Xingsi Han - Academia.edu (original) (raw)

Papers by Xingsi Han

Journal of Fluid Mechanics, 2013

The flow over a flat plate at a Reynolds number of 750 is numerically investigated via fine large... more The flow over a flat plate at a Reynolds number of 750 is numerically investigated via fine large-eddy simulation (LES), first at normal (90 • ) and then at oblique (45 • ) incidence flow direction with a uniform steady inlet. The results are in complete agreement with the direct numerical simulation (DNS) and experimental data, thereby serving as a validation for the present simulations. For the normal (90 • ) uniform inflow case, coherent vortices are alternately shed from both leading edges of the plate, whereas for the oblique (45 • ) uniform inflow case the vortices shed from the two sides of the plate interact strongly resulting in a quasi-periodic force response. The normal flat plate is then analysed with an incident gust signal with varying amplitude and time period. For these incident coherent gust cases, a reference test case with variable coherent inlet is first studied and the results are compared to a steady inlet simulation, with a detailed analysis of the flow behaviour and the wake response under the incident gust. Finally, the flat plate response to 16 different gust profiles is studied. A transient drag reconstruction for these incident coherent gust cases is then presented based on a frequency-dependent transfer function and phase spectrum analysis.

Among various hybrid RANS/LES methodologies, Speziale's Very Large Eddy Simulation (VLES) is one ... more Among various hybrid RANS/LES methodologies, Speziale's Very Large Eddy Simulation (VLES) is one that was early proposed and is a unified simulation approach that can change seamlessly from RANS to DNS depending on the numerical resolution. The present study proposes a new improved variant of the original VLES model. The advantages are achieved in two ways: (1) RANS simulation can be recovered near the wall which is similar to the Detached Eddy Simulation (DES) concept; (2) An LES subgrid scale model can be reached by the introduction of a third length scale, i.e. integral turbulence length scale. Thus the new model can provide a proper LES mode between the RANS and DNS limits. This new methodology is implemented in the standard k − ε model and Wilcox's k − ω model. Applications are conducted for the turbulent channel flow at Re τ = 395 and turbulent flow past a square cylinder at Re = 22000. Results are compared with previous studies. It is demonstrated that the new method is quite effective in resolving the large flow structures, and can give satisfactory predictions on a very coarse mesh.

Proceeding of THMT-12. Proceedings of the Seventh International Symposium On Turbulence, Heat and Mass Transfer Palermo, Italy, 24-27 September, 2012, 2012

Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2012

Among various hybrid RANS/LES methodologies, Speziale's Very Large Eddy Simulation (VLES) is one ... more Among various hybrid RANS/LES methodologies, Speziale's Very Large Eddy Simulation (VLES) is one that was early proposed and is a unified simulation approach that can change seamlessly from RANS to DNS depending on the numerical resolution. The present study proposes a new improved variant of the original VLES model. The advantages are achieved in two ways: (1) RANS simulation can be recovered near the wall which is similar to the Detached Eddy Simulation (DES) concept; (2) An LES subgrid scale model can be reached by the introduction of a third length scale, i.e. integral turbulence length scale. Thus the new model can provide a proper LES mode between the RANS and DNS limits. This new methodology is implemented in the standard k − ε model and Wilcox's k − ω model. Applications are conducted for the turbulent channel flow at Re τ = 395 and turbulent flow past a square cylinder at Re = 22000. Results are compared with previous studies. It is demonstrated that the new method is quite effective in resolving the large flow structures, and can give satisfactory predictions on a very coarse mesh.

Journal of Fluids Engineering, 2013

The numerical study reported here deals with the passive flow control around a two-dimensional D-... more The numerical study reported here deals with the passive flow control around a two-dimensional D-shaped bluff body at a Reynolds number of Re ¼ 3:6 Â 10 4 . A small circular control cylinder located in the near wake behind the main bluff body is employed as a local disturbance of the shear layer and the wake. 3D simulations are carried out using a newly developed very large eddy simulation (VLES) method, based on the standard k À e turbulence model. The aim of this study is to validate the performance of this method for the complex flow control problem. Numerical results are compared with available experimental data, including global flow parameters and velocity profiles. Good agreements are observed. Numerical results suggest that the bubble recirculation length is increased by about 36% by the local disturbance of the small cylinder, which compares well to the experimental observations in which the length is increased by about 38%. A drag reduction of about 18% is observed in the VLES simulation, which is quite close to the experimental value of 17.5%. It is found that the VLES method is able to predict the flow control problem quite well.

