Some of the most interesting areas in aerospace science and technologies are on either higher,faster,and larger systems or lower,slower,and smaller flying capabilities.In this paper,we present our perspectives on the ...Some of the most interesting areas in aerospace science and technologies are on either higher,faster,and larger systems or lower,slower,and smaller flying capabilities.In this paper,we present our perspectives on the aerodynamics related to small,fixed-wing as well as flapping-wing flight vehicles.From an evolutionary viewpoint,flyers have gone through many iterations,adaptations,and optimizations to balance their biological functions,including flight.In the low-Reynolds-number regime,the aerodynamic characteristics around a solid object differ from those observed at the scale of passenger-airplanes.Consequently,the optimal airfoil and wing shapes vary with vehicle size.As vehicle dimensions vary,non-proportional scaling between surface areas and weight shifts the dominance of physical mechanisms,leading to distinct operational parameters and technical requirements.With smaller flight vehicles,structural flexibility as well as anisotropic material properties become more pronounced,which causes qualitative changes in aerodynamics.The flapping motion of the wings,the interactions between wings,the synergistic characteristics of wing and tail,and the development of soft structures for better agility and flight performance are discussed.Low-Reynolds-number aerodynamics require collaborative innovation to optimize shape,motion,and structure of vehicles in accordance with the scaling laws.Together,progress on these fronts is reshaping the design paradigm of air vehicles and other types of robots with shrinking physical dimensions and more versatile capabilities to meet wider ranges of missions.展开更多
Dynamics of a spherical particle and the suspending low-Reynolds-number fluid confined by a cubic cavity were studied numerically.We calculated the particle’s hydrodynamic mobilities along x-,y-,and zdirections at va...Dynamics of a spherical particle and the suspending low-Reynolds-number fluid confined by a cubic cavity were studied numerically.We calculated the particle’s hydrodynamic mobilities along x-,y-,and zdirections at various locations in the cavity.The mobility is largest in the cavity center and decays as the particle becomes closer to no-slip walls.It was found that mobilities in the entire cubic cavity can be determined by a minimal set in a unit tetrahedron therein.Fluid vortices in the cavity induced by the particle motion were observed and analyzed.We also found that the particle can exhibit a drift motion perpendicular to the external force.Magnitude of the drift velocity normalized by the velocity along the direction of the external force depends on particle location and particle-to-cavity sizes ratio.This work forms the basis to understand more complex dynamics in microfluidic applications such as intracellular transport and encapsulation technologies.展开更多
We propose a simple model for turbulent contribution to the frictional drag in a wall-bounded turbulent flow based on the characteristic parameters of turbulent bursting events, it is verified on water and drag-reduci...We propose a simple model for turbulent contribution to the frictional drag in a wall-bounded turbulent flow based on the characteristic parameters of turbulent bursting events, it is verified on water and drag-reducing surfactant solution flows investigated by particle image velocimetry in experiments. It is obtained that the turbulent contribution to the skin friction factor is linearly proportional to the product of the spatial frequency and strength of turbulent bursts originated from the wall.展开更多
The swimming performance of rod-shaped microswimmers in a channel was numerically investigated using the two-dimensional lattice Boltzmann method(LBM).We considered variable-length squirmer rods,assembled from circula...The swimming performance of rod-shaped microswimmers in a channel was numerically investigated using the two-dimensional lattice Boltzmann method(LBM).We considered variable-length squirmer rods,assembled from circular squirmer models with self-propulsion mechanisms,and analyzed the effects of the Reynolds number(Re),aspect ratio(ε),squirmer-type factor(β)and blockage ratio(κ)on swimming efficiency(η)and power expenditure(P).The results show no significant difference in power expenditure between pushers(microswimmers propelled from the tail)and pullers(microswimmers propelled from the head)at the low Reynolds numbers adopted in this study.However,the swimming efficiency of pushers surpasses that of pullers.Moreover,as the degree of channel blockage increases(i.e.,κincreases),the squirmer rod consumes more energy while swimming,and its swimming efficiency also increases,which is clearly reflected whenε≤3.Notably,squirmer rods with a larger aspect ratioεand aβvalue approaching 0 can achieve high swimming efficiency with lower power expenditure.The advantages of self-propelled microswimmers are manifested whenε>4 andβ=±1,where the squirmer rod consumes less energy than a passive rod driven by an external field.These findings underscore the potential for designing more efficient microswimmers by carefully considering the interactions between the microswimmer geometry,propulsion mechanism and fluid dynamic environment.展开更多
