Search Results (134)

Presented in this work is a harmonic balance (HB)-based pseudo-time code-coupling approach applied to a one-degree-of-freedom vortex-induced vibration (VIV) problem of a circular cylinder in a low-Reynolds-number laminar flow regime. Unlike physical time coupling used in traditional time-accurate methods, this novel approach updates both of the fluid and structure fields by integrating respective HB forms of governing equations in pseudo-time, and then couples the two fields in pseudo-time using a partitioned approach. A separate procedure is adopted to determine the VIV frequency at every code-coupling iteration, which enables the simultaneous convergence of variables of both fields in a single run of the solver. For the cases considered here, lock-in vibrations are predicted over a range of Reynolds numbers, inside and outside the resonant range. The results are verified by a time-accurate method and also validated against earlier experimental data, demonstrating the efficiency and robustness of the pseudo-time code-coupling approach. Full article

This study investigates the aerodynamic performance of a symmetric NACA 0018 airfoil under harmonic pitching motions at low Reynolds numbers, a regime characterized by the presence of laminar separation bubbles and their impact on aerodynamic forces. The analysis encompasses oscillation frequencies of 1 Hz, 2 Hz, and 13.3 Hz, with amplitudes of 4° and 8°, along with steady-state simulations conducted for angles of attack up to 20° to validate the numerical model. The results reveal that the γ-Re_θ turbulence model provides improved predictions of aerodynamic forces at higher Reynolds numbers but struggles at lower Reynolds numbers, where laminar flow effects dominate. The inclusion of the 13.3 Hz frequency, relevant to Darrieus vertical-axis wind turbines, demonstrates the effectiveness of the model in capturing dynamic hysteresis loops and reduced oscillations, in contrast to the k-ω SST model. Comparisons with XFOIL further highlight the challenges in accurately modeling laminar-to-turbulent transitions and dynamic flow phenomena. These findings offer valuable insights into the aerodynamic behavior of thick airfoils under low Reynolds number conditions and contribute to the advancement of turbulence modeling, particularly in applications involving vertical-axis wind turbines. Full article

Soil particle size distribution is a critical parameter in geotechnical and hydraulic engineering, particularly in applications such as dam seepage monitoring, building foundation assessments, and sediment transport. This study presents a novel algorithm for estimating soil particle sizes by analyzing their falling velocities in water, combining optical flow computation with chaotic motion analysis. To address the limitations of the classical Horn and Schunck method, particularly its sensitivity to large displacements and brightness variations, we introduced a coarse-to-fine warping strategy, an image decomposition step to separate dominant structures from fine textures, and the Charbonnier penalty function. The improved model achieved competitive accuracy compared to advanced optical flow algorithms. To manage turbulence and motion noise during particle settling, we incorporated a global flow analysis framework using streaklines, streak flow, and potential functions. This enabled the segmentation of laminar, turbulent, and rebound flow regions without requiring individual particle tracking. Soil particle sizes were then back-calculated from laminar flow velocities using Stokes’ Law. Experimental results confirmed the method’s accuracy for particle sizes ranging from 20 mm to 0.7 mm, significantly extending the measurable range of Sedimaging systems. The proposed approach shows strong potential for integration into dam-related particle monitoring applications and building-related monitoring systems requiring fine-resolution analysis. Full article

Aerodynamic characteristics of a pitching NACA 0012 airfoil, including the load performance and flow field features, are studied using numerical simulations in this paper. Large Eddy Simulations (LESs) have been performed, and the chord-based Reynolds number is set to $6.6\times {10}^4$. Pitching frequency varies from 3 to 20 Hz, corresponding to a reduced frequency of 0.094–0.628 ($k=\pi f_{p}c/U_{\infty }$, where $f_p$ is the pitching frequency, c is the chord length, and $U_{\infty }$ refers to the incident flow speed). As the pitching frequency increases, the maximum lift coefficient achieved in one pitching cycle decreases, and the direction of the lift hysteresis loop changes as the pitching frequency exceeds a certain value, leading to a change in the lift of the sign at the zero-incidence moment, which is a result of the instantaneous flow patterns on the airfoil surface. As the pitching frequency increases, flow unsteadiness develops less in one pitching cycle, and the time duration in which the turbulence boundary layer can be detected in one pitching cycle shrinks. Additionally, for the pitching airfoil, combinations of the flow patterns on the upper and lower sides, such as laminar separation and the turbulent boundary layer, or laminar separation and the laminar separation bubble, were observed on the airfoil surface, and these were not detected on a static airfoil at the corresponding Reynolds number. This is considered an effect of the pitching motion that is in addition to the phase-lag effect. Full article