International Journal of Heat and Fluid Flow, 2013

Flow control has shown a potential in reducing the drag in vehicle aerodynamics. The present nume... more Flow control has shown a potential in reducing the drag in vehicle aerodynamics. The present numerical study deals with active flow control for a quasi-2D simplified vehicle model using a synthetic jet (zero net mass flux jet). Recently developed near-wall Partially-Averaged Navier-Stokes (PANS) method, based on the f-f RANS turbulence model, is used. The aim is to validate the performance of this new method for the complex flow control problem. Results are compared with previous studies using LES and experiments, including global flow parameters of Strouhal number, drag coefficients and velocity profiles. The PANS method predicts a drag reduction of approximately 15%, which is closer to the experimental data than the previous LES results. The velocity profiles predicted by the PANS method agree well with LES results and experimental data for both natural and controlled cases. The PANS prediction showed that the nearwake region is locked-on due to the synthetic jet, and the shear layer instabilities are thus depressed which resulted in an elongated wake region and reduced drag. It demonstrates that the PANS method is able to predict the flow control problem well and is thus appropriate for flow control studies.

International Journal for Numerical Methods in Fluids, 2013

Among the various hybrid methodologies, Speziale's very large eddy simulation (VLES) is one that ... more Among the various hybrid methodologies, Speziale's very large eddy simulation (VLES) is one that was proposed very early. It is a unified simulation approach that can change seamlessly from Reynolds Averaged Navier-Stokes (RANS) to direct numerical simulation (DNS) depending on the numerical resolution. The present study proposes a new improved variant of the original VLES model. The advantages are achieved in two ways: (i) RANS simulation can be recovered near the wall which is similar to the detached eddy simulation concept; (ii) a LES subgrid scale model can be reached by the introduction of a third length scale, that is, the integral turbulence length scale. Thus, the new model can provide a proper LES mode between the RANS and DNS limits. This new methodology is implemented in the standard k " model. Applications are conducted for the turbulent channel flow at Reynolds number of Re D 395, periodic hill flow at Re D 10, 595, and turbulent flow past a square cylinder at Re D 22, 000. In comparison with the available experimental data, DNS or LES, the new VLES model produces better predictions than the original VLES model. Furthermore, it is demonstrated that the new method is quite efficient in resolving the large flow structures and can give satisfactory predictions on a coarse mesh.

International Journal for Numerical Methods in Fluids, 2013

The paper describes the validation of a newly developed very LES (VLES) method for the simulation... more The paper describes the validation of a newly developed very LES (VLES) method for the simulation of turbulent separated flow. The new VLES method is a unified simulation approach that can change seamlessly from Reynolds-averaged Navier-Stokes to DNS depending on the numerical resolution. Four complex test cases are selected to validate the performance of the new method, that is, the flow past a square cylinder at Re D 3000 confined in a channel (with a blockage ratio of 20%), the turbulent flow over a circular cylinder at Re D 3900 as well as Re D 140, 000, and a turbulent backward-facing step flow with a thick incoming boundary layer at Re D 40, 000. The simulation results are compared with available experimental, LES, and detached eddy simulation-type results. The new VLES model performs well overall, and the predictions are satisfactory compared with previous experimental and numerical results. It is observed that the new VLES method is quite efficient for the turbulent flow simulations; that is, good predictions can be obtained using a quite coarse mesh compared with the previous LES method. Discussions of the implementation of the present VLES modeling are also conducted on the basis of the simulations of turbulent channel flow up to high Reynolds number of Re D 4000. The efficiency of the present VLES modeling is also observed in the channel flow simulation. From a practical point of view, this new method has considerable potential for more complex turbulent flow simulations at relative high Reynolds numbers.

