Search Results (1,153)

Transverse modification of step geometry has been repeatedly proposed as a means of increasing energy dissipation in stepped spillways, with numerically reported gains ranging from 5.6% to 34.7% for labyrinth configurations, in unresolved contradiction with air–water experimental evidence that detects no measurable difference. The objective of this study is threefold: to verify whether the transverse alternation of the tread length increases the net energy dissipation of the coupled chute–stilling basin system, to quantify its effect on the kinematic and energetic state of the flow delivered to the terminal energy dissipator, and to delimit its domain of applicability. That premise is subjected to verification through 38 three-dimensional Reynolds-averaged Navier–Stokes (RANS) simulations (k–$\omega$ shear-stress transport (SST) closure, homogeneous volume-of-fluid (VOF) formulation) performed in ANSYS CFX 2025 R2, comparing a three-section stepped spillway with a uniform rectilinear footprint against a configuration with transverse L–$L/2$ alternation, under 19 geometric–hydraulic combinations spanning the nappe, transition, and skimming flow regimes ($0.57\le d_c/h\le 2.93$). The model was verified through a mesh-convergence analysis of the uniform configuration (grid convergence index, GCI $=0.66\%$, on the approach depth) and validated against a physical scale model (0.38% discrepancy, exceeding the propagated experimental uncertainty); a three-level mesh study of the variable-tread configuration shows that its toe-flow response develops as the transverse tread strips become resolved and is not yet mesh-independent at the finest level, which is stated as a limitation of the quantitative results. Because the homogeneous multiphase formulation does not include an air-entrainment submodel, all results correspond to the modeled non-aerated flow conditions. The results do not support the hypothesis of a net dissipative gain: the global energy balance of the two topologies is equivalent within the numerical resolution of the study (bias $+0.13$ pp; root-mean-square error (RMSE) 0.26 pp), of the order of the discretization uncertainty of the study itself. The actual effect is a redistribution of the dissipative partition that conditions the flow delivered to the energy dissipator: the toe Froude number is reduced in all 19 paired cases (16.5–88.4%; mean: 34.1%). In nine scenarios the hydraulic jump is conditioned without being suppressed ($F{r}_1$ from 3.95–4.57 to 1.19–3.71), the energy delivered to the stilling basin drops by 18.3–57.9%, and the residual energy decreases by up to 14.98%; under subcritical toe flow, the same thickening increases the delivered energy by 12.3–19.4% and penalizes the residual energy by up to 18.08%; in two intermediate-discharge scenarios the jump is suppressed, yielding no benefit whatsoever. The transition is expressed through a critical threshold, nominally ${(d_c/h)}_{\mathrm{crit}}\approx 1.48$ within the observed separation interval $1.44<d_c/h<1.53$: the thirteen resolvable scenarios preserve the predicted sign separation without exception. The variable footprint is not a dissipation intensifier but a chute–dissipator coupling element, applicable only to high relative discharges. Full article

Accurate prediction of the dynamic response of wheeled amphibious vehicles during slope water entry is critical for operational safety, as this process involves intense gas–liquid–solid multi-phase coupling with highly transient and nonlinear characteristics. This paper establishes a bidirectional multi-physics co-simulation framework integrating STAR-CCM+ (version 2506) and ABAQUS (version 2024). For the fluid domain, the Reynolds-averaged Navier–Stokes (RANS) equations are solved in conjunction with the Realizable k-ε turbulence model, the volume of fluid (VOF) interface capturing method, and the over-set mesh technique. For the solid domain, rigid body dynamics and contact mechanics formulations are employed, with hull–slope contact constraints explicitly incorporated. Closed-loop coupling between the two domains is achieved via time-step-synchronized data exchange. The proposed method is validated against water entry experiments, with favourable agreement between numerical predictions and experimental measurements. Parametric analysis quantifies the effects of slope angle and initial entry velocity on peak submergence depth, pitch response, and slamming loads. By explicitly incorporating hull–slope contact and friction into the bidirectional coupling process, the proposed framework offers a model- and condition-specific numerical basis for preliminary operating condition screening and safety assessment during cross-medium transition; a generally validated operational envelope remains to be established in future work. Full article

