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Intermittent swimming and schooling individually offer the possibility of energy economy for fish. To study the role of the simultaneous use of both behaviors, the intermittent swimming of two self-propelled plates in tandem configuration is investigated numerically. Two intermittent swimming modes, i.e., the multiple-tail beat (MT) mode and the half-tail beat (HT) mode, are considered. For solitary plate swimming intermittently while maintaining fixed bout period, the propulsion velocity and energy consumption decrease monotonically as the duty cycle increases, consistent with the prediction of the reduced-order model. A pair of plates swimming with independent duty cycle can form orderly configurations, without separating or colliding, when their duty cycles are similar. For the MT mode, the asymmetric wake of the leader enhances the drag on the following plate by inducing an additional oncoming flow against it, making the follower harder to follow. For the HT mode, the symmetric wake of the leader reduces the drag on the follower, making it easier to follow. No significant difference was found in the propulsive performance of intermittent swimming between the leading plate in orderly configurations and solitary plate. The results of efficiency indicate that the HT mode is more economical than the MT mode for the follower. Usually, the smaller the equilibrium gap spacing is, the more efficient the follower is. The results of the cost of transport show that the follower achieves better energy economy for higher propulsion velocity. The results provide some insight into the collective intermittent swimming of fish and are helpful for bionic design. [ABSTRACT FROM AUTHOR]

Fish schooling with stable configurations is intriguing. How individuals benefit from hydrodynamic interactions is still an open question. Here, fish are modeled as undulatory self-propelled foils, which is more realistic. The collective locomotion of two foils in a tandem configuration with different amplitude ratios Ar and frequency ratios Fr is considered. Depending on Ar and Fr, the two foils without lateral or yaw motion may spontaneously form stable configurations, separate, or collide with each other. The phase diagram of the locomotion modes in the (Fr, Ar) plane is obtained, which is significantly different from that in Newbolt et al. ["Flow interactions between uncoordinated flapping swimmers give rise to group cohesion," Proc. Natl. Acad. Sci. U. S. A. 116, 2419 (2019)]. For stable configurations, the gap spacing may be almost constant [stable position (SP) mode] or change dynamically and periodically [stable cycle (SC) mode]. In our diagram, the fast SP mode is found. Besides, the border between the separation and SP/SC modes is more realistic. In the fast SP cases, analyses of hydrodynamic force show the phenomenon of inverted drafting, in which the leader achieves hydrodynamic advantages. For the SC mode, the cruising speed increases piecewise linearly with FrAr. When Ar < 1, the linear slope is identical to that of the isolated leader, and the follower-control mechanism is revealed. Our result sheds some light on fish schooling and predating. [ABSTRACT FROM AUTHOR]

The topological evolution near the turbulent/non-turbulent interface (TNTI) in turbulent mixing layer is studied by means of statistical analysis of the invariants of velocity gradient tensor (VGT) based on direct numerical simulation data. The dynamics of topological evolution is investigated in terms of the source terms of the evolution equations for the invariants, including the pressure effect term, viscous effect term and interaction term among the invariants. It is found that the local topology of fluid particles at the TNTI evolves from non-focal region to focal region in the plane of the second (Q) and the third (R) invariants of the VGT. The topological evolution is mainly associated with the pressure effect term in the TNTI region. According to the analysis of the evolution of enstrophy and dissipation, the enstrophy increase and the dissipation decrease are revealed in the TNTI region, which are caused by viscous vorticity diffusion near the TNTI. A weak correlation between the strain rate and the rotation rate is found in the TNTI region which is related to the reduction of enstrophy production. The alignments between vorticity and strain near the TNTI are investigated and a strong alignment of the vorticity with the extensive eigenvector direction is identified in the TNTI region. [ABSTRACT FROM AUTHOR]

The pressure Hessian tensor is entangled with the inherent nonlinearity and nonlocality of turbulence; thus, it is of crucial importance in modeling the Lagrangian evolution of the velocity gradient tensor (VGT). In the present study, we introduce the functional modeling strategy into the classic structural modeling strategy to model the pressure Hessian tensor based on deep neural networks (DNNs). The pressure Hessian tensor and its contributions to the VGT evolution are set as, respectively, the structural and functional learning targets. An a priori test shows that the present DNN-based model accurately establishes the mapping from the VGT to the pressure Hessian tensor and adequately models the physical effect of the pressure Hessian tensor on VGT invariants. An a posteriori test verifies that the present model reproduces well the principal features of turbulence-like skewness and vorticity strain-rate alignments obtained via direct numerical simulations. Importantly, the flow topology is accurately predicted, particularly for the strain-production-dominant regions in the invariant space. Moreover, an extrapolation test shows the generalization ability of the present model to higher Reynolds number flows that have not been trained. [ABSTRACT FROM AUTHOR]

Abstract: This study presents model extensions for a lattice Boltzmann (LB) approach to thermal axisymmetric flow including swirl or rotation. An incompressible axisymmetric lattice Boltzmann D2Q9 model was applied to solve the axial and radial velocities through inserting source terms into the two-dimensional lattice Boltzmann equation. The equations governing azimuthal (or swirling) velocity and the temperature were also solved by the LBM. It is found that this scheme is much more stable and consistent compared to previous hybrid schemes. It provides a significant advantage in simulation of melt flows with high Reynolds number and high Grashof number. The present scheme was validated by comparing the LB results with benchmark solutions for melt flow in Czochralski crystal growth. Unsteady flows with high Grashof numbers were studied in detail. The critical Grashof number for the onset of the oscillation is found to be about . The oscillation amplitude is proportional to for . The frequencies and flow patterns of the unsteady flows are also analyzed. The distributions of the mean quantities of the temperature and rms of temperature at Grashof number as high as is found to be similar to those obtained by 3D simulations. [Copyright &y& Elsevier]

