The experimental investigation is conducted with LDV and hydrogen bubble technique in water flow. The shear layer thickness. the vorticity thickness. the maximulll value of turbulence intensities. the turbulent coher...The experimental investigation is conducted with LDV and hydrogen bubble technique in water flow. The shear layer thickness. the vorticity thickness. the maximulll value of turbulence intensities. the turbulent coherent structure. the variations of wall shear stress and the boundary layer shape factor are obtained. In the redevelopment region. the detailed analysis is first made for the streak structures in the near wall region and the turbulent boundary layer is formed at (x-xr) / h = 20.展开更多
Compressible flow past a circular cylinder at an inflow Reynolds number of 2×105 is numerically investigated by using a constrained large-eddy simulation(CLES)technique.Numerical simulation with adiabatic wall bo...Compressible flow past a circular cylinder at an inflow Reynolds number of 2×105 is numerically investigated by using a constrained large-eddy simulation(CLES)technique.Numerical simulation with adiabatic wall boundary condition and at a free-stream Mach number of 0.75 is conducted to validate and verify the performance of the present CLES method in predicting separated flows.Some typical and characteristic physical quantities,such as the drag coefficient,the root-mean-square lift fluctuations,the Strouhal number,the pressure and skin friction distributions around the cylinder,etc.are calculated and compared with previously reported experimental data,finer-grid large-eddy simulation(LES)data and those obtained in the present LES and detached-eddy simulation(DES)on coarse grids.It turns out that CLES is superior to DES in predicting such separated flow and that CLES can mimic the intricate shock wave dynamics quite well.Then,the effects of Mach number on the flow patterns and parameters such as the pressure,skin friction and drag coefficients,and the cylinder surface temperature are studied,with Mach number varying from 0.1 to 0.95.Nonmonotonic behaviors of the pressure and skin friction distributions are observed with increasing Mach number and the minimum mean separation angle occurs at a subcritical Mach number of between 0.3 and 0.5.Additionally,the wall temperature effects on the thermodynamic and aerodynamic quantities are explored in a series of simulations using isothermal wall boundary conditions at three different wall temperatures.It is found that the flow separates earlier from the cylinder surface with a longer recirculation length in the wake and a higher pressure coefficient at the rear stagnation point for higher wall temperature.Moreover,the influences of different thermal wall boundary conditions on the flow field are gradually magnified from the front stagnation point to the rear stagnation point.It is inferred that the CLES approach in its current version is a useful and effective tool for simulating wall-bounded compressible turbulent flows with massive separations.展开更多
Dilute gas-particle turbulent flows over a backward-facing step are numerically simulated by Large Eddy Simulation (LES) for the continuous phase and Lagran- gian particle trajectory method for the particle phase. Pre...Dilute gas-particle turbulent flows over a backward-facing step are numerically simulated by Large Eddy Simulation (LES) for the continuous phase and Lagran- gian particle trajectory method for the particle phase. Predicted results of mean velocities and fluctuating velocities of both phases agree well with the experimental data, and demonstrate that the main characteristics of the flow are accurately captured by the simulations. Characteristics of separation and reattachments as well as essential features of the coherent structure are obtained, in which the processes of vortex roll up, growth, pairing and breaking up are shown in details. Particle dispersions are then investigated through particles’ instantaneous distri- butions in coherent structure as well as the mean and fluctuating properties of particle number density (PND). The predicted mean PND agree well with experiment results. For small particles, the instantaneous distributions show much preferential concentration, while their mean PND shows more uniform distribution in down- stream region. On the contrary, for large particles, their instantaneous distributions are much uniform (without clear preferential concentration) due to less effect of large eddy coherent, while their mean PND across the section is not uniform for more particles are distributed in the main flow region. The preferential concentra- tion of particles by the large-scale eddies can lead to a high fluctuating PND.展开更多
A numerical model has been developed for computing turbulent flow in plane-wall diffuser.The model solves the weakly compressible flow equations with the application of the Smagorinsky's subgrid-scale turbulence m...A numerical model has been developed for computing turbulent flow in plane-wall diffuser.The model solves the weakly compressible flow equations with the application of the Smagorinsky's subgrid-scale turbulence model and the boundary conditions of partial slip and no slip at the solid wall.Application examples include two-dimensional calculations of unstalled and stalled flows in diffusers of small diverging angle,as well as transitory stall flow in a symmetric diffuser of 16°total angle.For the unstalled and stalled flows,the calculated pressure recovery and velocity profile are compared with experimental data.For the transitory stall flow,the com- putational result shows the unsteady flow features including the vortex shedding and stall washout phenomena that have been experimentally observed.展开更多
The present study deals with the study of the velocity distribution and the separation phenomenon of flow of air over a two dimensional backward facing step. The flow of air over a backward facing step has been invest...The present study deals with the study of the velocity distribution and the separation phenomenon of flow of air over a two dimensional backward facing step. The flow of air over a backward facing step has been investigated numerically using FLUENT. Flow simulation has been carried out in a backward facing step having an expansion ratio （ratio of the height before and after the step） of 1：1.94 and the results obtained are compared with the published experimental results. Comparison of flow characteristics between steps with three different transitions is made. The variation ofreattachment length for all the three cases are analyzed for wide range of Reynolds number ranging from 100 to 7000 which covers the laminar, transition and turbulent flow of air. Simulation of the flow over steps with expansion ratios of 1：1.24, 1：1.38, 1：1.47, 1：1.53, 1：1.94, 1：2.20 are also carried out to examine the effect of different expansion ratios on the reattachment length. It is found that the primary reattachment length increases with increase in the expansion ratio. The primary reattachment length at the bottom wall downstream of the step is minimum for the step with round edged transition and maximum for the step with a vertical drop transition.展开更多
文摘The experimental investigation is conducted with LDV and hydrogen bubble technique in water flow. The shear layer thickness. the vorticity thickness. the maximulll value of turbulence intensities. the turbulent coherent structure. the variations of wall shear stress and the boundary layer shape factor are obtained. In the redevelopment region. the detailed analysis is first made for the streak structures in the near wall region and the turbulent boundary layer is formed at (x-xr) / h = 20.