Ship maneuverability in waves is governed by coupled hull–rudder–propeller interactions under wave-disturbed inflow. This study simulates the self-propelled maneuvering of the KRISO Container Ship (KCS) using the unsteady Reynolds-averaged Navier–Stokes (URANS) equations, the volume-of-fluid (VOF) method, dynamic fluid–body interaction (DFBI), an overset grid, and a geometry-resolved multiple-reference-frame (MRF) propeller. The numerical method is assessed against standard open-water propeller data and KCS zigzag and turning-circle data. Wave heading redistributed the response rather than changing all components uniformly: the following-sea case produced the largest reported resistance variation (51.99 N), the beam-sea case produced the largest heave amplitude (0.0150 m), and bow-quartering and following seas produced nearly equal pitch amplitudes (0.378° and 0.376°). Increasing the rudder angle from 17.5° to 22.5° reduced the tactical diameter from 11.25 m to 7.14 m (36.5%) but increased the estimated transient roll-growth slope from 0.013°/s to 0.035°/s (about 2.7 times), while heave amplitude changed by only 0.0035 m. The unequal response rates reveal that tighter turning is obtained at a disproportionate roll and propulsion-unsteadiness penalty, whereas vertical translation remains comparatively weakly coupled to steering intensity. Full article

In pressurized irrigation networks, water-intake hydrants combine flow regulation, shutoff and metering in a single cast body, and the resulting coupled resistance obscures how much each function costs. The conventional body of a DN100 (nominal diameter 100 mm) hydrant and a redesigned production variant were solved with an incompressible Reynolds-averaged Navier–Stokes (RANS) model in three functional configurations (bare body, +regulator, +metering line) at five inlet velocities (1–5 m s⁻¹, Re ≈ 1.0 × 10⁵–5.0 × 10⁵). The mesh was selected after a four-level refinement study, and the quarter-symmetry assumption was verified against two full-domain calculations with both symmetry planes removed: the reduced domain costs 1.0% in pressure drop, and the transverse forces and the net moment on the regulator vanish in the time mean, leaving the axial thrust as the only physically meaningful regulator load. The redesigned body lowers the bare-body loss coefficient by 30.9% and raises K_v by 20.3%. Adding the regulator removes that advantage—ξ becomes 3.0% worse and the full-device axial thrust 11.9% higher—and adding the metering line widens the gap to 37.0% in loss and 60.8% in thrust. Loss and mechanical load therefore reverse sign together at integration, so a body improvement demonstrated in isolation cannot be assumed to survive assembly. The decomposition also acted as a consistency check: a negative metering-line increment in the conventional device, impossible for a passive addition, was traced to a 23.5% passage enlargement present in the supplied full-device geometry rather than to the flow. Wall data give a minimum static pressure of 0.20 bar and a local cavitation number of 1.39, a margin five times narrower than a lumped estimate suggests. Flow-bench measurements on the finished product provide an independent order-of-magnitude reference rather than a validation. The configuration-additive decomposition and the axial-thrust indicator together form an evaluation approach transferable to other devices that combine sub-functions in one body, subject to confirmation on further geometries and Reynolds ranges. Full article

Precise prediction of pressure drop and drag-reduction performance is essential for improving the hydraulic efficiency and reducing the energy demand of crude-oil pipeline transportations. Therefore, this study aims to develop an integrated computational fluid dynamics (CFD)–machine learning (ML) framework for predicting pressure drop (∆p), drag reduction (DR), pumping power reduction (PPR), energy savings (ES), and flow-rate enhancement (Q) in turbulent crude-oil pipeline flow containing drag-reducing agents (DRAs). The investigated system considers the effect of pipeline length (L), diameter (D), surface roughness (ε), operating temperature (T), and DRA concentration (25–200 ppm). The Reynolds-average Navier–Stokes equations (RANS) were solved using the shear stress transport (SST) k-ω turbulence model approaching near-wall resolution of y⁺ ≈ 1 for DRA3 at 20 ppm. The CFD modelling was first used to validate an experimental benchmark and subsequently used to expand the available dataset over the investigated operating conditions. The combined experimental–CFD dataset was then employed to develop a multi-output Kolmogorov–Arnold network (KAN) surrogate model. The proposed framework predicted DR up to 44.2%, PPR of approximately 55 W, ES of 30%, and flow-rate enhancement up to 5–10(${\displaystyle \frac{\mathrm{L}}{\mathrm{d}\mathrm{a}\mathrm{y}}}$). The KAN model effectively captured the nonlinear relationships among DRA characteristics, pipeline geometry, and operating conditions, achieving R² = 0.9318 for PPR prediction. The novelty of the proposed work lies in integrating a validated, near-wall-resolved SST k-ω CFD model with a multi-output KAN surrogate model, combining physics-based flow analysis with rapid data-driven prediction of hydraulic and energy-performance indicators. Full article