We study the propulsion matrix of bacterial flagella numerically using slender body theory and the regularized Stokeslet method in a biologically relevant parameter regime. All three independent elements of the matrix...We study the propulsion matrix of bacterial flagella numerically using slender body theory and the regularized Stokeslet method in a biologically relevant parameter regime. All three independent elements of the matrix are measured by computing propulsive force and torque generated by a rotating flagellum, and the drag force on a translating flagellum. Nu- merical results are compared with the predictions of resistive force theory, which is often used to interpret micro-organism propulsion. Neglecting hydrodynamic interactions between different parts of a flagellum in resistive force theory leads to both qualitative and quantitative discrepancies between the theoretical prediction of resistive force theory and the numerical results. We improve the original theory by empirically incorporating the effects of hydrodynamic interactions and propose new expressions for propulsive matrix elements that are accurate over the parameter regime explored.展开更多
The near-wall domain decomposition method(NDD)has proved to be very efficient for modeling near-wall fully turbulent flows.In this paper the NDD is extended to non-equilibrium regimeswith laminar-turbulent transition(...The near-wall domain decomposition method(NDD)has proved to be very efficient for modeling near-wall fully turbulent flows.In this paper the NDD is extended to non-equilibrium regimeswith laminar-turbulent transition(LTT)for the first time.The LTT is identified with the use of the e^(N)-method which is applied to both incompressible and compressible flows.TheNDD ismodified to take into account LTT in an efficientway.In addition,implementation of the intermittency expands the capabilities of NDD to model non-equilibrium turbulent flows with transition.Performance of the modified NDD approach is demonstrated on various test problems of subsonic and supersonic flows past a flat plate,a supersonic flow over a compression corner and a planar shock wave impinging on a turbulent boundary layer.The results of modeling with and without decomposition are compared in terms of wall friction and show good agreement with each other while NDD significantly reducing computational resources needed.It turns out that the NDD can reduce the computational time as much as three times while retaining practically the same accuracy of prediction.展开更多
基金supported by the Research Grants Council(RGC)of the Government of Hong Kong Special Administrative Region(HKSAR)with RGC/GRF Project(Grant Nos.16206321 and 14113824).
文摘Some of the most interesting areas in aerospace science and technologies are on either higher,faster,and larger systems or lower,slower,and smaller flying capabilities.In this paper,we present our perspectives on the aerodynamics related to small,fixed-wing as well as flapping-wing flight vehicles.From an evolutionary viewpoint,flyers have gone through many iterations,adaptations,and optimizations to balance their biological functions,including flight.In the low-Reynolds-number regime,the aerodynamic characteristics around a solid object differ from those observed at the scale of passenger-airplanes.Consequently,the optimal airfoil and wing shapes vary with vehicle size.As vehicle dimensions vary,non-proportional scaling between surface areas and weight shifts the dominance of physical mechanisms,leading to distinct operational parameters and technical requirements.With smaller flight vehicles,structural flexibility as well as anisotropic material properties become more pronounced,which causes qualitative changes in aerodynamics.The flapping motion of the wings,the interactions between wings,the synergistic characteristics of wing and tail,and the development of soft structures for better agility and flight performance are discussed.Low-Reynolds-number aerodynamics require collaborative innovation to optimize shape,motion,and structure of vehicles in accordance with the scaling laws.Together,progress on these fronts is reshaping the design paradigm of air vehicles and other types of robots with shrinking physical dimensions and more versatile capabilities to meet wider ranges of missions.
基金supported by the Young Elite Scientists Sponsorship Program by the Chinese Society of Theoretical and Applied Mechanics(CSTAM).
