQL showed the most superior performance in terms of surface roughness Ra with 54.3% and 34% reduction, followed by MoS2 and Al2O3 nanofluid MQL conditions. Regarding the active cutting force Fa, Al2O3 nanofluid MQL achieves the largest reduction of about 18.4% and 22.1% when compared to dry and pure MQL, followed by GrP nanofluid MQL, MoS2 nanofluid MQL, and then SiC nanofluid MQL. Meanwhile, GrP nanofluid MQL shows the highest percentage of Fz reduction at about 13.4% and 26% when compared to the dry and pure MQL conditions, followed by MoS2 nanofluid MQL. Furthermore, the application of NF MQL also significantly improves tool life and extends about 36.4 ÷ 61.1% and 18.2 ÷ 50% compared to dry and pure MQL, respectively. Notably, through in-depth analysis of the variation of cutting forces, the study has elucidated the superior lubrication and cooling mechanism of the NF MQL method, confirming its potential application in machining advanced materials.

Fluids doi: 10.3390/fluids11050122

Authors: Muhammad Iqbal Andi Trimulyono Ammarunissa Noor Asiyah Raihannanda Azka Maulana Widestra Berlian Arswendo Adietya Ahmad Firdhaus

Improving ship hydrodynamic efficiency is an important strategy for reducing fuel consumption and operational costs. This study investigates the optimisation of ship resistance through a combined approach involving hull form modification and operational trim adjustment. The research focuses on a pioneer vessel model, where hydrodynamic performance is analysed using Computational Fluid Dynamics (CFD) simulations coupled with Central Composite Design (CCD) and the Response Surface Methodology (RSM). Prior to the optimisation analysis, the CFD model was verified through a grid convergence study and validated against towing tank experimental data, showing good agreement. The optimisation was conducted through three scenarios: hull form optimisation, trim optimisation, and integrated optimisation, which combined both strategies. The statistical analysis revealed that longitudinal parameters play a dominant role in resistance reduction. In particular, the longitudinal centre of buoyancy (LCB) was identified as the most influential parameter in hull form optimisation, while the longitudinal centre of gravity (LCG) was the dominant parameter in trim optimisation. The results show that hull form optimisation alone reduced resistance by approximately 6%, while trim optimisation achieved a reduction of about 4%. The integrated optimisation strategy produced the greatest improvement, resulting in resistance reduction of nearly 10% compared with the baseline configuration. The findings highlight the importance of integrating design-stage optimisation and operational optimisation in improving ship hydrodynamic performance. However, the optimisation was limited to calm-water conditions.

Fluids doi: 10.3390/fluids11050121

Authors: Li Sheng Kaimin Wang Xiaodong Chen Yujun Liu Tanghong Liu

High-speed train aerodynamics have mainly been improved by passive design methods, such as streamlined noses, local fairings, and surface smoothing. These methods have achieved clear benefits, but several important aerodynamic problems remain difficult to solve by geometry optimization alone. Open-air drag is still affected by tail flow separation, base-pressure recovery, and disturbances around bogies and the underbody; crosswind safety is influenced by unsteady leeward-side separation and wake asymmetry; slipstream behavior depends on wake vortices, boundary-layer development, and complex near-ground underbody flow; and tunnel-related pressure transients arise from compression-wave generation, propagation, and reflection. These coupled effects mean that one fixed train shape cannot perform optimally in all operating conditions. For this reason, this review proposes that active flow control (AFC) should not be regarded only as a drag-reduction or stability-improvement technique for high-speed trains. Instead, it should be understood as a mission-adaptive aerodynamic control framework, in which different control actions are used for different operating scenarios. This paper first clarifies that passive optimization is increasingly subject to diminishing returns under multi-objective and engineering constraints. It then reviews AFC studies on drag reduction, base-pressure recovery, wake and slipstream control, underbody flow conditioning, crosswind mitigation, and tunnel pressure-wave suppression. Related AFC studies on bluff bodies, road vehicles, and other separated flows are included only when their physical relevance to trains is clear. The review further distinguishes gross aerodynamic improvement from net energy gain and identifies actuator power, durability, maintainability, acoustic impact, validation level, and full-scale transferability as decisive feasibility factors. Current research is still dominated by open-loop numerical studies with simplified actuation. Future work should therefore move toward multi-objective, closed-loop, energy-aware, sensor–actuator-integrated, and explainable machine-learning-assisted AFC. The main message is that the next step in train aerodynamics is not simply a better fixed shape, but a control-enabled train that can selectively redistribute aerodynamic authority across its mission profile.

Fluids doi: 10.3390/fluids11050120

Authors: Anirban Saha Michael Poirier Dwayne McDaniel

Real-Time In-Line Monitoring (RTIM) of rheological properties such as slurry yield stress is important in different industries for its various benefits such as significant time savings and increased safety/efficiency of processes while reducing secondary waste due to sampling or inaccurate procedures. This paper discusses two methods for characterizing yield stress in real time: the Pressure Loss method and the Liquid Rise method. The Liquid Rise method uses the height of the slurry in a vertical column and the pressure difference to quantify the yield stress. The Pressure Loss method uses the drop of pressure in a laminar flow of slurry to determine the yield stress. Kaolin–water slurry is used as a simulant of the non-Newtonian fluid. An experimental setup is built to demonstrate the methods, and data obtained from the experimental setup is compared with the yield stress obtained from a conventional table-top rheometer (baseline rheology). The results show a good agreement between the experimental yield stress and baseline rheology.