Nitrogen injection under conditions close vicinity of the liquid-gas critical point is studied numerically. The fluid thermodynamic and transport properties vary drastically and exhibit anomalies in the near-critical regime. These anomalies can cause distinctive effects on heat-transfer and fluid-flow characteristics. To focus on the influence of thermodynamics on the flow field, a relatively low injection Reynolds number of 1 750 is adopted. For comparisons, a reference case with the same configuration and Reynolds number is simulated in the ideal gas regime. The model accommodates full conservation laws, real-fluid thermodynamic and transport phenomena. Results reveal that the flow features of the near-critical fluid jet are significantly different from their counterpart. The near-critical fluid jet spreads faster andmixes more efficiently with the ambient fluid along with a more rapidly development of the vortex pairing process. Detailed analysis at different streamwise locations including both the flat shear-layer region and fully developed vortex region reveals the important effect of volume dilatation and baroclinic torque in the near-critical fluid case. The former disturbs the shear layer and makes it more unstable. The volume dilatation and baroclinic effects strengthen the vorticity and stimulate the vortex rolling up and pairing process. [ABSTRACT FROM AUTHOR]

A nitrogen jet with subcritical and supercritical injection temperatures injected into a supercritical environment, in which both the pressure and temperature exceed those of the thermodynamic critical state, has been investigated numerically using large-eddy simulation technique. The effects of the injection temperature on the flow evolution are studied. We find that the jet surface is more unstable with the instability waves growing up and rolling into a succession of ring vortices for supercritical injection temperature, and the jet surface is nearly straight with the strong density stratification suppressing the development of the instability waves for subcritical injection temperature. With increasing injection temperature, the spatial growth rate of the surface instability wave strengthens and the frequency of the most unstable mode increases. This behavior is of importance in enhancing the fluid mixing effect. The results obtained in this study provide physical insight into the understanding of fundamental mechanisms of the jet evolution under supercritical conditions. [ABSTRACT FROM AUTHOR]

Isothermal gas flow in microtubes with a sudden expansion or contraction is studied numerically by lattice Boltzmann method. An axisymmetric D2Q9 model is used to simulate gas slip flow in micro-circular pipes. With the boundary condition combined specular and bounce-back schemes, the computed results are in excellent agreement with analytical solution for straight microtube. For the gas flow in the expanded or constricted tubes, we carried out simulations of several Knudsen numbers with inlet/outlet pressure ratio 3. It is found the pressure drop in each section can be predicted well by the theory of straight tubes. For smaller Knudsen number, flow separation in the expanded tube is observed. While for large Knudsen number, there is no vortex at corner and the streamlines are attached to boundary. In the constricted tube, the vortex at corner is very weak. These results are consistent with some experimental conclusions. [ABSTRACT FROM AUTHOR]

Abstract: Non-Newtonian fluid flow in a stenosed coronary bypass is investigated numerically using the Carreau–Yasuda model for the shear thinning behavior of the blood. End-to-side coronary bypass anastomosis is considered in a simplified model geometry where the host coronary artery has a 75% severity stenosis. Different locations of the bypass graft to the stenosis and different flow rates in the graft and in the host artery are studied. Particular attention is given to the non-Newtonian effect of the blood on the primary and secondary flow patterns in the host coronary artery and the wall shear stress (WSS) distribution there. Interaction between the jet flow from the stenosed artery and the flow from the graft is simulated by solving the three-dimensional Navier–Stokes equation coupled with the non-Newtonian constitutive model. Results for the non-Newtonian flow, the Newtonian flow and the rescaled Newtonian flow are presented. Significant differences in axial velocity profiles, secondary flow streamlines and WSS between the non-Newtonian and Newtonian fluid flows are revealed. However, reasonable agreement between the non-Newtonian and the rescaled Newtonian flows is found. Results from this study support the view that the residual flow in a partially occluded coronary artery interacts with flow in the bypass graft and may have significant hemodynamic effects in the host vessel downstream of the graft. Non-Newtonian property of the blood alters the flow pattern and WSS distribution and is an important factor to be considered in simulating hemodynamic effects of blood flow in arterial bypass grafts. [Copyright &y& Elsevier]

The investigation of a flapping foil, which is used as a basic mode of the flapping-based locomotion in insects, birds, and fish, is performed by solving the Navier-Stokes equations numerically. In this Brief Communication we provide insight into the understanding of dynamics of a flapping foil. A critical flapping Reynolds number based on the flapping frequency and amplitude, above which a forward flapping movement occurs, is predicted. The dynamics of the flapping foil are analyzed in two dynamic responses, i.e., an oscillatory movement and a steady movement, which depend on the density ratio between the foil and the surrounded fluid. The steady movement response is related to the forward flapping motion. The Strouhal number that governs a vortex shedding for the forward flapping foil is calculated and lies in the range where flying and swimming animals will be likely to tune for high propulsive efficiency. [ABSTRACT FROM AUTHOR]

Abstract: The pulsatile flow of non-Newtonian fluid in a bifurcation model with a non-planar daughter branch is investigated numerically by using the Carreau–Yasuda model to take into account the shear thinning behavior of the analog blood fluid. The objective of this study is to deal with the influence of the non-Newtonian property of fluid and of out-of-plane curvature in the non-planar daughter vessel on wall shear stress (WSS), oscillatory shear index (OSI), and flow phenomena during the pulse cycle. The non-Newtonian property in the daughter vessels induces a flattened axial velocity profile due to its shear thinning behavior. The non-planarity deflects flow from the inner wall of the vessel to the outer wall and changes the distribution of WSS along the vessel, in particular in systole phase. Downstream of the bifurcation, the velocity profiles are shifted toward the flow divider, and low WSS and high shear stress temporal oscillations characterized by OSI occur on the outer wall region of the daughter vessels close to the bifurcation. Secondary motions become stronger with the addition of the out-of-plane curvature induced by the bending of the vessel, and the secondary flow patterns swirl along the non-planar daughter vessel. A significant difference between the non-Newtonian and the Newtonian pulsatile flow is revealed during the pulse cycle; however, reasonable agreement between the non-Newtonian and the rescaled Newtonian flow is found. Calculated results for the pulsatile flow support the view that the non-planarity of blood vessels and the non-Newtonian properties of blood are an important factor in hemodynamics and may play a significant role in vascular biology and pathophysiology. [Copyright &y& Elsevier]