This study investigates the hydrodynamic behavior of an amphibious rescue and operation platform equipped with detachable floating boxes and a front bucket system, and it further develops a control-oriented turning optimization framework for a two-bucket steering configuration. Calm-water resistance, free-surface evolution, running attitude, and roll decay were analyzed using a Reynolds-averaged Navier–Stokes/volume-of-fluid solver with overset grids and dynamic fluid–body interaction. Straight-ahead non-rotating cases were computed using a symmetry-based half-domain model, whereas roll- and turning-related cases were simulated in the full domain. The numerical method was validated against towing-tank data for a benchmark amphibious vehicle, and the predicted resistance showed an overall deviation of 2.11%. The results show that the detachable floating boxes slightly increase resistance at 2 km/h, but reduce resistance by approximately 11.4% at 8 km/h owing to favorable wave interference. They also reduce trim and heave over the investigated speed range and markedly improve transverse stability, with the roll motion decaying to nearly zero within about 20 s. By contrast, the installation of the bucket substantially increases hydrodynamic resistance; at the design cruising speed of 8 km/h, the resistance increase reaches about 74.4%, while a bucket-induced bow-down moment modifies the running attitude and suppresses heave. At cruising speed, the bucket swing-arm angle has a non-monotonic influence: the resistance reaches a local peak near 6°, the minimum resistance is obtained at 20°, and the smallest trim is achieved at 4°. Based on these findings, a symmetry-preserving hydrodynamic surrogate and a constrained optimization strategy were established for bucket-controlled turning-radius allocation. The results indicate that differential bucket motion is the primary steering mechanism, whereas the bucket-arm angle provides secondary steering amplification at the cost of additional drag. The present study provides an integrated hydrodynamic basis for the design, operation, and steering-oriented control allocation of amphibious rescue platforms. Full article

This study presents a hybrid CFD–ε–NTU–neural network framework for evaluating the thermal performance of compact heat exchangers installed in confined nacelle environments under severe space and weight constraints. Airflow was simulated using steady Reynolds-Averaged Navier–Stokes (RANS) equations with the shear stress transport (SST) k–ω turbulence model, while heat-exchanger performance was evaluated using the effectiveness–number of transfer units (ε–NTU) method. To provide a continuous functional interface between the analytical model and the CFD solver, a neural network was employed as a representation of ε–NTU-based thermal performance. The proposed framework was validated against wind tunnel experiments, showing good agreement with measured data, with a maximum deviation of approximately 2.2 °C in coolant outlet temperature. Using the validated framework, a multi-objective redesign of a compact heat exchanger was performed under practical installation constraints. The selected design achieved a 28.4% reduction in heat exchanger mass while maintaining cooling performance comparable to the baseline design. Installation performance was evaluated numerically in a modified nacelle configuration, where the redesigned heat exchanger increased the air mass flow rate by approximately 13% under identical fan operating conditions because of its reduced air-side flow resistance. The proposed framework provides a practical and efficient approach for system-level design and installation analysis of compact heat exchangers for aircraft thermal management systems. Full article

Circulation control is an efficient active flow control technique that enhances aerodynamic lift by injecting a tangential jet near the trailing edge, altering the circulation around the airfoil. This study investigates the influence of three key geometric parameters—Coanda surface radius, slot height, and jet angle—on the aerodynamic performance of a NACA4309-based circulation-control airfoil (NACA4309-CCA) operating under a fixed ground clearance (hg/c = 0.14), representative of high-speed aerodynamically alleviated marine vehicles. Numerical simulations are performed using the Reynolds-averaged Navier–Stokes equations with the SST k-ω turbulence model. The results show that increasing r/c enhances lift up to a limit (r/c ≈ 0.017), beyond which flow separation occurs, reducing lift. For a fixed momentum coefficient (=0.01), an optimal h/c = 0.0007 balances jet momentum and mass flow, yielding the highest lift. The jet angle study reveals that the maximum lift (CL = 2.684) is achieved at θ ≈ 10°, but θ = 0° (CL = 2.581) is recommended for practical implementation due to simpler geometry and stable attachment, with only a 3.84% loss in lift relative to the maximum. The findings provide comparative numerical trends for the design of circulation-control systems on aerodynamically alleviated marine vehicles under the investigated conditions. Full article