Fluids doi: 10.3390/fluids11050119

Authors: Daniela Delia Alic Imre Zsolt Miklos Cristina Carmen Miklos

Highway merging and platoon formation are critical scenarios in heavy-duty vehicle aerodynamics. This study presents a transient computational fluid dynamics (CFD) analysis of two trucks undergoing a merging maneuver and subsequent platoon formation. A three-dimensional unsteady Reynolds-Averaged Navier–Stokes (uRANS) approach with the SST k–ω turbulence model is employed under zero-crosswind and yawed inflow conditions. The present work provides a time-resolved characterization of truck–truck aerodynamic interactions during dynamic spacing evolution, enabling the capture of unsteady wake effects that are not accessible in steady-state formulations commonly used in cooperative driving studies. Unlike previous steady analyses, the approach resolves transient wake development, vortex shedding, and their direct impact on instantaneous aerodynamic loads. Results identify three interaction regimes: weak interaction, strong wake interaction during wake impingement, and wake recovery at larger spacing. Under zero-crosswind conditions, significant drag reduction is observed, confirming platooning benefits. However, crosswind conditions substantially reduce this benefit and increase lateral loads due to asymmetric pressure distribution and wake deflection. A non-linear spacing–drag relationship is observed, governed by wake evolution and shear-layer interaction. These findings provide quantitative insight into transient aerodynamic interactions and highlight the importance of accounting for unsteady and crosswind effects in platoon performance assessment.

Fluids doi: 10.3390/fluids11050118

Authors: Bharath Sharma William Fox Jianhua Chen Daniel M. Espino Marco Castellani

Blood is a multiphase fluid, constituted of a plasma phase and a red blood cell (RBC) phase. Predicting the distribution of the RBC phase has applications in terms of medical device design, and for the characterisation of the risk of thrombus formation where atherosclerosis is present on coronary arteries. Computational fluid dynamics (CFD) can be used to simulate the multiphase flow of blood, but is time-consuming and requires a high level of technical expertise. This study evaluates the use of artificial neural networks (ANNs), as an alternative to CFD, to predict RBC distribution as part of blood flow through a coronary artery bifurcation model, both including and excluding stenosis. ANNs were trained on a dataset of 80 simulations generated using steady-state multiphase CFD. The initial data-driven ANNs encountered issues with overfitting and high errors in velocity component predictions. A physics-informed neural network (PINN) was employed, using a reduced order model (ROM), to enhance velocity component predictions, achieving average percentage error (APE) within 8.5% of CFD. These improved predictions were integrated into a hybrid model combining the PINN and the data-driven ANN to predict RBC distribution more effectively. The hybrid model achieved APEs ranging from 0.04% to 0.05%. Moreover, the hybrid model’s predictions were 14 times faster than CFD transient runs, demonstrating potential for translation into clinical use. In conclusion, a combined ROM-PINN and data-driven approach enables fast high-accuracy predictions of flow for multiphase fluids such as blood when compared to CFD.

Fluids doi: 10.3390/fluids11050117

Authors: Pu Liu Chuanhua Ge Ruiqi Zhang Ruifeng Tan Shanquan Fan

Gas-lift reverse circulation drilling technology is one of the typical “bottom-hole negative pressure” drilling technologies. This technology can significantly reduce wellbore circulation pressure loss, alleviate the bottom-hole pressure holding effect, and effectively lower the probability of lost circulation. The core theory underlying this technology is multiphase flow in the wellbore. Based on gas–liquid two-phase flow theory, this paper develops a method for calculating bottom-hole pressure during gas-lift reverse circulation. The effects of key operational parameters on bottom-hole pressure were analyzed. The results show that bottom-hole pressure decreases as gas injection rate increases and as the gas injection point deepens. Moreover, the deeper the gas injection point, the greater the pressure reduction. Compared with the results from gas-lift reverse circulation drilling design and monitoring software applied to a shale gas well in southern Sichuan, the two sets of data differ by approximately 3%. The proposed calculation method can predict bottom-hole pressure under gas-lift reverse circulation conditions, overcoming the low accuracy of empirical formulas traditionally used in such operations. This has significant implications for advancing gas-lift reverse circulation technology in oil and gas well drilling.

Fluids doi: 10.3390/fluids11050116

Authors: Enrique Guzmán Leonardo Di G. Sigalotti Lizeth Torres Jaime Klapp

This review summarizes the latest research concerning the horizontal flow of two-phase mixtures with viscosities ranging from 0.2 Pa·s to 6.4 × 104 Pa·s. Although our survey is concerned with Newtonian fluids, a short section is included to briefly discuss certain rheological aspects that should be generally considered. In contrast with previous work reporting on the progress in specific domains (e.g., in the oil and gas, chemical, or geophysical contexts), we seek to provide a comprehensive overview of the methods and results used in different contexts. Accordingly, the scope is widened to encompass a broader range of industrial applications and naturally occurring flows. The interest in high-viscosity flows is motivated by the operational challenges occurring in certain systems, most notably in the oil and gas industry, where the production of heavy and extra-heavy crude oils reduces the margins for a safe and efficient operation. Furthermore, this review underlines the cross-field analogies appearing in a broad range of scales and applications. It emphasizes the fundamental role of viscosity in determining the flow patterns, as experimental evidence suggests that the transition boundaries are largely altered at higher viscosities. Some gaps that could be addressed in future work are briefly discussed.

