The finite difference method (FDM) is applied in the present paper to solve the unsteady NHS equations for incompressible fluids. ADI and SLOR methods are served for the vorticity equation and the Poisson equation for...The finite difference method (FDM) is applied in the present paper to solve the unsteady NHS equations for incompressible fluids. ADI and SLOR methods are served for the vorticity equation and the Poisson equation for ψ respectively. The upwind scheme is used for the convective terms. The moving boundary conditions are specially treated, and the effects of outlet conditions on the flow field are abo examined. Numerical results obtained show that the spoiler's oscillation induces forming, growing and shedding of the vortices. The shedding frequency of vortices is equal to that of the spoiler's oscillation. The forced unsteady separated flows under the present investigation depend mainly on the reduced frequency. At low reduced frequency, the vortices shed from the spoiler interact weakly with each other, and move downstream at an almost uniform speed of 038 V∞. At high reduced frequency, the interaction between the adjacent vortices strengthens. They close up to and rotate around each other, and eventually, merge into one vortex.展开更多
The present paper proposes a Lagrangian criterion of unsteady flow separation for two-dimensional periodic flows based on the principle of weighted averaging zero skin-friction given by Haller （HALLER, G. Exact theor...The present paper proposes a Lagrangian criterion of unsteady flow separation for two-dimensional periodic flows based on the principle of weighted averaging zero skin-friction given by Haller （HALLER, G. Exact theory of unsteady separation for two-dimensional flows. Journal of Fluid Mechanics, 512, 257-311 （2004））. By analyzing the distribution of the finite-time Lyapunov exponent （FTLE） along the no-slip wall, it can be found that the periodic separation takes place at the point of the zero FTLE. This new criterion is verified with an analytical solution of the separation bubble and a numerical simulation of lid-driven cavity flows.展开更多
Flow visualization was used to investigate experimentally the evolution process from symmetrical shedding to staggered shedding of the starting vortex and the phenomenon of secondary separation on an elliptic cylinder...Flow visualization was used to investigate experimentally the evolution process from symmetrical shedding to staggered shedding of the starting vortex and the phenomenon of secondary separation on an elliptic cylinder at moderate Reynolds numbers.The vortex structure of the flow separation was studied.The temporal variation of separation angle and length of wake vortex were given. The photographs and experimental results provided basis for further investigation of the complicated feature of the starting process of unsteady separated flows around an elliptic cylinder.展开更多
In the present paper,numerical solution of the two-dimensional unsteady Navier-Stokes equations is used to study the forced shear flow induced by a spoiler's periodical up and down oscillation on a flat plate.The ...In the present paper,numerical solution of the two-dimensional unsteady Navier-Stokes equations is used to study the forced shear flow induced by a spoiler's periodical up and down oscillation on a flat plate.The paper studies the evolution of growing,shedding,merging and decaying of vortices due to the spoiler's oscillation,particularly the dependence of the forced shear flow on the re- duced frequency.Results show that the reduced frequency is a key factor in controlling the growing and the shedding of vortices in the shear layer.The instantaneous streamlines and the equi-vorticity con- tours,as well as the surface pressure distributions,have also been investigated.Numerical results agree well with corresponding experimental ones.The study is helpful for understanding the physical mecha- nism of shear flow control.展开更多
During the initial stage of vertical launch,a missile may exhibit an uncertain roll angle(φ)and a high angle of attack(α).This study focuses on examining the impact of roll angle variations on the flow field and the...During the initial stage of vertical launch,a missile may exhibit an uncertain roll angle(φ)and a high angle of attack(α).This study focuses on examining the impact of roll angle variations on the flow field and the unsteady aerodynamics of a canard-configured missile atα=75°.Simulations were performed using the validated k-ωSST turbulence model.The analysis encompasses the temporal development of vortices,the oscillatory characteristics of the lateral force,and the fluctuation of kinetic energy distribution within the framework of proper orthogonal decomposition(POD).The results indicate that the flow field surrounding the canardconfigured missile is characterized by inconsistent shedding cycles of Kármán-like and canard-separated vortices.A distinct transition zone is identified between these vortices,where vortex tearing and reconnection phenomena occur.With increasing roll angles from 0°to 45°,there is an observed shift in the dominant frequency of the lateral force from the higher frequency associated with Kármán-like vortex shedding to the lower frequency of canard vortex shedding.The shedding frequency of Kármán-like vortices corresponds to the harmonics of the canard vortex shedding frequency,indicative of a higher-order harmonic resonance.The frequency of the lateral force is observed to decrease with an increase in roll angle,except in configurations lacking distinct canard-separated vortices,which are characterized by a“+”shape.The POD analysis reveals that the majority of the fluctuation energy is concentrated in the oscillations and shedding of the canard-separated vortices,leading to pressure fluctuations that are primarily observed on the canard and the downstream region of the canard.展开更多
基金The project is supported by the National Nature Science Foundation of China(NNSFC)
文摘The finite difference method (FDM) is applied in the present paper to solve the unsteady NHS equations for incompressible fluids. ADI and SLOR methods are served for the vorticity equation and the Poisson equation for ψ respectively. The upwind scheme is used for the convective terms. The moving boundary conditions are specially treated, and the effects of outlet conditions on the flow field are abo examined. Numerical results obtained show that the spoiler's oscillation induces forming, growing and shedding of the vortices. The shedding frequency of vortices is equal to that of the spoiler's oscillation. The forced unsteady separated flows under the present investigation depend mainly on the reduced frequency. At low reduced frequency, the vortices shed from the spoiler interact weakly with each other, and move downstream at an almost uniform speed of 038 V∞. At high reduced frequency, the interaction between the adjacent vortices strengthens. They close up to and rotate around each other, and eventually, merge into one vortex.