Direct numerical simulation (DNS) is carried out to study turbulence characteristics in a wall-normal rotating channel with the rotation number N τ from 0 to 0.12 and the Reynolds number 194 based on the friction velocity of the non-rotating case and the half-height of the channel. Based on the present calculated results, two typical rotation regimes are identified. When in weak rotation regime with 0 0.05, all the turbulence statistics decrease as the effect of the Coriolis force plays a dominant role. The budgets of transport equations for the Reynolds stresses are calculated to reveal the effect of the Coriolis force on the dynamic process of turbulent kinetic energy production, dissipation and redistribution. With the increase of N τ in weak rotation regime, the main mechanism for the generation of the streamwise turbulent energy is gradually altered from the shear production effect related to the streamwise mean flow to the energy redistribution due to the pressure strain correlation. Correspondingly, the generation of the spanwise turbulent energy also changes from the energy redistribution effect to the shear production of the spanwise mean flow. In strong rotation regime, the mean flow shear rate is found to be a key factor to the turbulence production and dissipation. The redistribution between the streamwise and spanwise components of turbulent kinetic energy due to the effect of the Coriolis force becomes weak. A remarkable change of the direction of the near-wall vortical structures, nearly in alignment with the absolute mean flow direction, is observed. An attempt to evaluate the mean spacing between the streaky structures and the angle between the wall structures and streamwise direction has been examined based on the two-point correlations of the velocity fluctuations to reveal the change of the near-wall structures. [ABSTRACT FROM AUTHOR]

Abstract: Turbulent flow through a concentric annular channel rotating around its axis is investigated numerically by use of large eddy simulation (LES) coupled with a localized one-equation dynamic subgrid-scale (SGS) model (LDM). The objective of this study is to investigate the behavior of turbulent flow near the inner and outer walls of the rotating concentric annular channel and to examine the effectiveness of the LES technique for predicting the turbulent flow subjected to system rotation. The Reynolds number, based on the global friction velocity u τ and the annular channel gap width d, is 640, and the rotation number N =0–20, which is defined as N =2Ωd/u τ with Ω being the angular velocity of the rotating annular channel. To validate the present computation, LES on turbulent flows in a rotating pipe and in a concentric annular channel are carried out, which shows that the LES results are in good agreement with available experimental data and direct numerical simulation (DNS) results. Then, turbulent statistics in the rotating annular channel, including the resolved velocity, turbulence intensities, Reynolds stresses, turbulence skewness and flatness, and flow structures near the walls, are investigated. The budgets of turbulent kinetic energy and Reynolds stresses are calculated to examine the turbulence production rate, velocity and pressure-gradient correlation, turbulence diffusion, dissipation rate and Coriolis force term near the outer and inner walls. [Copyright &y& Elsevier]

Abstract: Large eddy simulation of thermally stratified turbulent open channel flows with low- to high-Prandtl number is performed. The three-dimensional filtered Navier–Stokes and energy equations under the Boussinesq approximation are numerically solved using a fractional-step method. Dynamic subgrid-scale (SGS) models for the turbulent SGS stress and heat flux are employed to close the governing equations. The objective of this study is to reveal the effects of both the Prandtl number (Pr) and Richardson (Ri τ ) number on the characteristics of turbulent flow, heat transfer, and large-scale motions in weakly stratified turbulence. The stably stratified turbulent open channel flows are calculated for Pr from 0.1 up to 100, Ri τ from 0 to 20, and the Reynolds number (Re τ ) 180 based on the wall friction velocity and the channel height. To elucidate the turbulent flow and heat transfer behaviors, some typical quantities, including the mean velocity, temperature and their fluctuations, turbulent heat fluxes, and the structures of the velocity and temperature fluctuations, are analyzed. [Copyright &y& Elsevier]

The non-Newtonian fluid flow in a bifurcation model with a non-planar daughter branch is investigated by using finite element method to solve the three-dimensional Navier–Stokes equations coupled with a non-Newtonian constitutive model, in which the shear thinning behavior of the blood fluid is incorporated by the Carreau–Yasuda model. The objective of this study is to investigate the influence of the non-Newtonian property of fluid as well as of curvature and out-of-plane geometry in the non-planar daughter vessel on wall shear stress (WSS) and flow phenomena. In the non-planar daughter vessel, the flows are typified by the skewing of the velocity profile towards the outer wall, creating a relatively low WSS at the inner wall. In the downstream of the bifurcation, the velocity profiles are shifted towards the flow divider. The low WSS is found at the inner walls of the curvature and the lateral walls of the bifurcation. Secondary flow patterns that swirl fluid from the inner wall of curvature to the outer wall in the middle of the vessel are also well documented for the curved and bifurcating vessels. The numerical results for the non-Newtonian fluid and the Newtonian fluid with original Reynolds number and the corresponding rescaled Reynolds number are presented. Significant difference between the non-Newtonian flow and the Newtonian flow is revealed; however, reasonable agreement between the non-Newtonian flow and the rescaled Newtonian flow is found. Results of this study support the view that the non-planarity of blood vessels and the non-Newtonian properties of blood are an important factor in hemodynamics and may play a significant role in vascular biology and pathophysiology. [Copyright &y& Elsevier]

Thermally stratified turbulent channel flow with temperature oscillation on the bottom wall of the channel is investigated by use of large eddy simulation (LES) approach coupled with dynamic subgrid-scale (SGS) models. The objective of this study is to deal with the influence of the temperature oscillation on characteristics of thermally stratified turbulent flow and to examine the effectiveness of the LES technique on predicting unsteady turbulent flow driven by time-varying buoyancy force. To validate the present calculation, thermally stratified turbulent channel flow is computed and compared with available data obtained by direct numerical simulation (DNS), which confirm that the present approach can be used to study stably and unstably stratified turbulent flow satisfactorily. Further, to elucidate the effect of the temperature oscillation on turbulent flow with different Richardson numbers and oscillation periods, some statistical quantities including the phase-averaged resolved velocities, temperature and their fluctuations, turbulent heat fluxes, and turbulence structures based on instantaneous velocity and temperature fluctuations are analyzed. [Copyright &y& Elsevier]

Oscillating turbulent channel flow with heat transfer has been investigated by use of large eddy simulation (LES) technique. Three-dimensional filtered Navier–Stokes and energy equations are numerically solved by fractional-step method. Dynamic subgrid-scale (SGS) models for the turbulent SGS stresses and heat fluxes are employed to closure the governing equations. The objective of this study is to examine the reliability of the LES technique for predicting the turbulent oscillatory heat transfer and to analyze the behavior of turbulent heat diffusion from a solid boundary to the adjacent oscillatory shear flow. In this study, oscillating turbulent channel flow with a constant difference of temperature imposed on both the walls is calculated for the Prandtl number (Pr) from 0.1 up to 100 and the Reynolds number (Reτ) 350 based on the friction velocity and the half-width of the channel. The frequency of driving pressure gradient for the oscillating turbulent channel flow ranges low, medium and high values. Some typical turbulence statistical quantities and structures of velocity and temperature fluctuations are analyzed. [Copyright &y& Elsevier]