Flow, Turbulence and Combustion, 2014

The concept of Momentum Injection (MI) through Moving Surface Boundary layer Control (MSBC) appli... more The concept of Momentum Injection (MI) through Moving Surface Boundary layer Control (MSBC) applied to a cubic structure is numerically studied using Large Eddy Simulation at a Reynolds number of 6.7 × 10 4 . Two small rotating cylinders are used to add the momentum at the front vertical edges of the cube. Two configurations are studied with the yaw angle of 0 • and 30 • , respectively, with ratio of the rotation velocity of cylinders and the freestream velocity of 2. The results suggest that MI delays the boundary layer separation and reattachment, and thus reduces the drag. A drag reduction of about 6.2 % is observed in the 0 • yaw angle case and about 44.1 % reduction in the 30 • yaw angle case. In the case of 0 • yaw angle, the main change of the flow field is the disappearance of the separation regions near the rotating cylinders and the wake region is slightly changed due to MI. In the 30 • yaw angle case, the flow field is changed a lot. Large flow separations near one rotating cylinder and in the wake is significantly reduced, which results in the large drag reduction. Meanwhile, the yaw moment is increased about 50.5 %.

International Journal of Computational Fluid Dynamics, 2012

The present study aims at analysing the sensitivity of two-dimensional flow past a flat plate to ... more The present study aims at analysing the sensitivity of two-dimensional flow past a flat plate to uncertain inflow conditions in the laminar flow regime. Both the Reynolds number and angle of incidence are treated as random inflow variables. The methodology consists of a stochastic collocation method based on generalised polynomial chaos (gPC) theory coupled with standard deterministic numerical simulations. With respect to the two random inputs, sensitivity analysis of global integral parameters such as Strouhal number, drag and lift coefficients and the time-averaged flow fields is performed, resulting in the construction of their response surfaces. The stochastic response of the full spectrum of the drag coefficient is also obtained. It is noticed that integral parameters are sensitive to the two random parameters. There is a peak in the probability density function (PDF) of mean drag coefficient. Two additional high frequencies are predicted in the spectrum of drag coefficients. They are about two and four times the primary vortex shedding frequency respectively, corresponding to first and second harmonics of the primary frequency. For the flow fields, the analysis demonstrates that the most probable solutions are significantly different from the deterministic ones and the solution sensitivity is localised near the regions transitioned to large scale fluid structure movements. Non-linear coupling between the two uncertainties is also studied thanks to the Sobol's decomposition. The angle of incidence is found to be the most influential variable to the mean flow fields.

Journal of Fluid Mechanics, 2013

The flow over a flat plate at a Reynolds number of 750 is numerically investigated via fine large... more The flow over a flat plate at a Reynolds number of 750 is numerically investigated via fine large-eddy simulation (LES), first at normal (90 • ) and then at oblique (45 • ) incidence flow direction with a uniform steady inlet. The results are in complete agreement with the direct numerical simulation (DNS) and experimental data, thereby serving as a validation for the present simulations. For the normal (90 • ) uniform inflow case, coherent vortices are alternately shed from both leading edges of the plate, whereas for the oblique (45 • ) uniform inflow case the vortices shed from the two sides of the plate interact strongly resulting in a quasi-periodic force response. The normal flat plate is then analysed with an incident gust signal with varying amplitude and time period. For these incident coherent gust cases, a reference test case with variable coherent inlet is first studied and the results are compared to a steady inlet simulation, with a detailed analysis of the flow behaviour and the wake response under the incident gust. Finally, the flat plate response to 16 different gust profiles is studied. A transient drag reconstruction for these incident coherent gust cases is then presented based on a frequency-dependent transfer function and phase spectrum analysis.

Among various hybrid RANS/LES methodologies, Speziale's Very Large Eddy Simulation (VLES) is one ... more Among various hybrid RANS/LES methodologies, Speziale's Very Large Eddy Simulation (VLES) is one that was early proposed and is a unified simulation approach that can change seamlessly from RANS to DNS depending on the numerical resolution. The present study proposes a new improved variant of the original VLES model. The advantages are achieved in two ways: (1) RANS simulation can be recovered near the wall which is similar to the Detached Eddy Simulation (DES) concept; (2) An LES subgrid scale model can be reached by the introduction of a third length scale, i.e. integral turbulence length scale. Thus the new model can provide a proper LES mode between the RANS and DNS limits. This new methodology is implemented in the standard k − ε model and Wilcox's k − ω model. Applications are conducted for the turbulent channel flow at Re τ = 395 and turbulent flow past a square cylinder at Re = 22000. Results are compared with previous studies. It is demonstrated that the new method is quite effective in resolving the large flow structures, and can give satisfactory predictions on a very coarse mesh.

