Fluid Dynamics最新文献

In this study, direct numerical simulation (DNS) is carried out to investigate flow around a wall-mounted hemisphere at a Reynolds number Re = 1000. The generation and transport characteristics of vorticity are analyzed based on the simulation results, deepening the understanding of the evolution mechanisms of vortex structures. The main flow features include near-wall recirculation vortices wrapping around the hemisphere, a large recirculation zone formed by flow separation at the apex, and hairpin vortices shedding downstream of the recirculation region along with several secondary vortical structures. From a vortex dynamics perspective, spanwise vorticity consistently dominates, contributing more than 60% to the total enstrophy. In the upstream recirculation vortices, spanwise vorticity is mainly amplified by stretching, while streamwise vorticity is generated through transfer from spanwise vorticity, manifested as spanwise stretching, spreading, and streamwise twisting of the recirculation structures. Vorticity generation occurs primarily on the windward face and upstream of the separation points due to fluid–surface interaction, followed by redistribution under the influence of surface curvature. In the near-wake evolution of arch vortices, strong transfer from wall-normal and spanwise vorticity to streamwise vorticity is identified as the key mechanism for the formation of hairpin vortices.

The trapezoidal slope-coupled terrain model that comprises a trapezoidal obstacle and a two-layer slope based on the topography of guyots and gentle continental slope shelves in the South China Sea is proposed. The large eddy simulation (LES) technology is employed to investigate the propagation characteristics of internal waves (IWs) over complex terrain and their impact on cylindrical structures. The results indicate that complex terrain induces a multistage response in the forces exerted on the cylinder. As compared with the single-slope terrain, the trapezoidal slope terrain, influenced by the secondary bank slope terrain, transforms the force response from a single-negative peak stage to a double-negative peak stage, with the second-stage negative peak exceeding the first. The trapezoidal obstacle effectively weakens the impact of internal waves on the cylinder, particularly in the first stage, when it impedes the flow of the lower layer, thereby reducing the internal wave energy. In the second stage, influenced by the secondary slope terrain, internal waves undergo reflection and interact with trailing waves, leading to drastic changes in the flow field structure and a reversal of the force direction on the upper and lower parts of the cylinder. Furthermore, as wave amplitude increases, the weakening effect of the trapezoidal barrier becomes more pronounced, especially in the second stage.

The aerodynamic performance of staggered supersonic biplane airfoils equipped with leading-edge flaps is studied using numerical simulation. A parametric analysis is carried out to estimate the effects of the stagger distance and the flap deflection angles on the lift-to-drag ratio, the shock wave interactions, and the overall aerodynamic efficiency. The results show that the optimum stagger configurations significantly reduce the wave drag by up to 18%, while the introduction of leading-edge flaps enhances the lift-to-drag ratio by approximately 22% as compared to the conventional biplane designs without flaps. The optimized flap deflection angles effectively smooth the pressure distributions and delay flow separation, leading to improved aerodynamic stability.

Stress response hysteresis behavior exists in the stress controlling by the electroosmotic effect. In this study, the response of microfluidic chip is investigated by modeling a three-dimensional resistive network, and the evolution of a number of randomly distributed viscous particles in the Polydimethylsiloxane (PDMS) matrix is calculated demonstrating that the change in the conductance of the microfluidic chip is determined by the amount of tunneled particles of solution in the reservoir. Besides, when the interval of agglomerated particles is below the cutoff distance of 150.58 nm, it has a wide range of the relative resistance rate of change. Therefore, the high sensitivity of the microfluidic chip is effectively obtained by adding PDMS matrix material particles to the conductive liquid to fully fill the spacing of the agglomerated particles. Moreover, by comparing the sensitivity and hysteresis of conductive liquids containing different proportions of KCl, it is proved that the conductive liquid containing 2 mol % KCl has the higher sensitivity and the lower hysteresis. On this basis, the corresponding stress control process is analyzed. It is proved that the stress response characteristic of microfluidic chip is proportional to the control voltage. Moreover, under the control of voltage with continuous waveform, by overshooting the control voltage and advancing the phase by π/12, the phase compensation of the unknown time loss in the control method is carried out to ensure that the stress response hysteresis is reduced. When the phase difference between the character of stress output and the waveform of input voltage is within 0.5 s, the character of stress output has ideal and stable waveform characteristic. Therefore, this study realizes the fast response in the process of stress regulation by electroosmotic effect, which provides the technical support and innovative application for precise stress control by the electroosmotic effect.