基金supported by the National Natural Science Foundation of China(Nos.11372340 and 11732016)
文摘The present paper proposes a Lagrangian criterion of unsteady flow separation for two-dimensional periodic flows based on the principle of weighted averaging zero skin-friction given by Haller （HALLER, G. Exact theory of unsteady separation for two-dimensional flows. Journal of Fluid Mechanics, 512, 257-311 （2004））. By analyzing the distribution of the finite-time Lyapunov exponent （FTLE） along the no-slip wall, it can be found that the periodic separation takes place at the point of the zero FTLE. This new criterion is verified with an analytical solution of the separation bubble and a numerical simulation of lid-driven cavity flows.
基金The project supported by the National Natural Science Foundation of China.
文摘Flow visualization was used to investigate experimentally the evolution process from symmetrical shedding to staggered shedding of the starting vortex and the phenomenon of secondary separation on an elliptic cylinder at moderate Reynolds numbers.The vortex structure of the flow separation was studied.The temporal variation of separation angle and length of wake vortex were given. The photographs and experimental results provided basis for further investigation of the complicated feature of the starting process of unsteady separated flows around an elliptic cylinder.
文摘In the present paper,numerical solution of the two-dimensional unsteady Navier-Stokes equations is used to study the forced shear flow induced by a spoiler's periodical up and down oscillation on a flat plate.The paper studies the evolution of growing,shedding,merging and decaying of vortices due to the spoiler's oscillation,particularly the dependence of the forced shear flow on the re- duced frequency.Results show that the reduced frequency is a key factor in controlling the growing and the shedding of vortices in the shear layer.The instantaneous streamlines and the equi-vorticity con- tours,as well as the surface pressure distributions,have also been investigated.Numerical results agree well with corresponding experimental ones.The study is helpful for understanding the physical mecha- nism of shear flow control.
基金Fund of Science and Technology on Underwater Information and Control Laboratory,Grant/Award Number:2021-JCJQ-LB-030-05。
文摘During the initial stage of vertical launch,a missile may exhibit an uncertain roll angle(φ)and a high angle of attack(α).This study focuses on examining the impact of roll angle variations on the flow field and the unsteady aerodynamics of a canard-configured missile atα=75°.Simulations were performed using the validated k-ωSST turbulence model.The analysis encompasses the temporal development of vortices,the oscillatory characteristics of the lateral force,and the fluctuation of kinetic energy distribution within the framework of proper orthogonal decomposition(POD).The results indicate that the flow field surrounding the canardconfigured missile is characterized by inconsistent shedding cycles of Kármán-like and canard-separated vortices.A distinct transition zone is identified between these vortices,where vortex tearing and reconnection phenomena occur.With increasing roll angles from 0°to 45°,there is an observed shift in the dominant frequency of the lateral force from the higher frequency associated with Kármán-like vortex shedding to the lower frequency of canard vortex shedding.The shedding frequency of Kármán-like vortices corresponds to the harmonics of the canard vortex shedding frequency,indicative of a higher-order harmonic resonance.The frequency of the lateral force is observed to decrease with an increase in roll angle,except in configurations lacking distinct canard-separated vortices,which are characterized by a“+”shape.The POD analysis reveals that the majority of the fluctuation energy is concentrated in the oscillations and shedding of the canard-separated vortices,leading to pressure fluctuations that are primarily observed on the canard and the downstream region of the canard.