A systematically numerical study of the sinusoidally oscillating viscous flow around a circular cylinder was performed to investigate vortical instability by solving the three-dimensional incompressible Navier-Stokes equations. The transition from two- to three-dimensional flow structures along the axial direction due to the vortical instability appears, and the three-dimensional structures lie alternatively on the two sides of the cylinder. Numerical study has been taken for the Keulegan-Carpenter (KC) numbers from 1 to 3.2 and frequency parameters from 100 to 600. The force behaviors are also studied by solving the Morison equation. Calculated results agree well with experimental data and theoretical prediction. [ABSTRACT FROM AUTHOR]

The hydrodynamics of a two-dimensional self-propelled flexible plate in a uniform shear flow is explored using a penalty-immersed boundary method. The leading edge of the plate is enforced into a prescribed harmonic oscillation in the vertical direction but free to move in the horizontal direction. It is found that as the shear rate increases, the input power, the propulsive velocity, and the efficiency increase. This finding means that the plate enables to get substantial hydrodynamic benefits from the shear flow. Using the force decomposition method based on the weighted integral of the second invariant of the velocity gradient tensor, the hydrodynamic force exerted on the plate is decomposed into a body-acceleration force, a vortex-induced force, and forces due to viscous effects. The results show that the body-acceleration force is the main driving force of the self-propelled motion, and that it is almost invariant with the shear rate. The vortex-induced force offers a significant contribution to the drag, and it decreases with the shear rate. The viscous friction force provides a pure drag, and it increases with the propulsion velocity. Further investigation on the vortex evolution and the vortex-induced force shows that the incoming shear flow destroys the trailing-edge vortex that sheds during the downward half period and, therefore, reduces the vortex-induced drag, which is the reason for the enhanced propulsive performance in the shear flow. The result obtained in this study provides new insight into the self-propulsion mechanism in complex incoming flows. [ABSTRACT FROM AUTHOR]

Rationale : In human beings and experimental animals, maladaptive impulsivity is manifested by the acute injection of psychostimulants, such as amphetamine. Cannabinoid CB1 receptors have been implicated in the regulation of stimulant-induced impulsive action, but the role of CB1 receptors in timing-related impulsive action by amphetamine remains unknown. Methods: Male rats were used in evaluating the effects of CB1 receptor antagonist and agonist (SR141716A and WIN55,212–2, respectively) systemically administered individually and combined with d-amphetamine on a differential reinforcement of low-rate response (DRL) task, an operant behavioral test of timing and behavioral inhibition characterized as a type of timing impulsive action. Results: A distinct pattern of DRL behavioral changes was produced by acute d-amphetamine (0, 0.5, 1.0, and 1.5 mg/kg) treatment in a dose-dependent fashion, whereas no significant dose effect was detected for acute SR141716A (0, 0.3, 1, and 3 mg/kg) or WIN55,212–2 (0, 0.5, 1, and 2 mg/kg) treatment. Furthermore, DRL behavior altered by 1.5 mg/kg d-amphetamine was reversed by a noneffective dose of SR141716A (3 mg/kg) pretreatment. The minimally influenced DRL behavior by 0.5 mg/kg d-amphetamine was affected by pretreatment with a noneffective dose of WIN55,212–2 (1 mg/kg). Conclusion: These findings reveal that the activation and blockade of CB1 receptors can differentially modulate the timing impulsive action of DRL behavior induced by acute amphetamine treatment. Characterizing how CB1 receptors modulate impulsive behavior will deepen our understanding of the cannabinoid psychopharmacology of impulsivity and may be helpful in developing an optimal pharmacotherapy for reducing maladaptive impulsivity in patients with some psychiatric disorders. [ABSTRACT FROM AUTHOR]

The mechanism behind stable aggregations of active swimmers is not fully understood. In particular, the in-depth quantitative explanations are notably lacking. To address this, a vorticity-based force expression is proposed to study the collective behaviors of two two-dimensional tandem flapping plates. The hydrodynamic force is directly related to the generation of vorticity on the plate and viscous forces resulting from its subsequent evolution. For the first time, the physical process by which the interactions between the rear plate and the wake vortices of the leader affects the hydrodynamic force is quantitatively elucidated and not based on simplified theory. The wake vortices of the leader influence the hydrodynamic force on the rear plate indirectly by inducing an additional oncoming flow. The flow affects the generation of vorticity, which mainly occurs on the head of the rear plate. The results show that, if the trajectory of the rear plate passes through the vortex cores, vorticity generation on the rear plate is suppressed and the thrust effect is weakened. If the rear plate slaloms between the vortices, the vorticity generation and the thrust effect are enhanced. We also found that the wake vortices of the leader has a certain ability to trap the rear plate into orderly configurations—no matter the rear plate has a dissimilar flapping amplitude or is applied an external horizontal loading—by adjusting the equilibrium position of the rear plate in it. The findings may shed some light on the understanding of collective behaviors in swimming animals. [ABSTRACT FROM AUTHOR]

The locomotion of a flapping flexible plate with different shapes and non-uniform chordwise stiffness distribution in a stationary fluid is studied numerically. The normalized effective bending stiffness K ∗ for three-dimensional plates with arbitrary stiffness distribution and shape parameters is proposed, and the overall bending stiffness of non-uniform plates with different shapes is reasonably characterized. It is found that the propulsion performance in terms of cruising speed and efficiency of the self-propelled flapping plate mainly depends on the effective bending stiffness. Plates with moderate flexibility K ∗ show better propulsion performance. Meanwhile, both a large area moment of the plate and a flexible anterior are favorable to significantly improve their propulsive performance. The evolution of vortical structures and the pressure distribution on the upper and lower surfaces of the plate are analyzed, and the inherent mechanism is revealed. These findings are of great significance to the optimal design of propulsion systems with different fins or wings. [ABSTRACT FROM AUTHOR]