This report uses computational fluid dynamics (CFDs) to investigate the aerodynamics of a rubber O-ring, with a focus on assessing the influence of fluid velocity and surface topology whilst providing a detailed methodology that promotes correct procedures. A steady state scenario was set up, modelling laminar airflow across two O-rings with 5 μm and 100 μm surface finishes, respectively. Analysis showed that increasing the fluid velocity from 0.01 m/s to 2 m/s significantly translates the separation points downstream, consolidating wake regions behind the airfoil. The CFD simulations also infer that as the fluid velocity increases, the frictional drag coefficients decrease from 3.13 to 0.11, and the pressure drag coefficients increase from 0.55 to 0.6, implying that the recirculation of flowlines behind the O-ring becomes the most hindering factor on aerodynamics. Conversely, variations in surface roughness showed negligible effects on the flow field. This insensitivity is attributed to the low Reynolds number (Re) used in all simulations, where a roughness of 5 μm or 100 μm remains well within the laminar sublayer, therefore minimising their impact on boundary layer disruption and flow separation. Full article

The objectives of reducing and increasing pollutant emissions during the glass production process also apply to the glass industry, meaning that the accurate modeling of a glass furnace is of critical strategic value. In the available literature, several CFD studies have proposed various models with different levels of complexity. Two basic aspects are shared by the existing models, limiting their accuracy and their impact on furnace design: the combustion space is usually solved with reliance on simplified models (e.g., Flamelet and global kinetic mechanisms); and the glass tank is solved separately, using an iterative approach to couple two (or more) simulated domains. This work presents the development of an innovative CFD model to overcome these limitations and to perform accurate simulations of industrial glass furnaces. The reactive flow is solved using a reduced chemical kinetic mechanism and the EDC (eddy dissipation concept) turbulence–chemistry interaction model to properly reproduce the complex combustion development. The glass bath is solved as a laminar flow with the appropriate temperature-dependent glass properties. The two domains are simulated simultaneously and thermally coupled through an interface. This procedure allows for the more accurate calculation of the heat flow and the temperature distributions on the glass bath, accounting for their subsequent influence on the glass convective motions. The simulation of an existing glass furnace, along with selected comparisons with experimental data, are presented to demonstrate the validity of the proposed model. The developed model provides a contribution that allows us to advance the wider understanding of glass furnace dynamics. Full article

The accurate, robust, and efficient prediction of transition in viscous flows is a significant challenge in computational fluid dynamics. We present a coupled high-fidelity iterative approach that leverages the FUN3D flow solver and the LASTRAC stability code to predict transition in low-disturbance environments, initiated by the linear growth of boundary-layer instability modes. Our method integrates the ability of FUN3D to compute mixed laminar–transitional–turbulent mean flows via transition-sensitized Reynolds-Averaged Navier–Stokes equations with the ability of LASTRAC to perform linear stability analysis, all within an automated framework that requires no intermediate user involvement. Unlike conventional frameworks that rely on classical stability theory or reduced-order metamodels, our approach employs parabolized stability equations to provide more accurate and reliable estimates of disturbance growth for multiple instability mechanisms, including Tollmien–Schlichting, Kelvin–Helmholtz, and crossflow modes. By accounting for the effects of mean-flow nonparallelism as well as the surface curvature, this approach lays the foundation for improved N-factor correlations for transition onset prediction in a broad class of flows. We apply this method to three distinct flow configurations: (1) flow over a zero-pressure-gradient flat plate, (2) the NLF-0416 airfoil with both natural and separation-induced transition, and (3) a 6:1 prolate spheroid, where transition is primarily driven by crossflow instability. For two-dimensional cases, a formulated intermittency distribution is used to model the transition zone between the laminar and fully turbulent flows. The results include comparisons with experimental measurements, similar numerical approaches, and transport-equation-based models, demonstrating good agreement in surface pressure coefficients, transition onset locations, and skin-friction coefficients for all three configurations. In addition to contributing a couple of new insights into boundary-layer transition in these canonical cases, this study presents a powerful tool for transition modeling in both research and design applications in aerodynamics. Full article