文摘Dynamics of a spherical particle and the suspending low-Reynolds-number fluid confined by a cubic cavity were studied numerically.We calculated the particle’s hydrodynamic mobilities along x-,y-,and zdirections at various locations in the cavity.The mobility is largest in the cavity center and decays as the particle becomes closer to no-slip walls.It was found that mobilities in the entire cubic cavity can be determined by a minimal set in a unit tetrahedron therein.Fluid vortices in the cavity induced by the particle motion were observed and analyzed.We also found that the particle can exhibit a drift motion perpendicular to the external force.Magnitude of the drift velocity normalized by the velocity along the direction of the external force depends on particle location and particle-to-cavity sizes ratio.This work forms the basis to understand more complex dynamics in microfluidic applications such as intracellular transport and encapsulation technologies.
文摘We propose a simple model for turbulent contribution to the frictional drag in a wall-bounded turbulent flow based on the characteristic parameters of turbulent bursting events, it is verified on water and drag-reducing surfactant solution flows investigated by particle image velocimetry in experiments. It is obtained that the turbulent contribution to the skin friction factor is linearly proportional to the product of the spatial frequency and strength of turbulent bursts originated from the wall.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.12372251 and 12132015)the Fundamental Research Funds for the Provincial Universities of Zhejiang(Grant No.2023YW69)。
文摘The swimming performance of rod-shaped microswimmers in a channel was numerically investigated using the two-dimensional lattice Boltzmann method(LBM).We considered variable-length squirmer rods,assembled from circular squirmer models with self-propulsion mechanisms,and analyzed the effects of the Reynolds number(Re),aspect ratio(ε),squirmer-type factor(β)and blockage ratio(κ)on swimming efficiency(η)and power expenditure(P).The results show no significant difference in power expenditure between pushers(microswimmers propelled from the tail)and pullers(microswimmers propelled from the head)at the low Reynolds numbers adopted in this study.However,the swimming efficiency of pushers surpasses that of pullers.Moreover,as the degree of channel blockage increases(i.e.,κincreases),the squirmer rod consumes more energy while swimming,and its swimming efficiency also increases,which is clearly reflected whenε≤3.Notably,squirmer rods with a larger aspect ratioεand aβvalue approaching 0 can achieve high swimming efficiency with lower power expenditure.The advantages of self-propelled microswimmers are manifested whenε>4 andβ=±1,where the squirmer rod consumes less energy than a passive rod driven by an external field.These findings underscore the potential for designing more efficient microswimmers by carefully considering the interactions between the microswimmer geometry,propulsion mechanism and fluid dynamic environment.
基金supported by the National Natural Science Foundation of China(Grant No.11104179)the Shanghai Pujiang Program,China(Grant No.12PJ1405400)+1 种基金the Program for Professor of Special Appointment(Eastern Scholar)at Shanghai Institutions of Higher Learning,China(Grant No.SHDP201301)the Innovation Program of Shanghai Municipal Education Commission,China(Grant No.14ZZ030)
文摘We study the propulsion matrix of bacterial flagella numerically using slender body theory and the regularized Stokeslet method in a biologically relevant parameter regime. All three independent elements of the matrix are measured by computing propulsive force and torque generated by a rotating flagellum, and the drag force on a translating flagellum. Nu- merical results are compared with the predictions of resistive force theory, which is often used to interpret micro-organism propulsion. Neglecting hydrodynamic interactions between different parts of a flagellum in resistive force theory leads to both qualitative and quantitative discrepancies between the theoretical prediction of resistive force theory and the numerical results. We improve the original theory by empirically incorporating the effects of hydrodynamic interactions and propose new expressions for propulsive matrix elements that are accurate over the parameter regime explored.
文摘The near-wall domain decomposition method(NDD)has proved to be very efficient for modeling near-wall fully turbulent flows.In this paper the NDD is extended to non-equilibrium regimeswith laminar-turbulent transition(LTT)for the first time.The LTT is identified with the use of the e^(N)-method which is applied to both incompressible and compressible flows.TheNDD ismodified to take into account LTT in an efficientway.In addition,implementation of the intermittency expands the capabilities of NDD to model non-equilibrium turbulent flows with transition.Performance of the modified NDD approach is demonstrated on various test problems of subsonic and supersonic flows past a flat plate,a supersonic flow over a compression corner and a planar shock wave impinging on a turbulent boundary layer.The results of modeling with and without decomposition are compared in terms of wall friction and show good agreement with each other while NDD significantly reducing computational resources needed.It turns out that the NDD can reduce the computational time as much as three times while retaining practically the same accuracy of prediction.