In this study we evaluate how an insulated vertical strip functions as a heat regulating element to govern fluid motion together with heat transfer in square cavities. In previous studies, the researchers have given a limited attention to investigating the insulated strips with the use of the multi-relaxation time lattice Boltzmann method. The vertical strip divides the cavity exactly in the middle as both walls of the enclosure operate with oppositely moving lids having the upper wall heated and the lower one cold. The remaining walls within the enclosure hold adiabatic characteristics. The behavior of flow and heat transfer within the enclosure are governed by the principles of mass, momentum, and energy conservation, stated via nonlinear partial differential equations along with relevant boundary conditions. To simulate these phenomena, the D2Q9 lattice methods of the multi-relaxation time lattice Boltzmann method are employed, considering the key dimensionless parameters which include the Grashof numbers from 10⁴ to 5 × 10⁵, the Richardson numbers varying between 0.1 and 100, and the Prandtl numbers ranging from 0.7 to 7. As the Grashof number increases, it promotes greater separation between the dominant vertical structures, pushing them toward the cavity walls and inducing secondary re-circulation regions in the central area. When the Richardson number receives becomes higher, it generates the strengthened buoyancy forces that squeeze the temperature contours while reshaping the thermal distribution pattern over the entire cavity domain. With rising the Grashof number, the average local Nusselt number displays a general trend for increase but exhibits low variation with changes in the Richardson number.

The droplet trapping dynamics in an L-shaped constricted microchannel are investigated using three-dimensional numerical simulations and theoretical analysis. The observed droplet regimes include trapping and squeezing. Based on the theoretical balance of the hydrostatic pressure of flow exerted on the droplet and the net Laplace pressure of the droplet generated by contraction when entering the constricted microchannel, a theoretical model is proposed to predict the critical capillary number Ca governing the transition between the two regimes. The theoretical model considers the effects of the viscosity ratio (lambda ) and microchannel geometry, including the width ratio ({{C}_{I}}) and the contraction ratio ({{C}_{{II}}}). The results from the predictive equation closely match the numerical simulations, confirming the model’s accuracy. The study also explains how geometry, flow, and fluid properties affect the droplet behavior in constricted microchannels at low Reynolds numbers. It offers insights into controlling droplet trapping and release for biomedical and chemical applications, and serves as a useful reference for designing the microfluidic systems.

The impacts of two synthetic jets ejected transversely on two-degree-of-freedom vortex induced vibrations (VIVs) of a circular cylinder with the mass ratio m* = 5.78 in turbulent flow are numerically studied. Synthetic jets are placed on the upper and lower shoulders of the cylinder, and the momentum coefficient C_u is equal to 1.0, 2.0, and 4.0, respectively. The Reynolds number of uniform free-stream flow varies between 1803 and 7212, corresponding to the reduced velocity range 3.0 ≤ U* ≤ 12.0. The oscillation characters, the hydrodynamic force coefficients, and the wake structures are compared and analyzed in various cases. The results indicate that the synthetic jets could enhance both streamwise and transverse oscillations of the cylinder at U* ≥ 6.0, and the strengthening effect on oscillations is improved with increase in C_u. In the controlled cases, the synchronization region of the transverse oscillations is extended, and the streamwise oscillation frequencies are close to  f_{n, x}  at the most reduced velocities. The “dual-resonance” phenomenon is observed at C_u = 4.0 when 3.0 ≤ U* ≤ 8.0. The synthetic jets could promote vortex shedding on the cylinder’s shoulders, and the 2S mode, the 4P mode, the special 2S mode (with additional small vortex pairs), and the 2P + 2S mode are observed in the controlled cases. Vortex shedding is unstable in some special cases, which cause the appearance of abnormal oscillation behaviors and irregular oscillation trajectories.