Current research on the transition mechanisms induced by moderate-height roughness elements remains insufficiently explored. Hence, direct numerical simulation (DNS) and BiGlobal stability analysis are employed in this study to investigate boundary layer transition from laminar to turbulent flow induced by moderate-height isolated roughness elements and roughness strips under a supersonic freestream at Mach 3.5. Analysis of DNS results reveals that the isolated roughness element induces transition within the boundary layer, characterized by two high-speed streaks in the wake. This transition is attributed to the coupling between the separated shear layer at the roughness apex and the downstream counter-rotating vortex pair (CVP). BiGlobal stability analysis further identifies that symmetric eigenmodes dominate the transition process in the wake, actively promoting flow destabilization. Conversely, the roughness strip configuration suppresses transition, with only attenuated high-speed streaks persisting in the near wake before complete dissipation. The wake flow exhibits multiple CVPs and adjacent horseshoe vortex pairs interacting with the shear layer, with antisymmetric modes dominating this process. These findings provide technical foundations and theoretical frameworks for predicting and controlling roughness-induced transition. Full article

A numerical simulation of the circular cylinder as an obstacle in a double forward-facing (DFFS) step was performed. The size and position of the upstream cylinder (c₁) and downstream cylinder (c₂) were varied to explore their role in heat transfer in both laminar and turbulent conditions. Comparative results of the upper and lower half of the downstream cylinder were plotted as results to understand the heat transfer and flow characteristics around the downstream cylinder due to the effect of the upstream cylinder’s dimension and position. For Re = 800, when the c₁ is placed near the bottom of the wall, it results in a pair of rear-side symmetrical vortices, and the c₂ cylinder vortices become larger when the c₁ is shifted towards the top wall. Additional flow separation happens adjacent to the steps when c₁ is greater than c₂. These vortices strongly influence the convection heat transfer from the step. However, when Reynolds number (Re) is increased from 800 to 80,000, these vortices’ size is decreased. When c₁ moves from 0.375H to 0.75H, the average Nusselt number is increased significantly. Moreover, a hike in Re results in a higher average Nusselt number irrespective of the position of obstacles. The upstream cylinder significantly enhances the Nusselt number when it is placed near the top wall rather than the bottom wall. Full article

We developed a membraneless-microfluidic biofuel cell (MBFC) for the quantification of ethanol. The system employs anolyte and catholyte solutions, each containing a biocatalyst and redox mediator. The laminar flow conditions in the microfluidic chip minimize the mixing between anolyte and catholyte and obviate the need for a membrane to separate them. When ethanol is added to the anolyte, alcohol dehydrogenase (ADH) catalyzes its oxidation to acetaldehyde, releasing electrons to the anode. On the cathode, electrons are transferred to horseradish peroxidase (HRP), which reduces hydrogen peroxide in the catholyte to water. We optimized key design factors and operating conditions. We also studied the incorporation of glycerol as a viscosity modifier, which improved the power and current density supplied by the MBFC, with a maximum power output of 307 µW cm⁻² and an open circuit voltage of 0.733 V. The proposed ethanol/hydrogen peroxide MBFC was successfully applied as a biofuel cell-based sensor for the quantification of ethanol in a commercial liquor. Full article

The current study investigates the performance of implicit Large Eddy Simulation (iLES) incorporating an unstructured third-order Weighted Essentially Non-Oscillatory (WENO) reconstruction method, alongside conventional Large Eddy Simulation (LES) using the Wall-Adapting Local Eddy Viscosity (WALE) model, for wall-bounded flows. Specifically, iLES is applied to the flow around a NACA0012 airfoil at a Reynolds number which involves key flow phenomena such as laminar separation, transition to turbulence, and flow reattachment. Simulations are conducted using the open-source computational fluid dynamics package OpenFOAM, with a second-order implicit Euler scheme for time integration and the Pressure-Implicit Splitting Operator (PISO) algorithm for pressure–velocity coupling. The results are compared against direct numerical simulation (DNS) for the same flow conditions. Key metrics, including the pressure coefficient and reattached turbulent velocity profiles, show excellent agreement between the iLES and DNS reference results. However, both iLES and LES predict a thinner separation bubble in the transitional flow region then DNS. Notably, the iLES approach achieved a 35% reduction in mesh resolution relative to wall-resolving LES, and a 70% reduction relative to DNS, while maintaining satisfactory accuracy. The study also captures detailed instantaneous flow evolution on the airfoil’s upper surface, with evidence suggesting that three-dimensional disturbances arise from interactions between separating boundary layers near the trailing edge. Full article