Fluids doi: 10.3390/fluids11050115

Authors: Claudius Stockinger Antonio Raiolo Ulrich Nieken Abdellah Hadjadj Mostafa Safdari Shadloo

In this paper, the formulation and method of implementation of boundary conditions for heterogeneous reactions in porous media are elaborated. These are implemented into a previously validated lattice Boltzmann model for the simulation of heterogeneous reactions in porous media, extending it on multiple fronts. The formulation of the boundary conditions is validated thoroughly. The conversion of solid carbon to CO and CO2 is chosen as a specific case of application. An extensive parametric study is conducted with a specific geometry consisting of spherical substrate particles, coated with a reactive soot layer, to highlight the capability of the code. The code was able to capture the expected evolution of a combustion front and the influence of process parameters onto its propagation velocity. The propagation speed linearly increased with an increase in the reactant mass fraction and exponentially increased with Péclet number. Also, the CO/CO2 ratio obtained from experimental data could be reproduced with good accuracy. Furthermore, an algorithm for the correct evaluation of the specific surface is presented, which is necessary for evolving solid domains based on realistic geometries containing enclosed cavities. The method of implementation, computational overhead and acceleration technique are discussed. Finally, the model and all boundary conditions are extended to 3D and validated.

Fluids doi: 10.3390/fluids11050114

Authors: Stoyan Nedeltchev

Gas–liquid reactors (especially bubble columns (BCs)) are the most widely used in both the chemical and biochemical industries [...]

Fluids doi: 10.3390/fluids11050113

Authors: Celia Miguel-González Aitor Vega-Valladares Manuel García-Díaz Alejandro Rodrígurez de Castro José González Pérez Bruno Pereiras

Air knives are extensively employed in many cold rolling or tin plate production lines for drying purposes. Generally, these systems are oversized, resulting in excessive energy consumption, a consequence of insufficient understanding of their performance. Considering this deficiency, an empirical exploration was initiated to analyze the functionality of an air knife oriented perpendicularly to a given surface. Given the scarcity of information within the current body of literature, particular emphasis was placed on the regions affected by the finite dimensions of the device. Impingement pressure distributions were measured at the midspan plane and planes parallel to the midspan but extending beyond the projection of the air knife. The midspan impingement pressure profile aligned with the established bell-shaped distribution, whereas the outcomes beyond the air knife’s projection conformed to an analytically fitted similarity principle. Consequently, the mathematical formulations introduced in this study facilitate the mapping of the impingement pressure within the whole impingement plane, encompassing areas influenced by the finite length of the air knife, thereby representing the innovative contribution of this research.

Fluids doi: 10.3390/fluids11050112

Authors: Tianle Li Peng Wang Wang Zheng Donghua Lu Xin Xia Hanghui Zhou Qiaorui Si

This study investigates the factors influencing the performance characteristics of annular jet pumps (AJPs) conveying non-Newtonian fluids, to enhance their suction capability for marine organisms such as jellyfish, which exhibit properties close to non-Newtonian fluids. Based on the power-law fluid model, realizable k-ε model, and volume of fluid (VOF) model, shear-thinning carboxymethyl cellulose (CMC) was selected to simulate marine organisms like jellyfish. Fluent software was employed to numerically simulate the performance characteristics and internal flow field of the annular jet pumps. The results demonstrate that the shear-thinning effect of non-Newtonian fluids reduces the maximum efficiency point of annular jet pumps and decreases the flow rate ratio corresponding to this efficiency point. As the concentration of CMC solution increased to 0.5%, the maximum efficiency point decreased by 5.5%, and the flow rate ratio corresponding to this efficiency point dropped from 1 to 0.8. These findings provide reference and insights for analyzing the full flow field of annular jet pumps pumping shear-thinning non-Newtonian fluids and for structural design of such pumps.

Fluids doi: 10.3390/fluids11050111

Authors: Rachid Chebbi

Bingham fluids, also called Bingham plastics, are used in different industries including the production of food, pharmaceuticals, household products, construction and oil and gas drilling. The behavior of Bingham fluids is viscous above a critical shear stress and rigid-body below the threshold stress value. Knowledge of the size of the entrance region has several applications including hemodynamics and microfluidics. A model for steady Bingham fluid flow in the entrance region between parallel plates is developed using the inlet-filled region concept. A boundary layer model is used to solve the fluid flow dynamics in the inlet region up to the point where the critical shear stress is reached at the edge of the boundary layer. Beyond that point, the boundary layer does not grow, while the velocity profile keeps readjusting in the filled region to asymptotically reach the fully developed flow. The results include boundary layer thickness profiles, dimensionless pressure drop, centerline velocity, friction factor and inlet and entrance region sizes as functions of the Bingham number. The results are validated against the results for the Newtonian fluid case (Bingham fluid yield stress equal to zero) and CFD results, using the finite element method, for nonzero Bingham numbers. In addition, the results are found to asymptotically reach the fully developed flow values for the general Bingham fluid flow case. The effects of the Bingham number are addressed and compared with the literature. The present model is largely analytical, requiring minor numerical tasks.