Inspired by the wings of owls and the tubercles present on humpback whales' flippers, leading-edge serrations have demonstrated the potential to mitigate airfoil–turbulence interaction noise. To deepen our understanding of the underlying mechanisms driving this noise reduction, we conducted compressible large-eddy simulations on a rod-airfoil configuration equipped with wavy leading edges (WLEs) of varying amplitudes. All tested serrations exhibited some degree of noise reduction, with the amplitude of the WLE exerting a significant influence on the overall noise reduction effect. Notably, the wavy airfoil with the largest amplitude demonstrated the most substantial noise reduction in the mid-frequency range, achieving a remarkable decrease in up to 2.2 dB in noise levels. Applying multi-process acoustic theory, we delved into sound production on surfaces and near-field structures responsible for generating noise sources. Our findings underscore a crucial mechanism contributing to noise reduction—the source cutoff effects manifested through the significant weakening of noise sources at hill regions along the serrations' surface. Stronger source cutoff effects were observed with larger WLE amplitudes. Furthermore, our study reveals that destructive relationships among sources also play a pivotal role in reducing flow noise. The reduction in mid-frequency noise results from a synergy of the source cutoff effect and destructive source relationships induced by WLEs, while the decrease in low-frequency noise primarily emanates from the source cutoff effect. [ABSTRACT FROM AUTHOR]

The non-normal effects of the velocity gradient tensor (VGT) in a compressible turbulent boundary layer are studied by means of the Schur decomposition of the VGT into its normal and non-normal parts. Based on the analysis of the relative importance of them, it is found that the non-normal part significantly affects the dynamics of the VGT in the wall-bounded turbulent flow and the relevant non-normal effect has a dominant influence on the enstrophy and dissipation. It is revealed that the deviatoric part of the pressure Hessian is associated with the non-normal effect and the isotropic part is associated with the normal effect. The pressure Hessian significantly influences the vortex stretching. The non-normal effect reinforces the preferences for the vorticity vector to align with the intermediate strain-rate eigenvector and to be perpendicular to the extensive and compressive strain-rate eigenvector in the near-wall region. The non-normal effect also reduces the intermediate eigenvalue of the strain-rate tensor. Furthermore, a subgrid scale (SGS) model that separately considers the normal and non-normal effects is proposed based on the above characters and is verified to give a better prediction of the SGS dissipations in the wall-bounded turbulent flow. [ABSTRACT FROM AUTHOR]

The rheology of a capsule suspension in two-dimensional confined Poiseuille flow is studied numerically using an immersed-boundary lattice Boltzmann method. The effects of capsule volume fraction ϕ and bending stiffness Eb on the rheology of the suspension are investigated first. The apparent viscosity does not monotonically increase with ϕ: the variation curve can be divided into four flow regimes. In each regime, there is a distinct equilibrium spatial configuration. The overall lateral position of the capsules is directly connected with the apparent viscosity. Then, we propose to investigate the effect of inertia on the capsule configuration in dilute cases and the capsule transport in concentrated cases. For dilute cases, phase diagrams of flow regimes on the (ϕ, Eb) plane are plotted. It is found that, as the Reynolds number (Re) increases, the range of values for regime I, with a single-line configuration, reduces, while the range for regime II (transition configuration) increases. It is highly correlated with the equilibrium lateral position of a single capsule. For even larger Re, the range for regime IV (random configuration) increases rapidly and dominates because the larger inertia makes the arrangement more random. For concentrated cases, we observe that the optimal volume fraction, at which the transport of capsules is a maximum, increases with Re. This study may help to understand the collective behavior of capsules in Poiseuille flows. [ABSTRACT FROM AUTHOR]

Investigations on shock train dynamics and relevant turbulence features in a backpressured supersonic channel flow are carried out using direct numerical simulation for three inflow Mach numbers of M a 0 = 1.61, 2.0, and 2.45. As Ma0 increases, the shock train undergoes a structural change characterized by the leading shock which changes from the symmetric "λ" (M a 0 = 1.61) to the symmetric "X" (M a 0 = 2.00) and then to the asymmetric "X" pattern (M a 0 = 2.45). The symmetry breaking of shock structures induces asymmetric separation, which significantly alters the distribution characteristics of wall variables such as wall pressure and friction. To examine the unsteady behaviors of the shock train, a mode decomposition technique, namely, reduced-order variational mode decomposition [Liao et al., J. Fluid Mech. 966, A7 (2023)], is adopted taking its merit of adaptively extracting time-frequency features of dynamic systems. The modal analysis reveals that the shock train system exhibits significant centralization of low-frequency energy. Specifically, two basic types of low-frequency oscillation modes dominate the unsteady motion of the shock train: one depicts overall translating oscillation while another represents accordion-like oscillation. The analysis of turbulent kinetic energy shows that turbulence amplification is mainly dominated by the interaction of the decelerating mean flow with streamwise velocity fluctuations in the vicinity of the leading shock for all three cases, which is unaffected by the symmetry breaking of shock structures. [ABSTRACT FROM AUTHOR]

Improving the noise reduction capability of airfoil broadband noise through serrated trailing edge design is a challenging task. To address this, we propose a novel porous-serrated trailing edge design where the gaps between the serrations are filled with porous media. Implicit large eddy simulations were conducted at Mach number Ma = 0.1631 and Reynolds number Re = 96 000 under a zero incidence angle. In addition to straight trailing edges and conventional serrated trailing edges, cutting-type porous-serrated (CPS) and insert-type porous-serrated (IPS) trailing edges with different porosities were designed. The flow in the porous media is described by Darcy's law, which is related to the pressure and velocity. The results indicate that the CPS trailing edges offer limited noise reduction compared to conventional serrated trailing edges, while IPS trailing edges achieve a significant noise reduction of approximately 5.21 dB. However, the drag force increases by 8.0% in the IPS case with maximum noise reduction. The composite control mainly affects flow structures near the trailing edges, especially inducing the flow penetration across the porous surface. To investigate the noise reduction mechanism, dynamic mode decomposition was conducted to show that both the CPS and IPS designs promote energy transferring significantly from the energetic mode to the modes at other frequencies, which would partly explain the difference in the noise reduction performance to some extent. Furthermore, the analysis of the wall pressure fluctuations reveals that the reduced convection velocity on the porous surface and enhanced destructive interference between the porous and the solid surfaces in IPS cases could be identified as the key factors contributing to lower noise radiation efficiency. [ABSTRACT FROM AUTHOR]