Tip leakage degrades ducted fan performance and contributes to unsteady loading noise. This study investigates a wall-penetrating blade ring (WPBR) ducted fan for unmanned electric vertical takeoff and landing (eVTOL), replacing radial clearance with axial end face gaps. Six configurations of a 381 mm four-bladed rotor were evaluated at 5000 r/min under quasi-hover conditions: a conventional ducted fan, an internal blade ring rotor, and four WPBR cases with single-sided clearances of 0.8–2.0 mm. Sliding-mesh unsteady Reynolds-averaged Navier–Stokes simulations using the shear stress transport k-ω model were coupled with the Ffowcs Williams–Hawkings formulation. The WPBR formed a U-shaped cavity recirculation and redistributed the concentrated tip-region vortical structures. The 1.2 mm case retained 28.72 N of thrust, 2.1% above baseline, while reducing torque by 6.1% relative to the 0.8 mm case; its figure of merit remained lower. Its simulations predicted a reduction of 16.4 dB in the first blade-passing frequency level in the rotor plane and a predicted reduction of up to 15 dB in overall sound pressure level at 1 m. Thus, it represents a compromise among thrust, torque, and predicted acoustic performance rather than an aerodynamic optimum. A magnetically supported prototype operated up to 2000 r/min, demonstrating low-speed operability of the architecture for unmanned eVTOL propulsion. Full article

This study investigates the turbulent skin friction behaviour of original coated, slime/algae-covered, and laser-cleaned marine surfaces through a combined experimental and numerical approach under fully developed channel flow conditions. Numerical validation was performed against established smooth-wall experimental datasets over the investigated Reynolds number range using Reynolds-Averaged Navier–Stokes (RANS) and Large-Eddy Simulation (LES) approaches. LES provided the closest agreement with the reference data, while the two-dimensional wall function RANS $k$-$\omega$ SST model was selected for subsequent rough-wall simulations based on its balance between predictive accuracy and computational cost. The selected RANS approach was independently validated against rough-wall experimental data, showing good agreement in predicted skin friction behaviour. Measured surface elevation data were used to estimate equivalent sand grain roughness for the original coated, slime/algae-covered, and post-cleaning surfaces. The post-cleaning surfaces exhibited lower measured roughness and predicted skin friction coefficients than the slime/algae-covered surfaces, while the three investigated laser configurations produced broadly similar roughness characteristics and hydrodynamic responses. However, the original coating was characterised before immersion, whereas the fouled and post-cleaning surfaces had undergone water exposure. Because the coating is hydrolysing/self-polishing, the observed post-cleaning roughness reduction may be partly or entirely associated with immersion-induced coating changes and cannot be attributed uniquely to laser treatment. The results therefore demonstrate an association between the post-cleaning surface condition and reduced roughness and predicted skin friction, but do not establish a causal relationship between laser treatment and these reductions. Full article

The paper concerns the numerical optimization of the Tesla turbine rotor working with R1233zd(E) fluid to maximize isentropic efficiency. The calculations were carried out using the Reynolds-averaged Navier–Stokes approach based on the finite-volume method. The heat source temperature was assumed to be 368 K, and the fluid superheating level was 5 K. The heat sink was at 313 K. The inlet apparatus consisted of 10 converging nozzles with a minimum throat width equal to 0.2 mm, which corresponds to the partial admission coefficient of 0.029. The design variables included outlet radius, gap size, and rotational speed. The Kriging surrogate model was created on the basis of the calculated cases obtained from a design of experiments. The genetic algorithm method was used to find the optimal design points. The optimization resulted in an improvement of the isentropic efficiency from 8.8% to 12.5%. Optimal designs promoted smaller outlet radii and moderate inter-disk gap sizes. The conclusions might be helpful in the early-stage design of bladeless turbines. Full article

The influence of blade number and rotor solidity on cyclorotor hover performance remains insufficiently understood because previous studies have generally varied these parameters simultaneously or investigated them through separate one-factor analyses. This work examines their independent effects using a two-dimensional unsteady Reynolds–Averaged Navier–Stokes model in which blade number (2–8) and rotor solidity (0.24–0.60) are varied independently across 26 geometrically feasible design points, at constant rotor radius and rotational speed. The model is validated against published experimental data for the same rotor before the parametric analysis is performed. At fixed rotational speed, increasing solidity raises both the thrust and power coefficients and lowers power loading. Because power loading is disk-loading-dependent even for an ideal rotor, however, this apparent penalty largely reflects a change in operating point rather than a loss of aerodynamic efficiency: compared at matched disk loading, efficiency varies only weakly with solidity except in the corner of the design space that combines high solidity with a long blade chord, and an interior efficiency optimum emerges near $\sigma \approx 0.36$ for blade counts $N=4$–8, reconciling the present results with the chord-to-radius optimum reported in the literature. Blade number has only a secondary influence on mean performance at constant solidity, consistent with classical rotor theory; azimuthally resolved loads, however, show peak-to-mean thrust ratios of 3–4 for two- and three-bladed rotors, a design constraint invisible in cycle-averaged metrics. Full article