Fluids doi: 10.3390/fluids11050110

Authors: Matthias Ehrhardt Sergey Pereselkov Venedikt Kuz’kin Sergey Tkachenko Alexey Pereselkov

This paper presents a theoretical analysis of frequency shifts in broadband acoustic field interference structures caused by an internal soliton wave in shallow water. It analyzes the spectral signature of interference-maxima frequency shifts within a coupled-mode framework that describes the scattering of acoustic normal modes under soliton-induced perturbations. Using the weak coupling approximation, analytical expressions are obtained for modal phase variations and the spectral peak frequency associated with the temporal evolution of frequency shifts induced by internal soliton waves. The analytical estimates obtained in the weak coupling approximation are extensively validated using numerical simulations under realistic ocean conditions without invoking it. This paper’s theoretical analysis demonstrates that internal soliton wave-induced mode coupling produces frequency shift spectrum signatures that strongly depend on soliton parameters. These results suggest that it is potentially feasible to estimate key soliton parameters, such as propagation direction, velocity, and effective amplitude, from measured frequency shifts. Numerical simulations demonstrate the feasibility of solving this inverse problem. These findings highlight the potential of frequency shift analysis as a practical, robust tool for remote sensing of internal wave dynamics in ocean acoustics.

Fluids doi: 10.3390/fluids11050109

Authors: Omid Farghadani Abdolamir Bak Khoshnevis Morteza Bayareh

Skewness (S) and kurtosis (K) are statistical measures that provide insights into the characteristics of turbulence. This paper investigates the effects of adverse pressure gradients (APG) on S and K for mean and fluctuating velocities in the turbulent boundary layer (TBL), using the probability distribution function (PDF) and cumulative distribution function (CDF). The velocity distributions in the TBL are obtained experimentally. The experiments are conducted at Re ~ 1.12 × 105. According to the Clauser criterion, the APG parameter is β = 0.62. Two test sections are examined: a straight duct (zero pressure gradient) and a straight diffuser with a divergence angle of 6° and a cross-sectional area ratio of 1:4. Measurements are performed at five streamwise stations (x/c = 1, 1.5, 2, 3, and 4, where c = 100 mm). The results show that the APG does not influence the maximum or minimum values of the PDFs for mean and fluctuating velocities. Compared to the third and fourth moments, variations in the first and second moments are minimal. It is found that S values for the straight duct are lower than those for the straight diffuser. The largest difference is observed in the fourth moment of the PDF, i.e., K. Additionally, four PDF curve-fitting equations are presented for the mean velocity and velocity fluctuations in the TBL for both the straight duct and the straight diffuser. Differential entropy analysis indicates that the decrease in entropy resulting from wall shear and the turbulent boundary layer in the straight channel is more pronounced than the reduction in mean velocity entropy caused by the APG in the diffuser channel.

Fluids doi: 10.3390/fluids11050108

Authors: Georgios Tsonos Sotiria Kripotou Georgios Mavroeidis Christos Tsonos Lorenzo Guazzelli Luca Guglielmero Ilias Stavrakas Konstantinos Moutzouris

The effect of water on the dynamics and ionic conductivity of the ionic liquids 1-ethyl-1-methylpyrrolidinium levulinate ([C2C1Pyr]Lev) and 1-butyl-1-methylpyrrolidinium levulinate ([C4C1Pyr]Lev) was investigated using differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) over a wide temperature range. Although both ILs share the same levulinate anion, water induces markedly different dynamical responses depending on cation structure. In both systems, water acts as a plasticizer, lowering the glass transition temperature; however, the extent of plasticization and the resulting relaxation dynamics are cation-dependent. Stronger water–cation interactions are observed in [C2C1Pyr]Lev, whereas in [C4C1Pyr]Lev, water primarily disrupts alkyl-chain packing, enhancing ionic mobility. Increasing hydration shifts the main relaxation to higher frequencies and increases liquid fragility, while translational ionic motion remains decoupled from structural relaxation. These results demonstrate that water plays a cation-specific and mechanistically distinct role in levulinate-based ILs, providing new insights into hydration-controlled glassy dynamics and charge transport relevant to the design of IL-based electrolytes under non-anhydrous conditions.

Fluids doi: 10.3390/fluids11050107

Authors: Mirza Popovac Thomas Bäuml Dominik Dvorak Dragan Šimić

This paper presents a data-driven surrogate model for predicting cabin airflow and its integration into system-level electric vehicle simulations for energy management analysis. The model employs a graph-based neural network with a mirror-symmetric predictor–corrector architecture and is trained on a dataset generated using computational fluid dynamics (CFD) covering a defined range of inlet velocities and temperatures. The surrogate appropriately reconstructs temperature fields and captures the dominant airflow structures at significantly lower computational cost than CFD. Quantitative evaluation shows high accuracy in passenger-relevant regions, while localized discrepancies remain confined mainly to shear-layer zones. The model enables near-real-time inference and is coupled with a system-level modeling framework for control-oriented simulations that are impractical with CFD. The study is tailored to a specific geometry and operating range, showing that targeted training strategies and physics-based extensions improve robustness, particularly under limited data conditions.