This study presents a numerical simulation of hypersonic inlet flows across three geometries: a single ramp, a concave ramp, and a convex shoulder. It aims at understanding the formation and behavior of separation bubbles (SB) over a wide range of Mach numbers. The effects of the angle of attack and the wall temperature on separation bubbles are also analyzed. Due to the complexities associated with the separation bubbles, the simulation is divided into two steps: an initial inviscid simulation that followed by a viscous simulation. The inviscid simulation focuses on the interaction of geometry-induced shock waves, including cowl shock waves and shoulder expansion waves, to clearly characterize the adverse pressure gradients. The viscous simulation then investigates the impact of expansion waves from sharp and convex corners on the complex shock wave boundary layer interactions (CSWBLI) and the interaction of geometry-induced shock waves (GISW) with separation bubble-induced shock waves (SBISW). Computational details such as the inlet model, the numerical methods, the boundary conditions, the grid independence and code validation results are given. The key results highlight the dependency of separation bubble size and shape on geometric, thermal, and flow parameters, providing a deeper insight into the separation bubble behavior and the shock wave interactions in hypersonic flows. The findings contribute to the optimization of inlet design for hypersonic flows.

Three-phase flow is commonly present in oil and gas production pipelines. Three-fluid models are extensively used for numerical simulation. Accurately modeling and solving the three-phase flow model is fundamental for monitoring the flow parameters and ensuring production stability. However, the model may be ill-posed under certain initial conditions. No reliable methods have long been used to judge the well-posedness of three-fluid models. This study presents a universal method for determining the well-posedness of the three-fluid model, which can judge the model’s well-posedness of single-phase, two-phase, and three-phase flow under the stratified flow pattern. The model’s well-posedness range in the subsonic region becomes notably limited under the high pressure and the large fluid density ratios. A convenient eigenvalue map method with a wide range of liquid holdup and pressure to analyze the three-fluid model’s hyperbolicity is proposed, which can intuitively describe the well-posed range of the model under typical operating conditions and the distribution of model eigenvalue signs, as well as determine the boundary parameters on the pipelines' inlet and outlet boundaries. The three-fluid model has the largest well-posed range under low pressure and low liquid holdup. The proposed method provides a reference for well-posed analysis and accurate solution of the three-fluid model.

The vertical axis wind turbines (VAWTs) operate efficiently in the low wind speed regions and varying wind directions like urban environments. The current study experimentally examines the performance of a two-bladed hybrid Darrieus–Savonius wind turbine with auxiliary Darrieus blades at three different low wind speeds, namely, 3, 4, and 5 m/s. To test the impact of the auxiliary blades’ location relative to the central shaft, the auxiliary blades were positioned at three different distances, namely, 17, 22, and 27 cm. When investigating the effect of the overlap ratio, three different overlap ratios equal to 0, –0.5, and 0.5 were used to arrange the Savonius blades. According to the results, hybridisation enhances the rotor’s performance. The hybrid Darrieus rotor with main blades, and the Savonius rotor with a  –0.5 overlap ratio ensures the maximum static torque coefficient equal to 0.045 at 0° azimuthal position and the wind speed of 3 m/s. A negative overlap ratio enhances the hybrid rotor’s initial characteristics. The arrangement with auxiliary blades at 17 cm from the central shaft and a fixed overlap ratio of  –0.5 yields the highest coefficient of power, namely, 0.018, at a TSR of 0.78 and a wind speed of 5 m/s.