The filling-and-emptying valve and downstream shaft are crucial components of navigation-lock systems. Under insufficient downstream submergence, air can be drawn through the shaft and trapped in the post-valve culvert, altering the flow structure and compromising hydraulic stability. A three-dimensional Reynolds-averaged Navier–Stokes/volume-of-fluid model was developed to investigate shaft aeration and entrapped-air-pocket evolution under varying inlet velocities and downstream-submergence depths. The aeration process comprises three stages: jet establishment, air-pocket formation, and air-pocket breakup and reorganization. Downstream-submergence depth determines whether a continuous air-intake pathway forms, whereas inlet velocity primarily controls aeration intensity and air-pocket persistence once the pathway is established. With decreasing submergence depth, the flow transitions successively from a water-sealed regime to a transition regime, a stable entrapped-air-pocket regime, and a strongly unsteady hydraulic-jump-like regime. For the present geometry and fixed valve opening, the transition from transient to sustained shaft aeration is identified within the downstream-submergence interval of h_w = 2–5 m. Combined analyses of the air-pocket volume per unit width, pressure response, vortex structures, and shear-layer characteristics indicate that enhanced jet-induced shear is closely associated with shaft aeration and air entrapment, while pressure fluctuations are closely coupled with air-pocket formation, persistence, breakup, and reorganization. Full article

Computational fluid dynamics methods range from Reynolds-Averaged Navier–Stokes (RANS) simulations to Direct Numerical Simulations (DNSs). RANS offers low computational cost at the expense of accuracy, while DNS provides high accuracy but at a prohibitive cost. The aim of this study is to evaluate whether multi-fidelity neural networks can learn a corrective mapping from RANS to DNS for a small canonical dataset and to determine how training-set composition and model architecture influence generalisation across geometries. In this study, multi-fidelity neural networks for data fusion between low-fidelity RANS and high-fidelity DNS data were applied to turbulent flow (Re = 5600) over parameterised periodic hills, defined by a geometry parameter characterising the steepness ratio. The inputs to the models were the coordinates and the corresponding RANS velocity components, and the outputs were the DNS velocity components, with data from both fidelities mapped onto the same mesh. Both a single-branch and a two-branch architecture were considered. Generalisability was assessed within a small dataset of five periodic hills defined by different values of the geometry parameter $\alpha$ (0.5, 0.8, 1.0, 1.2, 1.5). Both model architectures were trained on data from different combinations of the geometry parameter to evaluate interpolation and extrapolation capabilities. Both networks successfully corrected RANS flow fields for unseen geometries in interpolation regimes. When interpolating, the single-branch architecture achieved more than a 69% reduction in error, while the two-branch architecture achieved more than a 60% reduction, with both improving key flow features such as recirculation zones and jet structures. A key finding is that the single-branch architecture consistently outperformed the two-branch formulation, particularly in low-data regimes. The results show that multi-fidelity neural networks can improve RANS predictions using small datasets and simple inputs, provided that the training set spans the relevant geometric space. As the model does not require the geometry parameter as an explicit input, it is applicable to geometries lacking straightforward parameterisation. The demonstrated advantage of the single-branch architecture highlights the importance of architectural simplicity when training data is limited. Full article

Archimedes Spiral Wind Turbines (ASWTs) are suitable for small-scale urban wind applications, but changing the blade angle can also change the rotor dimensions under different geometric constraints. This study examines these coupled effects by comparing five ASWT configurations with blade angles of 30°, 45°, and 60° under fixed-diameter (Fixed D) and fixed-axial-length (Fixed L) conditions. Steady three-dimensional Reynolds-averaged Navier–Stokes simulations using the Multiple Reference Frame method were conducted to evaluate the power coefficient ($C_p$), torque coefficient ($C_t$), and mid-plane pressure and velocity fields. The numerical setup was assessed through grid-independence and reference-data comparisons. Under the Fixed D constraint, the 60° configuration achieved the highest $C_p$ of 0.2915 at a tip-speed ratio (λ) of 1.9, whereas the 30° configuration reached a maximum $C_p$ of 0.1792 at λ = 1.0. Under the fixed L constraint, the corresponding $C_p$ values were 0.2869 at λ = 2.5 for the 60° configuration and 0.1713 at λ = 0.8 for the 30° configuration. The baseline 45° configuration achieved a maximum $C_p$ of 0.2444 at λ = 1.5. The torque coefficient decreased with increasing λ for all configurations, while the pressure and velocity fields differed between the two constraints at the same blade angle. These results indicate that blade angle should be evaluated together with rotor diameter, axial length, and the installation envelope, because a larger rotor or swept area does not necessarily produce a proportional increase in normalized aerodynamic efficiency. Full article