Fluids doi: 10.3390/fluids11050106

Authors: Marco Robert Herberg Stefano De Pinto Marco Donato de Tullio Giuseppe Pascazio

Active flow control represents a key enabling technology for advancing aerodynamic performance, offering significant potential improvements in drag reduction, lift enhancement, and overall efficiency. This paper reviews state-of-the-art active flow control techniques originally developed for aerospace applications and evaluates their applicability to automotive systems, considering constraints such as packaging, efficiency, cost, and integration. A structured classification of fluidic, surface-based (including morphing), and plasma-based approaches is presented, followed by a comparative and decision-oriented assessment of their performance, technological maturity, and feasibility. The results indicate that synthetic jet actuators and morphing-based solutions provide the most balanced compromise between aerodynamic effectiveness and practical implementation. In contrast, conventional fluidic methods are limited by low system efficiency, while plasma-based techniques, although highly responsive, face challenges related to scalability and integration.

Fluids doi: 10.3390/fluids11050105

Authors: Bruno Delhom Chaouki Habchi Olivier Colin Julien Bohbot

Fully compressible two-phase flow configurations present many challenges for numerical modelling, requiring the development of Real Fluid Models (RFMs) able to simulate flows in subcritical, transcritical and supercritical regimes. Such an RFM has been recently developed at IFPEN based on physical properties lookup tables, mainly for binary and ternary chemical systems. This paper proposes an Artificial Neural Network (ANN) approach to overcome the limitations of lookup tables of thermodynamic properties and to apply RFM to multi-species combustion. A methodology for generating an optimized data set by combining a vapor–liquid equilibrium (VLE) thermodynamic solver and the in situ adaptive tabulation (ISAT) method is developed. It aims to improve the neural network training process for two-phase combustion simulations where many species are present. This ANN methodology has been implemented in the CONVERGE CFD solver and validated using a mixing layer (LOX/GH2) benchmark from the literature relevant to rocket conditions, and an academic gaseous (H2/O2) case relevant to hydrogen combustion. The results show that this ANN approach makes H2 combustion simulation possible when coupled to the RFM framework and using a 10-species kinetic mechanism.

Fluids doi: 10.3390/fluids11040104

Authors: Juliana B. Araujo Kieran McDonald Kenneth C. Carroll Mark L. Brusseau

This work investigates the impact of sample size on the measurement of non-wetting/wetting interfacial areas in porous media by X-ray microtomography (XMT). Standard-sized small columns and significantly larger columns, both packed with sand, were imaged using the same industrial XMT system (IMT). Additional small columns were imaged using synchrotron XMT (SMT) to evaluate the comparability between the different systems. The mean bulk densities and porosities for all three imaged sets are statistically identical, indicating that column preparations were robust. The mean non-wetting/wetting interfacial areas measured for the large columns for the low and moderate NAPL saturations (Sn), were 11.3 cm−1 (Sn = 0.13) and 15.3 cm−1 (Sn = 0.22), respectively. The mean interfacial areas measured at the moderate Sn for the two small columns imaged by IMT (16.7 cm−1, Sn = 0.24) and imaged by SMT (16.9 cm−1, Sn = 0.21) are consistent with those of the larger column. In addition, the mean interfacial area measured at the lower Sn for the two small columns imaged by SMT (9.4 cm−1, Sn = 0.12) is consistent with that of the larger column. The results indicate that the small imaged volumes typically used for XMT are sufficient to establish REV conditions for measurement of fluid–fluid interfacial areas in this sand.

Fluids doi: 10.3390/fluids11040103

Authors: Giovanni Cecere Mats Andersson Simona Silvia Merola Adrian Irimescu

This paper studies the dynamics of a low-density gas directly injected onto a flat wall, focusing on the influence of different pressure ratios (PRs) and plate position. Due to safety reasons, Helium (He) was employed as substitute to reproduce the mixing characteristics of hydrogen. A Nd:YAG laser has been used to generate the luminous background in the constant volume chamber (CVC) and vegetable oil particles as trackers to identify the induced flow-field. Two configurations were investigated: the first, with a flat wall perpendicularly positioned at an axial distance of 10 mm from the injector tip, and the second with the same plate at 30 mm downstream of the injector, inclined at 30°. The pressure of injection was swept from 20 to 50 bar, while the backpressure inside the CVC ranged from 2 to 6 bar to enable the reproduction of five different values of PRs: 3, 4, 7, 10 and 17. The comparison of the results in the two configurations has highlighted the role of the plate at short distance in decelerating the jet speed (230 m/s to 160 m/s) while improving the vorticity intensity (+10%). In addition, a stagnation region was observed to form on the flat wall, downstream of the injector axis for 10 mm configuration. In this area the velocity ranged from 50% to 60% compared to the average jet speed. This phenomenon was noted to be less pronounced with the 30 mm, 30° configuration that led to a more contained speed reduction to 150–160%.

Fluids doi: 10.3390/fluids11040102

Authors: Aaron Albuja Juan Bacuy Fernando Almeida Luis Carrión Byron Cortez Josué Pazmiño César Portero Wilmer Suárez Christian Narváez-Muñoz

This study examines the electrohydrodynamic (EHD) behavior of air bubbles rising in deionized water under a non-uniform electric field, with particular emphasis on the influence of applied voltage (0.5–3.0 kV) and gas flow rates of 30 and 40 mL min−1 (corresponding to Reynolds numbers of Reg=107–142) on bubble dynamics. High-speed imaging reveals bubbles with equivalent diameters in the range of deq≈0.8–3.5 mm, enabling a detailed characterization of their deformation, trajectory, and interfacial response under coupled hydrodynamic and electric stresses. At Reg=107, bubbles exhibited stable vertical trajectories with negligible lateral displacement, whereas at Reg=142, inertial and wake effects induced deviations. Increasing BoE reduced lateral displacement, restoring alignment with the electric field. Bubble rise velocities increased by ∼20–30% with applied voltage due to polarization-driven EHD forces. A transition from hydrodynamically dominated to EHD-dominated regimes was identified. While polarization forces govern the initial bubble motion under a strong electric field, bubbles progressively transition downstream to a hydrodynamic regime as the electric field weakens, reducing the influence of polarization effects. These findings provide quantitative insight into coupled hydrodynamic–electrohydrodynamic interactions and support the development of predictive models for controlling bubble trajectories, with implications for electrically tunable multiphase and microfluidic systems.