Proceeding of THMT-12. Proceedings of the Seventh International Symposium On Turbulence, Heat and Mass Transfer Palermo, Italy, 24-27 September, 2012, 2012

Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2012

Among various hybrid RANS/LES methodologies, Speziale's Very Large Eddy Simulation (VLES) is one ... more Among various hybrid RANS/LES methodologies, Speziale's Very Large Eddy Simulation (VLES) is one that was early proposed and is a unified simulation approach that can change seamlessly from RANS to DNS depending on the numerical resolution. The present study proposes a new improved variant of the original VLES model. The advantages are achieved in two ways: (1) RANS simulation can be recovered near the wall which is similar to the Detached Eddy Simulation (DES) concept; (2) An LES subgrid scale model can be reached by the introduction of a third length scale, i.e. integral turbulence length scale. Thus the new model can provide a proper LES mode between the RANS and DNS limits. This new methodology is implemented in the standard k − ε model and Wilcox's k − ω model. Applications are conducted for the turbulent channel flow at Re τ = 395 and turbulent flow past a square cylinder at Re = 22000. Results are compared with previous studies. It is demonstrated that the new method is quite effective in resolving the large flow structures, and can give satisfactory predictions on a very coarse mesh.

Journal of Fluids Engineering, 2013

The numerical study reported here deals with the passive flow control around a two-dimensional D-... more The numerical study reported here deals with the passive flow control around a two-dimensional D-shaped bluff body at a Reynolds number of Re ¼ 3:6 Â 10 4 . A small circular control cylinder located in the near wake behind the main bluff body is employed as a local disturbance of the shear layer and the wake. 3D simulations are carried out using a newly developed very large eddy simulation (VLES) method, based on the standard k À e turbulence model. The aim of this study is to validate the performance of this method for the complex flow control problem. Numerical results are compared with available experimental data, including global flow parameters and velocity profiles. Good agreements are observed. Numerical results suggest that the bubble recirculation length is increased by about 36% by the local disturbance of the small cylinder, which compares well to the experimental observations in which the length is increased by about 38%. A drag reduction of about 18% is observed in the VLES simulation, which is quite close to the experimental value of 17.5%. It is found that the VLES method is able to predict the flow control problem quite well.

International Journal of Heat and Fluid Flow, 2013

Flow control has shown a potential in reducing the drag in vehicle aerodynamics. The present nume... more Flow control has shown a potential in reducing the drag in vehicle aerodynamics. The present numerical study deals with active flow control for a quasi-2D simplified vehicle model using a synthetic jet (zero net mass flux jet). Recently developed near-wall Partially-Averaged Navier-Stokes (PANS) method, based on the f-f RANS turbulence model, is used. The aim is to validate the performance of this new method for the complex flow control problem. Results are compared with previous studies using LES and experiments, including global flow parameters of Strouhal number, drag coefficients and velocity profiles. The PANS method predicts a drag reduction of approximately 15%, which is closer to the experimental data than the previous LES results. The velocity profiles predicted by the PANS method agree well with LES results and experimental data for both natural and controlled cases. The PANS prediction showed that the nearwake region is locked-on due to the synthetic jet, and the shear layer instabilities are thus depressed which resulted in an elongated wake region and reduced drag. It demonstrates that the PANS method is able to predict the flow control problem well and is thus appropriate for flow control studies.

International Journal for Numerical Methods in Fluids, 2013

Among the various hybrid methodologies, Speziale's very large eddy simulation (VLES) is one that ... more Among the various hybrid methodologies, Speziale's very large eddy simulation (VLES) is one that was proposed very early. It is a unified simulation approach that can change seamlessly from Reynolds Averaged Navier-Stokes (RANS) to direct numerical simulation (DNS) depending on the numerical resolution. The present study proposes a new improved variant of the original VLES model. The advantages are achieved in two ways: (i) RANS simulation can be recovered near the wall which is similar to the detached eddy simulation concept; (ii) a LES subgrid scale model can be reached by the introduction of a third length scale, that is, the integral turbulence length scale. Thus, the new model can provide a proper LES mode between the RANS and DNS limits. This new methodology is implemented in the standard k " model. Applications are conducted for the turbulent channel flow at Reynolds number of Re D 395, periodic hill flow at Re D 10, 595, and turbulent flow past a square cylinder at Re D 22, 000. In comparison with the available experimental data, DNS or LES, the new VLES model produces better predictions than the original VLES model. Furthermore, it is demonstrated that the new method is quite efficient in resolving the large flow structures and can give satisfactory predictions on a coarse mesh.