The propulsive performance and vortex dynamics of a two-dimensional model for the jellyfish-like propulsion with burst-and-coast strategy are investigated using a penalty-immersed boundary method. The simplified model comprises a pair of pitching flexible plates with their leading edges connected. The effects of two key parameters are considered, i.e., the duty cycle (DC, the ratio of the closing phase to the whole period) and the bending stiffness (K). Three different wake patterns, i.e., periodic symmetric, periodic asymmetric, and chaotic wakes, are identified in the DC–K plane. Numerical results indicate that a significant fast-close-slow-open motion is more likely to achieve higher speed, efficiency, and stability than a slow-close-fast-open motion, and proper higher bending stiffness is conducive to improving efficiency. A force decomposition based on the weighted integral of the second invariant of the velocity gradient tensor is performed to gain physics insight into the self-propulsive mechanism. It is found that the repulsive force induced by the strain-rate field between the body and the previous vortex pair is the main driving force of the jellyfish-like motion and that capturing the previous vortex pair during the closing phase can significantly enhance the strain rate as well as the thrust. This clarifies why the jellyfish can achieve thrust by pushing back vortex pairs. This study provides inspiration for the design and control of flexible jet propulsion devices. [ABSTRACT FROM AUTHOR]

In this study, large eddy simulations of two rectangular jets with different aspect ratios and one planar jet with spanwise periodic boundaries are carried out in an attempt to investigate and clarify some key issues regarding the flow dynamics and noise generation mechanisms for the screech tone. The substantial effects of the nozzle configuration on shock structures and noise are first revealed in detail. It is found that when the lateral confinement is weakened, the spacing of the shock cells increases and the jet oscillates more intensively in the minor-axis plane, increasing the noise level and altering the screeching frequency. By analyzing the pressure fluctuations of the shear layer in the wavenumber-frequency space, different kinds of waves in these supersonic jets are examined. Importantly, it is revealed that the guided jet wave should play a dominant role in closing the resonance loops rather than the acoustic wave. Moreover, the energy-containing structures with pertinent frequencies are extracted by employing the reduced-order variational mode decomposition, and some underlying flow dynamics are presented. Especially, a novel mechanism has been identified: the low-frequency stretching motions of shock cells have a significant modulation effect on the screeching amplitude in the planar jet. Furthermore, with the aid of the multi-process acoustic theory, the characteristics of physical noise sources are diagnosed, particularly the source mechanisms related to the screech tone. The general structures and distribution of the kinematic noise source Sβ and entropy noise source Se are presented. Sβ mainly exhibits a vortex-like structure near the nozzle, while Se exhibits a lamellar bilayer structure. The spatiotemporal correlations between the physical sources and far-field noise show that the dominant mechanisms for the screech tone rely on the nozzle configuration and the screech tone tends to be produced by multiple sources in all cases. [ABSTRACT FROM AUTHOR]

The role of the hydrodynamic effect in the collective locomotion of several birds or fish is an interesting topic. Taking a model of a self-propelled flapping plate, we numerically investigated the collective locomotion of a pair of plates with comparable but different propulsive capacities, e.g., one long and one short plates. The longer plate is supposed to have a stronger propulsive capacity. It is found that two typical equilibrium configurations, i.e., compact and sparse configurations, may emerge, which depend mainly on initial lateral and longitudinal gap spacing, i.e., H and G0, respectively. In the compact cases, when H is small, e.g., H < 0.6, in terms of cruising speed and efficiency, hydrodynamic advantages are found for both plates. In all sparse configurations, the propulsive performance of the leading plate is identical to that of the corresponding isolated one. The following short plate in the "long-short" (the longer in the front) sparse cases always takes hydrodynamic advantages in terms of cruising speed and efficiency. In the "short-long" (the shorter in the front) sparse cases, the follower's propulsive capacity is suppressed because the cruising speed and input power decrease significantly compared to its isolated case. The analyses of hydrodynamic force and corresponding potential energy show that the staggered sparse configuration with H ∈ (0.4, 1.0) is more stable than that with other H. The "hydrodynamic drafting" analyzed here may shed some light on understanding the coordinated collective behaviors in biological and natural systems. [ABSTRACT FROM AUTHOR]

We investigate by direct numerical simulation the active control of laminar-turbulent transition in a hypersonic flat-plate boundary layer at a freestream Mach number of 5.86. The control mechanism is a synthetic jet. Based upon the linear stability theory of Mack, in hypersonic flow the important path to transition involves a high-frequency, second-mode fundamental resonance. Through systematic investigation, we reveal that the forcing the boundary layer with a synthetic jet at appropriate combinations of amplitude and frequency suppresses the second mode and delays transition. To gain physical insights into the major control mechanism, we employ the momentum potential theory (MPT) to analyze the flows with and without control. Essentially, the underlying control mechanism relies on an intriguing effect of the synthetic jet via generating the outward radiated wave structures, which are identified to split the upstream acoustic and vortical components. The splitting treatment presents the second-mode energy to drop sharply after the flow passes through the synthetic jet slot. The MPT source-term analysis reveals that the significantly suppressed near-wall source terms are responsible for suppressing the second mode downstream. Compared with the vortical and thermal source terms, the acoustic source term is found to be suppressed most. The kinetic budget analysis further reveals that the splitting treatment is related to the non-parallel effect and the nonlinear interaction. [ABSTRACT FROM AUTHOR]

The stability of the inverted flexible plate with non-uniform stiffness distribution in a free stream is studied by numerical simulation and mathematical theory. In our study, the bending stiffness distribution is expressed as the function of the leading edge's bending stiffness K ∗ and the polynomial of the plate's coordinate. Based on the former theoretical work on the stability of inverted plates with uniform stiffness distribution, we derive the upper limit value of K ∗ at which the zero-deflection equilibrium loses its stability for the plate with non-uniform stiffness distribution. The critical K ∗ derived from the mathematical theory agrees well with that obtained from the numerical simulation. An effective bending stiffness is defined, which can be used to unify the regimes of the motion modes between uniform plates and non-uniform plates. Moreover, three orders of mass ratio [ O (10 − 2) , O (10 − 1) , and O (1) ] are investigated, and the underlying mechanism for large amplitude flapping is clarified for the inverted plate with different mass ratios. An appropriate bending stiffness distribution can greatly improve the deformation of the plate. The findings shed some light on the energy harvesting of the inverted plate. [ABSTRACT FROM AUTHOR]