Fluids doi: 10.3390/fluids11040101

Authors: Jianxin Peng Liwei Wang Xin Qiao Ju Liu Sixin Li Wen Zhang Yanyan Feng Zhanying Zheng Yu Zhou

Non-Newtonian weighted fracturing fluids are used to carry out hydraulic fracturing operations into the deep and ultra-deep earth for oil and gas extraction, though their flow and friction drag characteristics are largely unknown. This study aims to understand the abovementioned characteristics. An engineering-oriented cost-effective numerical scheme is deployed, incorporating LES with a generalized Newtonian fluid constitutive equation, for predicting the non-Newtonian pipe flow and friction drag coefficient Cf. The weighted fracturing fluid is described as a power-law fluid, i.e., viscosity μ(γ˙)=Kγ˙n−1, where both K and n are coefficients related to fluid rheology, and γ˙ is the shear rate. The influences of fluid density ρ, mean velocity U and pipe diameter D, as well as K and n on Cf were documented and compared with a water pipe flow. It was found that Cf = f1 (K, n, ρ, U, D) may be reduced to Cf = f2 (Reg), where the scaling factor Reg = ρU2−nDn/(K8n−1) is the generalized Reynolds number. This scaling law can reasonably well predict the friction drag variation in the pipe flow of non-Newtonian weighted fracturing fluids throughout a range of interests and engineering applications.

Fluids doi: 10.3390/fluids11040100

Authors: Oswaldo González-Gaxiola Milisha Hart-Simmons Husham M. Ahmed Anjan Biswas

This paper examines the sixth-order generalized Boussinesq equation, which describes the dynamics of shallow-water waves, including the effects of surface tension. The study introduces Kudryashov’s R-function neural network approach for the first time, aiming to provide exact solutions to the nonlinear differential equation that represents the mathematical model of the sixth-order generalized Boussinesq equation. This technique incorporates the solutions of a nonlinear differential equation into neural networks, using them as an activation function within the hidden layer. In line with previous research on this topic, two categories of solutions are derived: solitary wave and shock wave solutions. Additionally, this paper includes 3D and 2D graphs to visually represent the solutions obtained.

Fluids doi: 10.3390/fluids11040099

Authors: Sergio Gandía-Barberá Sergio Hoyas María J. Pérez-Quiles

A wall-dependent identification method for low-speed streamwise streaks in turbulent channel flows is presented to overcome a key limitation of uniform thresholds, which tend to produce either an excess of detected structures near the wall or an apparent lack of streaks farther from it. The method is constructed following wall-dependent threshold requirements established in the literature and is applied to DNS data of Couette–Poiseuille flows at Reτ≈250 using percolation analysis and clustering to extract three-dimensional objects. In addition, a percolation study at a significantly higher Reynolds number is included to assess the effect and robustness of the filter beyond the reference case. Although some global trends remain consistent with previous results obtained using uniform thresholds, the present work provides a direct comparison between both approaches and shows more clearly how the threshold definition affects the identification of streaks away from the wall. The detected structures are classified into wall-attached and wall-detached families, separated by a transition region at ymin+=20, and both exhibit self-similar geometric trends. Their spatial distribution is also analysed, showing differences mainly associated with the alignment of attached streaks in pure Poiseuille and pure Couette flows.

Fluids doi: 10.3390/fluids11040098

Authors: Hao Jia Hao Feng Yapeng Wang Jiuchun Sun Xiaopeng Sun Yunlong Sang Haitao Wang

The hydrojet-shield machine, a rapidly advancing shield machine type, uses slurry for excavation and muck removal via a pipeline system. The pipeline includes a flushed feeding system that injects slurry into areas at risk of obstruction. This study provides a computational fluid dynamics (CFD) analysis of the flow characteristics of a large hydraulic shield machine, proposing the Particle Lifting Coefficient (L) and Regional Improvement Ratio (I) as innovative criteria to evaluate the effects of flow rate distribution and cutting wheel rotational velocity. By adjusting the proportion of scouring flow in the lower part of the chambers to 30%, 50%, and 100%, three flow distribution strategies, labeled as FC1, FC2, and FC3, were obtained to suit normal slurry transport conditions, address cutterhead mud accumulation, and deal with the deposition of rock and soil particles at the bottom of the chamber, respectively. The FC3 strategy amplifies the flow of symmetric jets in the lower scouring region, strengthening the upward flow that entrains surrounding fluid, thereby significantly increasing the L and I values in the targeted area and showing great potential for inhibiting the settlement and deposition of rock and soil debris. This study also emphasizes the need to integrate slurry jet distribution strategies with real-time monitoring of cutterhead mud accumulation and chamber deposition, while adjusting cutterhead rotation speed based on geological conditions.