International Journal for Numerical Methods in Fluids, 2013

The paper describes the validation of a newly developed very LES (VLES) method for the simulation... more The paper describes the validation of a newly developed very LES (VLES) method for the simulation of turbulent separated flow. The new VLES method is a unified simulation approach that can change seamlessly from Reynolds-averaged Navier-Stokes to DNS depending on the numerical resolution. Four complex test cases are selected to validate the performance of the new method, that is, the flow past a square cylinder at Re D 3000 confined in a channel (with a blockage ratio of 20%), the turbulent flow over a circular cylinder at Re D 3900 as well as Re D 140, 000, and a turbulent backward-facing step flow with a thick incoming boundary layer at Re D 40, 000. The simulation results are compared with available experimental, LES, and detached eddy simulation-type results. The new VLES model performs well overall, and the predictions are satisfactory compared with previous experimental and numerical results. It is observed that the new VLES method is quite efficient for the turbulent flow simulations; that is, good predictions can be obtained using a quite coarse mesh compared with the previous LES method. Discussions of the implementation of the present VLES modeling are also conducted on the basis of the simulations of turbulent channel flow up to high Reynolds number of Re D 4000. The efficiency of the present VLES modeling is also observed in the channel flow simulation. From a practical point of view, this new method has considerable potential for more complex turbulent flow simulations at relative high Reynolds numbers.

Flow, Turbulence and Combustion, 2014

The concept of Momentum Injection (MI) through Moving Surface Boundary layer Control (MSBC) appli... more The concept of Momentum Injection (MI) through Moving Surface Boundary layer Control (MSBC) applied to a cubic structure is numerically studied using Large Eddy Simulation at a Reynolds number of 6.7 × 10 4 . Two small rotating cylinders are used to add the momentum at the front vertical edges of the cube. Two configurations are studied with the yaw angle of 0 • and 30 • , respectively, with ratio of the rotation velocity of cylinders and the freestream velocity of 2. The results suggest that MI delays the boundary layer separation and reattachment, and thus reduces the drag. A drag reduction of about 6.2 % is observed in the 0 • yaw angle case and about 44.1 % reduction in the 30 • yaw angle case. In the case of 0 • yaw angle, the main change of the flow field is the disappearance of the separation regions near the rotating cylinders and the wake region is slightly changed due to MI. In the 30 • yaw angle case, the flow field is changed a lot. Large flow separations near one rotating cylinder and in the wake is significantly reduced, which results in the large drag reduction. Meanwhile, the yaw moment is increased about 50.5 %.

International Journal of Computational Fluid Dynamics, 2012

The present study aims at analysing the sensitivity of two-dimensional flow past a flat plate to ... more The present study aims at analysing the sensitivity of two-dimensional flow past a flat plate to uncertain inflow conditions in the laminar flow regime. Both the Reynolds number and angle of incidence are treated as random inflow variables. The methodology consists of a stochastic collocation method based on generalised polynomial chaos (gPC) theory coupled with standard deterministic numerical simulations. With respect to the two random inputs, sensitivity analysis of global integral parameters such as Strouhal number, drag and lift coefficients and the time-averaged flow fields is performed, resulting in the construction of their response surfaces. The stochastic response of the full spectrum of the drag coefficient is also obtained. It is noticed that integral parameters are sensitive to the two random parameters. There is a peak in the probability density function (PDF) of mean drag coefficient. Two additional high frequencies are predicted in the spectrum of drag coefficients. They are about two and four times the primary vortex shedding frequency respectively, corresponding to first and second harmonics of the primary frequency. For the flow fields, the analysis demonstrates that the most probable solutions are significantly different from the deterministic ones and the solution sensitivity is localised near the regions transitioned to large scale fluid structure movements. Non-linear coupling between the two uncertainties is also studied thanks to the Sobol's decomposition. The angle of incidence is found to be the most influential variable to the mean flow fields.