Oxidative stress is closely related to the occurrence and development of ischaemic stroke. Natural plant polysaccharides have potential value in inhibiting oxidative stress and preventing ischaemic stroke. Here, a novel neutral polysaccharide named LICP009-3F-1a with a Mw of 10,780 Da was separated and purified from Lycium barbarum L. fruits. Linkage and NMR data revealed that LICP009-3F-1a has the following backbone: →4)-β-D-Glcp-(1→6)-β-D-Galp-(1→, with a branched chain of β-D-Galp-(1→3)-β-D-Galp-(1→, α-L-Araf-(1→ and →6)-α-D-Glcp-(1→ connected to the main chain through O-3 of →3,6)-β-D-Galp-(1→. X-ray and SEM analyses showed that LICP009-3F-1a has a semicrystalline structure with a laminar morphology. Thermal property analysis showed that LICP009-3F-1a is thermally stable. In vivo experiments suggested that LICP009-3F-1a could inhibit hypoxia-induced oxidative stress damage by eliminating ROS, reversing and restoring the activities of the antioxidant enzymes SOD, CAT, and GPx, and reducing the expression levels of the HIF-1α and VEGF genes. Blocking the apoptosis genes Bax and Caspase 3 and upregulating the expression of the antiapoptotic gene Bcl-2 protected PC12 cells from hypoxia-induced apoptosis. These results suggest that LICP009-3F-1a may have multiple potential uses in the treatment of IS. Full article

This article provides a comprehensive review of key approaches to suppressing stall flow in pumps, offering insights to enhance pump performance and reliability. It begins by outlining the formation mechanisms and characteristics of stalls, followed by an in-depth analysis of various stall types. The discussion highlights passive and active flow control methods, emphasizing their roles in suppressing stall phenomena. Passive flow-control strategies, including surface roughness, grooves, obstacles, fixed guide vanes, and vortex generators, are examined with a focus on their mechanisms and effectiveness in suppressing stall. Similarly, active flow-control techniques, such as jets and adjustable guide vanes, are explored for their capacity to regulate the flow field and suppress stall. The novelty of this review lies in its exploration of the effectiveness of passive and active flow-control methods in suppressing pump stall, with a focus on their mechanisms of action and the underlying principles of stall formation. The findings reveal that appropriate flow-control measures can mitigate laminar flow separation and reduce performance losses associated with stall. However, careful attention must be given to the optimal arrangement of control devices. Finally, the article highlights the limitations of current implementations of combined active and passive flow-control methods while offering insights into the future potential of advanced flow-control technologies in regard to suppressing stall. Full article

Microfluidic-chip based hydrodynamic filtration is one of the passive sorting techniques that can separate cell or particle suspensions into subpopulations of different sizes. As the branch channels and side channels play an important role in maintaining particle focusing, their rational design is necessary for highly efficient sorting. A model framework involving multiple side and multiple branch channels has been developed by extending the analytical analysis of three-dimensional laminar flow in channel networks, which was previously validated by comparison with numerical simulations. Objective parameters were identified as the number of branch channels and each length of individual branches. The presence of multiple side channels causes an increase in the average fluid velocity in main and branch channels as the branch point shifts toward the end of the main channel, which differs from the behavior observed in a single side channel. The number of branches and their individual lengths decrease distinctly in the case of branch channels consisting of narrow and wide sections, which enables the compact design of a microfluidic-chip, being operated by a lower pressure drop under the same throughput. Sorting of bidisperse particles was accomplished with an optimally designed chip to verify this framework by achieving very high recovery and purity. Full article

To achieve low-cost and on-demand launches of microsatellites, the authors have been researching and developing a micro hybrid rocket since 2014. In 2018, a ballistic launch experiment was performed using the developed hybrid rocket, where it reached an altitude of about 6.2 km. The rocket engine had a 3D-printed solid fuel grain made of acrylonitrile butadiene styrene (ABS) resin in combination with a nitrous oxide oxidizer. The fuel grain port had a star-fractal swirl geometry in order to increase the surface area of the port, to promote the laminar–turbulent transition by increasing the friction resistance, and to give a swirling velocity component to the oxidizer flow. This overcame the hybrid rocket’s drawback of a low fuel regression rate; i.e., it achieved a higher fuel gas generation rate compared with a classical port geometry. In 2021, the hybrid rocket engine was scaled up, and its total impulse was increased to over 50 kNs; it reached an altitude of 15 km. In addition to the engine, other components were also improved, such as through the incorporation of lightweight structures, low-shock separation devices, a high-reliability telemetry device, and a data logger, while keeping costs low. The rocket was launched and reached an altitude of about 10.1 km, which broke the previous Japanese altitude record of 8.3 km for hybrid rockets. This presentation will report on the developed components from the viewpoint of system design and the results of the ballistic launch experiments. Full article