Fluids doi: 10.3390/fluids11040097

Authors: Hiroyuki Kogawa Yoshiki Maeda Masatoshi Futakawa Yanrong Li

Pressure fluctuations caused by a jet in crossflow (JICF) can induce cavitation and potentially damage wall surfaces. In mercury targets for a pulsed spallation neutron source, where cavitation damage progresses due to thermal shock, mercury is confined within a vessel that incorporates a double-wall structure—comprising a narrow channel and a main flow channel—to form parallel flows and suppress damage. However, as the damage progressed, penetration holes were formed in the inner wall separating these flows, and characteristic damage patterns were observed that suggest accelerated damage progression caused by JICF, in which a jet flows from the narrow channel into the main channel. The mechanism underlying this phenomenon has not been fully clarified. Therefore, the flow field and pressure fluctuations around the penetration hole were evaluated using PIV measurements in a water loop and numerical simulations of single-phase flow, with varying jet velocity and jet width. The results revealed that inflow through the penetration in the inner wall generates JICF, which produces vortices downstream of the inflow jet and induces pressure fluctuations that may be associated with cavitation.

Fluids doi: 10.3390/fluids11040096

Authors: Krishna Kant Chaouki Habchi Martha Hajiw-Riberaud Al-Hassan Afailal Jean-Charles de Hemptinne

The growing need to mitigate climate change has accelerated the development of Carbon Capture, Utilization, and Storage (CCUS) technologies, where the safe transport of supercritical CO2 (sCO2) through pipelines is a key challenge. The flow behavior in such systems is strongly influenced by phase-change processes under transient conditions such as decompression and heat transfer and is further complicated by the presence of impurities (e.g., N2, CH4, and Ar). These impurities modify thermodynamic properties and phase boundaries, thereby affecting the overall flow dynamics. In this study, the influence of impurities on leakage, mass flow rate, and decompression wave propagation in sCO2 pipelines is investigated using computational fluid dynamics (CFD) simulations. A real-fluid model (RFM) implemented in the CONVERGE CFD solver is employed, with a tabulation-based approach to accurately capture thermodynamic and transport properties across multiphase regimes. The simulations were validated against available experimental data and performed for varying impurity concentrations to assess their impact on key flow variables, including pressure, temperature, and wave speed. Although simplifying assumptions were used, the results are in fairly good agreement with experimental observations and provide a better understanding of the phase behavior induced by impurities during transient decompression. Additionally, the effects of outlet geometry, pipeline configuration, and the choice of equation of state are examined, highlighting their influence on the predicted flow response. The validity of the RFM modeling framework is further demonstrated by simulations of a large-scale pipeline configuration representative of industrial conditions, which will serve as a benchmark for future improvements.

Fluids doi: 10.3390/fluids11040095

Authors: Batuhan Göçen

This study investigates the dripping and leakage problem in kitchenware known as the “teapot effect” through a multidisciplinary experimental approach encompassing fluid mechanics, material science, and ergonomic design. Unlike previous studies confined to idealized geometries and single-fluid analyses, this work systematically examines 32 distinct spout geometries from commercially available teapots, coffee pots, and milk jugs under realistic operating conditions. Experiments were performed using three fluids with contrasting rheological properties: boiling black tea, cow’s milk, and Turkish coffee on a precision rotating platform operating at quasi-static (1°/s) to isolate surface tension, gravitational, and geometric effects from inertial forces. Three quantitative parameters were measured for each specimen: capillary dome angle, teapot effect angle range, and optimum pouring angle. Results demonstrate that spout tip geometry is the dominant controlling parameter. Thin-lipped elliptical cross-sections effectively suppressed dripping, whereas triangular and wide curved geometries produced the teapot effect across broad pouring angle ranges reaching up to 70°. A spout outlet extension length of 4–5 mm combined with a spout tip radius below 4 mm was found necessary and sufficient for clean flow separation. Furthermore, suspended particles and proteins in milk and Turkish coffee were shown to intensify the teapot effect by disrupting contact line dynamics at the spout tip. These findings provide quantitative design thresholds directly applicable to industrial kitchenware development.

Fluids doi: 10.3390/fluids11040094

Authors: Stephanie Iris G. Pinto Jose G. Vasconcelos Alexandre K. Soares

Hydraulic transients in pressurized pipe systems are significantly influenced by the presence of entrapped air, which alters wave propagation through increased compressibility and energy dissipation. Traditional discrete cavity models, such as the Discrete Gas Cavity Model (DGCM), often assume a constant wave celerity, which limits their accuracy under high gas content conditions. This study evaluated different approaches for representing the effects of gas cavities and unsteady friction in closed pipe transients. The work introduces the Adjustable-celerity Gas Cavity Model (AGCM), a formulation that explicitly couples local air volume and pressure to dynamically adjusted celerity values. Two variants are considered, a non-iterative (AGCM.v1) and an iterative approach (AGCM.v2), the latter ensuring consistency between pressure head and celerity at each time step. The models were evaluated through numerical simulations using both experimental datasets and a hypothetical test case with increasing air fractions. Results show that the AGCM was able to represent celerity magnitudes in unsteady flows with large fractions of air. Also, while constant-celerity models perform well under low-air conditions, variable-celerity formulations offer superior consistency in predicting wave amplitudes and celerity dynamics as gas content increases. These findings underscore the importance of dynamic celerity coupling in transient flow modeling and validate the AGCM as a useful approach for transient modeling in conditions with higher air phase fractions.