When controlling the trailing-edge (TE) interference noise of airfoil, the design of the TE serration shape is still an open issue. To this end, the flow and noise generation for different TE serration shapes are explored by the wall-resolved implicit large-eddy simulation and acoustic analogy. The feather-like serrations are found to achieve the most prominent noise reduction among the four types of curved serrations, especially in the low-frequency range. With the aid of acoustic analogy, the coherence analysis of far-field noise produced by the dipole sources on the airfoil surface is performed. The results show that destructive interference is still the critical mechanism responsible for noise reduction. Considering only the dipole sources, we find that the feather-like serrated TE shape can obtain the best noise reduction performance among all the serrated cases. Furthermore, since flow structures are reorganized near the TE serrations, we investigated the flow noise sources quantitatively in the near field. In these cases, the noise source due to flow structures is suppressed to a greater extent in the feather-like serrated case near the TE serration roots. Consequently, the above findings indicate that the feather-like serration favors suppressing dipole and flow noise sources in the near field, which makes it an efficient configuration for reducing airfoil noise. [ABSTRACT FROM AUTHOR]

The dynamic performance and wake structure of flapping plates with different shapes were studied using multi-block lattice Boltzman and immersed boundary method. Two typical regimes relevant to thrust behavior are identified. One is nonlinear relation between the thrust and the area moment of plate for lower area moment region and the other is linear relation for larger area moment region. The tendency of the power variation with the area moment is reasonably similar to the thrust behavior and the efficiency decreases gradually as the area moment increases. As the mechanism of the dynamic properties is associated with the evolution of vortical structures around the plate, the formation and evolution of vortical structures are investigated and the effects of the plate shape, plate area, Strouhal number and Reynolds number on the vortical structures are analyzed. The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to flapping locomotion. [ABSTRACT FROM AUTHOR]

Abstract: In many lattice Boltzmann studies about bubble rising, mass conservation is not satisfactory and the terminal bubble rising shape or velocity is not so consistent with experimental data as those obtained through other CFD techniques. In this paper, based on the multiphase model (He et al., 1999 [1]), a mass-conserving axisymmetric multiphase lattice Boltzmann model is developed. In the model, a mass correction step and an effective surface tension formula are introduced into the model. We demonstrate how the macroscopic axisymmetric Cahn–Hilliard equation and Navier–Stokes equation are recovered from the lattice Boltzmann equations through Chapman–Enskog expansion. The developed model is applied to simulate the bubble rising in viscous fluid. The mass correction step in our scheme significantly improves the bubble mass conservation. The surface tension calculation successfully predicts the terminal bubble shapes and reproduces the effect of initial bubble shape. The terminal bubble rising velocities are very consistent with experimental and numerical data in the literature. Qualitatively, the wakes behind the bubbles also agree well with experimental data. This model is useful for predicting the axisymmetric two-phase flows. [Copyright &y& Elsevier]

Turbulent boundary layers at Mach 4.9 with the ratio of wall temperature to recovery temperature from 0.5 to 1.5 are investigated by means of direct numerical simulation. Various fundamental properties relevant to the influence of wall temperature on Morkovin’s scaling, standard and modified strong Reynolds analogies, and coherent vortical structures have been studied. It is identified that the scaling relations proposed for cool and adiabatic wall conditions, such as Morkovin’s scaling and the modified strong Reynolds analogy, are also applicable for hot wall condition. Moreover, the relation between the density and temperature fluctuations under the second-order approximation is derived and verified to provide a reliable prediction. Based on the analysis of coherent vortical structures, it is found that the orientation of vortex core can be quantitatively determined by means of the vector with its direction and modulus using the local strain direction and the imaginary part of the eigenvalue of velocity gradient tensor, respectively. As the increase of wall temperature, the spanwise distance between the two legs of hairpin vortex increases, and the mean swirling strength and the angle of vortical structure with respect to the wall plane also increase in the inner layer. The statistical properties relevant to vortical structures are nearly insensitive to the wall temperature in the outer layer. [ABSTRACT FROM PUBLISHER]

This paper presents an efficient implementation of the four-way coupled point-particle direct numerical simulation (PP-DNS) for compressible particle-laden wall turbulence, utilizing the open-source finite-difference compressible Navier–Stokes solver, STREAmS. The proposed design integrates a GPU-based two-phase collision detection algorithm known as the spatial subdivision method, along with specialized storage and MPI communication strategies for Lagrangian particles on multi-GPU platforms. Specifically, a 'page table' like data structure is designed to store the particle information compactly and to enable highly parallelized packing and unpacking procedures for GPU-GPU data exchange. These advancements significantly reduce the computational cost of four-way coupled particle-laden flow simulations, enabling efficient simulations involving over O (1 0 7) particles (an order of magnitude higher than that in the state-of-the-art simulations) on a single NVIDIA A100 GPU. To validate the proposed implementation, we perform simulations of compressible particle-laden wall-bounded turbulence using canonical configurations such as channel flows and zero-pressure-gradient boundary layers. The example results highlight the effects of inter-particle collisions and flow compressibility. Furthermore, we assess single-GPU performance and scalability by employing up to eight NVIDIA GPU devices. Even for four-way coupled simulations, the elapsed time per step scales approximately linearly with the number of particles (when the number of particles is large enough), and a parallel efficiency of 94.1% is achieved on 8 NVIDIA A100 GPUs. [Display omitted] • Multi-GPU acceleration strategies for PP-DNS are proposed. • Efficient four-way coupled simulations involving over 1 0 7 particles are realized. • The significance of four-way coupling effects are preliminarily demonstrated. [ABSTRACT FROM AUTHOR]