基金support from National Natural Science Foundation of China(Grants No.91130001 and No.11221061)supported by the National Basic Research Program of China(Grant No.2009CB724101)support from National Science Foundation for Postdoctoral Scientists of China(Grant No.2012M520109).
文摘Compressible flow past a circular cylinder at an inflow Reynolds number of 2×105 is numerically investigated by using a constrained large-eddy simulation(CLES)technique.Numerical simulation with adiabatic wall boundary condition and at a free-stream Mach number of 0.75 is conducted to validate and verify the performance of the present CLES method in predicting separated flows.Some typical and characteristic physical quantities,such as the drag coefficient,the root-mean-square lift fluctuations,the Strouhal number,the pressure and skin friction distributions around the cylinder,etc.are calculated and compared with previously reported experimental data,finer-grid large-eddy simulation(LES)data and those obtained in the present LES and detached-eddy simulation(DES)on coarse grids.It turns out that CLES is superior to DES in predicting such separated flow and that CLES can mimic the intricate shock wave dynamics quite well.Then,the effects of Mach number on the flow patterns and parameters such as the pressure,skin friction and drag coefficients,and the cylinder surface temperature are studied,with Mach number varying from 0.1 to 0.95.Nonmonotonic behaviors of the pressure and skin friction distributions are observed with increasing Mach number and the minimum mean separation angle occurs at a subcritical Mach number of between 0.3 and 0.5.Additionally,the wall temperature effects on the thermodynamic and aerodynamic quantities are explored in a series of simulations using isothermal wall boundary conditions at three different wall temperatures.It is found that the flow separates earlier from the cylinder surface with a longer recirculation length in the wake and a higher pressure coefficient at the rear stagnation point for higher wall temperature.Moreover,the influences of different thermal wall boundary conditions on the flow field are gradually magnified from the front stagnation point to the rear stagnation point.It is inferred that the CLES approach in its current version is a useful and effective tool for simulating wall-bounded compressible turbulent flows with massive separations.
基金the National Natural Science Foundation of China (Grant Nos. 19972036, 50172067) the Research Committee of The Hong Kong Polytechnic University (Grant No. A-DP99)
文摘Dilute gas-particle turbulent flows over a backward-facing step are numerically simulated by Large Eddy Simulation (LES) for the continuous phase and Lagran- gian particle trajectory method for the particle phase. Predicted results of mean velocities and fluctuating velocities of both phases agree well with the experimental data, and demonstrate that the main characteristics of the flow are accurately captured by the simulations. Characteristics of separation and reattachments as well as essential features of the coherent structure are obtained, in which the processes of vortex roll up, growth, pairing and breaking up are shown in details. Particle dispersions are then investigated through particles’ instantaneous distri- butions in coherent structure as well as the mean and fluctuating properties of particle number density (PND). The predicted mean PND agree well with experiment results. For small particles, the instantaneous distributions show much preferential concentration, while their mean PND shows more uniform distribution in down- stream region. On the contrary, for large particles, their instantaneous distributions are much uniform (without clear preferential concentration) due to less effect of large eddy coherent, while their mean PND across the section is not uniform for more particles are distributed in the main flow region. The preferential concentra- tion of particles by the large-scale eddies can lead to a high fluctuating PND.
文摘A numerical model has been developed for computing turbulent flow in plane-wall diffuser.The model solves the weakly compressible flow equations with the application of the Smagorinsky's subgrid-scale turbulence model and the boundary conditions of partial slip and no slip at the solid wall.Application examples include two-dimensional calculations of unstalled and stalled flows in diffusers of small diverging angle,as well as transitory stall flow in a symmetric diffuser of 16°total angle.For the unstalled and stalled flows,the calculated pressure recovery and velocity profile are compared with experimental data.For the transitory stall flow,the com- putational result shows the unsteady flow features including the vortex shedding and stall washout phenomena that have been experimentally observed.
文摘The present study deals with the study of the velocity distribution and the separation phenomenon of flow of air over a two dimensional backward facing step. The flow of air over a backward facing step has been investigated numerically using FLUENT. Flow simulation has been carried out in a backward facing step having an expansion ratio （ratio of the height before and after the step） of 1：1.94 and the results obtained are compared with the published experimental results. Comparison of flow characteristics between steps with three different transitions is made. The variation ofreattachment length for all the three cases are analyzed for wide range of Reynolds number ranging from 100 to 7000 which covers the laminar, transition and turbulent flow of air. Simulation of the flow over steps with expansion ratios of 1：1.24, 1：1.38, 1：1.47, 1：1.53, 1：1.94, 1：2.20 are also carried out to examine the effect of different expansion ratios on the reattachment length. It is found that the primary reattachment length increases with increase in the expansion ratio. The primary reattachment length at the bottom wall downstream of the step is minimum for the step with round edged transition and maximum for the step with a vertical drop transition.