Fluids doi: 10.3390/fluids11040093

Authors: Yaxiong Zheng Fei Peng Zhanzhi Wang Jianming Lei Shan Pian

To improve unified modeling of steady two-dimensional lid-driven cavity flow across a wide range of Reynolds numbers, this study proposes a Vorticity-Enhanced Physics-Informed Neural Network (VE-PINN). The method augments a standard velocity-pressure PINN with a vorticity-transport residual and uses a logarithmic Reynolds-number embedding, log10Re, for multi-regime training. Using CFD benchmark data as supervision and evaluation, we conduct systematic ablation studies on network architecture, loss weighting, sampling density, input embedding, and physical constraint over Re=1000−50000, together with out-of-range extrapolation tests. The results show that the logarithmic Reynolds-number embedding improves cross-regime training stability and reduces the multi-regime mean relative error, while the vorticity-transport constraint improves the reconstruction of velocity fields and secondary vortical structures with only a modest increase in training cost. Further comparisons based on contour fields, centerline velocity profiles, vortex-core locations, and vorticity intensity indicate that VE-PINN provides improved accuracy, physical consistency, and generalization relative to the baseline PINN in the present benchmark. These findings suggest that, for the steady cavity-flow problem considered here, combining logarithmic parameter embedding with derivative-level physical constraint is a practical and effective strategy for parametric PINN modeling.

Fluids doi: 10.3390/fluids11040092

Authors: Atsuyoshi Sato Arata Kioka Masami Nakagawa Takeshi Tsuji

This paper investigates the combustion dynamics of interacting lazy multi-component gas plumes (i.e., buoyancy-dominated gas releases with a low initial momentum flux), a configuration relevant to coal mining waste emissions. By coupling a three-dimensional large eddy simulation (mesh size of 10−2 m; paralleling with 2048 processors) with detailed chemical kinetics (GRI-Mech 3.0), we analyzed the sensitivity of the flow structure and plume stabilization to the vent spacing of twin hydrogen-rich multi-component gas plumes (H2-CO-CH4-air). The results identified a distinct topological transition. While gas plumes from vents spaced at δ/D=5 (δ and D are the spacing and width of gas vents, respectively) evolve independently, those at closely spaced sources (δ/D=5/4) exhibit rapid coalescence driven by hydrodynamic shielding. This hydrodynamic merging results in a unified column with an effective hydraulic diameter of Deff≈2D. This leads to a significant reduction in the surface-to-volume ratio available for ambient air entrainment, maintaining a coherent combustible-rich core to higher altitudes than isolated-source correlations would predict. However, despite this mass retention, the rapid vertical acceleration of buoyancy-dominated flows induces high strain rates, significantly disrupting the reaction zone structure. These findings establish that, for clustered emission sources, the dispersion hazard is governed by a coupling between hydrodynamic coalescence, which maintains reactant concentration, and finite-rate chemistry, restricting oxidation efficiency. This paper provides critical insights for designing gas capture infrastructure and assessing flammability limits in multi-vent systems.

Fluids doi: 10.3390/fluids11040091

Authors: Amir H. Hirsa

Nearly all biological fluids and a vast number of non-biological fluids are, to some extent, non-Newtonian, with many also exhibiting viscoelastic responses [...]

Fluids doi: 10.3390/fluids11040090

Authors: Ren Paulo Estaquio Ma Kevina Canlas Neil Astrologo Job Immanuel Encarnacion Joshua Agar Ken Bryan Fernandez Julius Rhoan Lustro Joseph Gerard Reyes

Indoor exposure to particulate matter (PM) depends on ventilation-driven transport, yet sensor placement in real rooms is often based on limited point data. This study develops and experimentally validates a transient CFD framework, using RANS airflow coupled with Lagrangian discrete phase tracking, to map PM2.5 and PM10 in a full-scale 2.0 × 3.0 × 2.5 m bedroom with a fixed, non-oscillating pedestal fan and an open window. Airflow was verified by grid independence and validated against 10-point velocity measurements (RMSE = 0.108 m·s−1). Incense experiments (≈31 min burn) provided PM time series over the first 60 min at 16 locations on two heights; emission rate, burning time, and air-change rate (1.96–5.39 ACH) were calibrated so that accepted models achieved aggregate R2 > 0.90. Spatial mapping on a 0.5 m grid shows that PM behavior is governed primarily by airflow-defined accumulation pockets rather than by source proximity alone. A near-source region consistently captured strong early-time peaks, whereas remote low-exchange pockets remained elevated during the decay phase. For PM2.5, the most persistent hotspot is a ceiling-adjacent recirculation pocket, while for PM10, gravitational settling shifted the dominant hotspots toward floor-layer, low-velocity regions. An exposure score combining normalized peak and time-averaged concentrations, interpreted together with particle-track persistence metrics, distinguished transiently traversed regions from true retention pockets. The results show that sensor placement should follow the monitoring objective: near-source regions are more responsive to peak events, ceiling pockets are more suitable for persistent PM2.5 monitoring, and floor hotspots are more critical for PM10. No single fixed sensor location adequately represents both particle sizes in the present bedroom and ventilation configuration.