Originally, the color-gradient model proposed by Rothman and Keller (R-K) was unable to simulate immiscible two-phase flows with different densities. Later, a revised version of the R-K model was proposed by Grunau et al. [D. Grunau, S. Chen and K. Eggert, Phys. Fluids A: Fluid Dyn. 5, 2557 (1993).] and claimed it was able to simulate two-phase flows with high-density contrast. Some studies investigate high-density contrast two-phase flows using this revised R-K model but they are mainly focused on the stationary spherical droplet and bubble cases. Through theoretical analysis of the model, we found that in the recovered Navier-Stokes (N-S) equations which are derived from the R-K model, there are unwanted extra terms. These terms disappear for simulations of two-phase flows with identical densities, so the correct N-S equations are fully recovered. Hence, the R-K model is able to give accurate results for flows with identical densities. However, the unwanted terms may affect the accuracy of simulations significantly when the densities of the two fluids are different. For the simulations of spherical bubbles and droplets immersed in another fluid (where the densities of the two fluids are different), the extra terms may not be important and hence, in terms of surface tension, accurate results can be obtained. However, generally speaking, the unwanted term may be significant in many flows and the R-K model is unable to obtain the correct results due to the effect of the extra terms. Through numerical simulations of parallel two-phase flows in a channel, we confirm that the R-K model is not appropriate for general two-phase flows with different densities. A scheme to eliminate the unwanted terms is also proposed and the scheme works well for cases of density ratios less than 10. [ABSTRACT FROM AUTHOR]

Abstract: A free energy (FE) model, the Shan–Chen (S–C) model, and the Rothman and Keller (R–K) model are studied numerically to evaluate their performance in modeling two-dimensional (2D) immiscible two-phase flow in porous media on the pore scale. The FE model is proved to satisfy the Galilean invariance through a numerical test and the mass conservation of each component in the simulations is exact. Two-phase layered flow in a channel with different viscosity ratios was simulated. Comparing with analytical solutions, we see that the FE model and the R–K model can give very accurate results for flows with large viscosity ratios. In terms of accuracy and stability, the FE model and the R–K model are much better than the S–C model. Co-current and countercurrent two-phase flows in complex homogeneous media were simulated and the relative permeabilities were obtained. Again, it is found that the FE model is as good as the R–K model in terms of accuracy and efficiency. The FE model is shown to be a good tool for the study of two-phase flows with high viscosity ratios in porous media. [Copyright &y& Elsevier]

A supersonic flow past a hemispherical nose with an opposing jet placed on its axis has been investigated using large eddy simulation. We find that the flow behaviors depend mainly on the jet total pressure ratio and can be classified into three typical flow regimes of unstable, stable and transition. The unstable flow regime is characterized by an oscillatory bow shock with a multi-jet-cell structure and the stable flow regime by a steady bow shock with a single jet cell. The transition regime lies between the unstable and stable ones with a complex flow evolution. Turbulence statistics are further analyzed to reveal the relevant turbulent behaviors in the three flow regimes. The results obtained in this study provide a physical insight into the understanding of the mechanisms underlying this complex flow. [ABSTRACT FROM AUTHOR]

Abstract: Turbulent open channel flows with heat transfer subjected to the control of a spanwise travelling wave have been investigated by means of direct numerical simulation (DNS). The three-dimensional Navier–Stokes and energy equations are numerically solved using a fractional-step method. The spanwise travelling wave is induced by a body force that is confined within the viscous layer with its maximum at the bottom wall and decaying exponentially away from it. The objective of this study is to reveal the near-wall turbulence behaviours, the turbulent heat transfer, and thermal structures under the control of the spanwise travelling wave. Three typical frequencies of the spanwise travelling wave, i.e., high-, middle- and low-frequency, corresponding to the exciting periods at T + =25, 50 and 100, are investigated to reveal the dynamics of turbulent motions and heat transfer. The Prandtl number (Pr) varies from 1 up to 100. To elucidate the behaviours of turbulence statistics and heat transfer, some typical quantities, including the mean velocity, velocity and vorticity fluctuations, temperature and its fluctuation, turbulent heat fluxes, and the structures of the temperature fluctuation, are exhibited and analyzed. [Copyright &y& Elsevier]

Abstract: Direct numerical simulation of wall-normal rotating channel flow with heat transfer has been performed for the rotation number N τ from 0 to 0.1, the Reynolds number 194 based on the friction velocity of non-rotating case and the half-height of the channel, and the Prandtl number 1. The objective of this study is to reveal the effects of rotation on the characteristics of turbulence and heat transfer. Some statistical turbulence and heat transfer quantities, including the mean velocity, temperature and their fluctuations, turbulent heat fluxes, and turbulence structures, are investigated. Based on the present calculated results, two typical rotation regimes are identified. When 0< N τ <0.06, the turbulence statistics correlated with the spanwise velocity fluctuation are enhanced since the shear rate of spanwise mean flow induced by Coriolis force increases; however, the other statistics are suppressed. When N τ >0.06, all the turbulence statistics are suppressed significantly. To elucidate the effects of rotation on the turbulent heat transfer, the budget terms in the transport equation of turbulent heat fluxes are analyzed. Remarkable change of the direction of near-wall streak structures of the velocity and temperature fluctuations, nearly in alignment with the absolute mean flow direction, is revealed. An attempt to evaluate the mean spacing and the direction of streaky structures near the wall has been examined based on the two-point correlations of the velocity and temperature fluctuations. [Copyright &y& Elsevier]

The self-organization of active swimmers is interesting but not fully understood. Lighthill conjectured that the orderly configurations may emerge passively from the hydrodynamic interactions rather than active control mechanism. To further test Lighthill's conjecture, the effect of active control on the propulsive performance of two self-propelled flapping plates in tandem configuration is studied. Different types of external horizontal forces are applied at the leading edge of the following plate. It is found that the collective dynamic and propulsive performance of the two-plate system are mainly affected by the mean value of the external horizontal force rather than its specific form. The two-plate self-propelled system has certain ability to counteract a limited external intervention and maintain the orderly configuration by adjusting the gap spacing between two plates. For a stable configuration, the external intervention hardly affects the propulsion velocity but has a significant monotonic effect on the gap spacing and input work. Further, a simplified model is proposed to relate the external horizontal force to the gap spacing between two plates and verified to be reliable by the numerical results. Moreover, the momentum and energy transferred to fluid are investigated in terms of local vortical structures. It is revealed that the impulse of the wake vortex pair is hardly affected by the external horizontal force, while its kinetic energy and the local dissipative energy vary monotonically with it. These results may shed some light on the understanding of collective behaviors of living swimmers and robotic fish. [ABSTRACT FROM AUTHOR]