Using Finite-Time Lyapunov Exponents (FTLE) method, Lagrangian coherent structures (LCSs) in a fully developed flat-plate turbulent boundary layer are successfully identified from a two-dimensional (2D) velocity field...Using Finite-Time Lyapunov Exponents (FTLE) method, Lagrangian coherent structures (LCSs) in a fully developed flat-plate turbulent boundary layer are successfully identified from a two-dimensional (2D) velocity field obtained by time-resolved 2D PIV measurement. The typical LCSs in the turbulent boundary layer are hairpin-like structures, which are characterized as legs of quasi-streamwise vor- tices extending deep into the near wall region with an inclination angle θ to the wall, and heads of the transverse vortex tube located in the outer region. Statistical analysis on the characteristic shape of typical LCS reveals that the probability density distribution of θ accords well with t-distribution in the near wall region, but presents a bimodal distribution with two peaks in the outer region, corresponding to the hairpin head and the hairpin neck, respectively. Spatial correlation analysis of FTLE field is im- plemented to get the ensemble-averaged inclination angle θ R of typical LCS. θ R first increases and then decreases along the wall-normal direction, similar to that of the mean value of θ. Moreover, the most probable value of θ saturates at y+=100 with the maximum value of about 24°, suggesting that the most likely position where hairpins transit from the neck to the head is located around y+=100. The ensem- ble-averaged convection velocity Uc of typical LCS is finally calculated from temporal-spatial correla- tion analysis of FTLE field. It is found that the wall-normal profile of the convection velocity Uc(y) ac- cords well with the local mean velocity profile U(y) beyond the buffer layer, evidencing that the down- stream convection of hairpins determines the transportation properties of the turbulent boundary layer in the log-region and beyond.展开更多
Basing on the commercial CFD metho d ,several models are adopted to simulate water model. The numerical simulation results are in good agreement with the experimental data of water model under the same experimental co...Basing on the commercial CFD metho d ,several models are adopted to simulate water model. The numerical simulation results are in good agreement with the experimental data of water model under the same experimental conditions. Then the numerical simulation method is presented to investigate the flow field, the vortex on gas-liquid surface and the uniform mixing time in hot metal ladles. This research shows: the vortex depth increases with increasing of immersion depth and rotation speed; Overflow appears in N=120rpm and h=3.2m; the turbulent kinetic energy increases with increasing immersion depth and rotation speed; uniform mixing time decreases with the decreasing of immersion depth and the rotation speed of stirring ; the optimal uniform mixing time is 23.5s on the conditions of N=100rpm and h=3.2m within the allowable range.展开更多
In the present paper, the unsteady, viscous, incompressible and 2-D flow around two side-by-side circular cylinders was simulated using a Cartesian-staggered grid finite volume based method. A great-source term techni...In the present paper, the unsteady, viscous, incompressible and 2-D flow around two side-by-side circular cylinders was simulated using a Cartesian-staggered grid finite volume based method. A great-source term technique was employed to identify the solid bodies （cylinders） located in the flow field and boundary conditions were enforced by applying the ghost-cell technique. Finally, the characteristics of the flow around two side-by-side cylinders were comprehensively obtained through several computational simulations. The computational simulations were performed for different transverse gap ratios （1.5≤T/D≤4） in laminar （Re=100,200） and turbulent （Re=104） regimes, where T and D are the distance between the centers of cylinders and the diameter of cylinders, respectively. The Reynolds number is based on the diameter of cylinders,D. The pressure field and vorticity distributions along with the associated streamlines and the time histories of hydrodynamic forces were also calculated and analyzed for different gap ratios. Generally, different flow patterns were observed as the gap ratio and Reynolds number varied. Accordingly, the hydrodynamic forces showed irregular variations for small gaps while they took a regular pattern at higher spacing ratios.展开更多
The unsteady turbulent cloud cavitation around a NACA66 hydrofoil was simulated using the filter-based density corrected model(FBDCM). The cloud cavitation was treated as a homogeneous liquid-vapor mixture and the eff...The unsteady turbulent cloud cavitation around a NACA66 hydrofoil was simulated using the filter-based density corrected model(FBDCM). The cloud cavitation was treated as a homogeneous liquid-vapor mixture and the effects of turbulent eddy viscosity were reduced in cavitation regions near the hydrofoil and in the wake. The numerical results(in terms of the vapor shedding structure and transient pressure pulsation due to cavitation evolution) agree well with the available experimental data, showing the validity of the FBDCM method. Furthermore, the interaction of vortex and cavitation was analyzed based on the vorticity transport equation, revealing that the cavitation evolution has a strong connection with vortex dynamics. A detailed analysis shows that the cavitation could promote the vortex production and flow unsteadiness by the dilatation and baroclinic torque terms in the vorticity transport equation.展开更多
基金Supported by the National Natural Science Foundation of China (Grant Nos. 10425207 and 10832001)
文摘Using Finite-Time Lyapunov Exponents (FTLE) method, Lagrangian coherent structures (LCSs) in a fully developed flat-plate turbulent boundary layer are successfully identified from a two-dimensional (2D) velocity field obtained by time-resolved 2D PIV measurement. The typical LCSs in the turbulent boundary layer are hairpin-like structures, which are characterized as legs of quasi-streamwise vor- tices extending deep into the near wall region with an inclination angle θ to the wall, and heads of the transverse vortex tube located in the outer region. Statistical analysis on the characteristic shape of typical LCS reveals that the probability density distribution of θ accords well with t-distribution in the near wall region, but presents a bimodal distribution with two peaks in the outer region, corresponding to the hairpin head and the hairpin neck, respectively. Spatial correlation analysis of FTLE field is im- plemented to get the ensemble-averaged inclination angle θ R of typical LCS. θ R first increases and then decreases along the wall-normal direction, similar to that of the mean value of θ. Moreover, the most probable value of θ saturates at y+=100 with the maximum value of about 24°, suggesting that the most likely position where hairpins transit from the neck to the head is located around y+=100. The ensem- ble-averaged convection velocity Uc of typical LCS is finally calculated from temporal-spatial correla- tion analysis of FTLE field. It is found that the wall-normal profile of the convection velocity Uc(y) ac- cords well with the local mean velocity profile U(y) beyond the buffer layer, evidencing that the down- stream convection of hairpins determines the transportation properties of the turbulent boundary layer in the log-region and beyond.
基金supported by the National Natural Science Foundation of China(No.50974035)National Natural Science Foundation of China(No.51074047)the Doctoral Fund of EDU Gov(20090407)
文摘Basing on the commercial CFD metho d ,several models are adopted to simulate water model. The numerical simulation results are in good agreement with the experimental data of water model under the same experimental conditions. Then the numerical simulation method is presented to investigate the flow field, the vortex on gas-liquid surface and the uniform mixing time in hot metal ladles. This research shows: the vortex depth increases with increasing of immersion depth and rotation speed; Overflow appears in N=120rpm and h=3.2m; the turbulent kinetic energy increases with increasing immersion depth and rotation speed; uniform mixing time decreases with the decreasing of immersion depth and the rotation speed of stirring ; the optimal uniform mixing time is 23.5s on the conditions of N=100rpm and h=3.2m within the allowable range.
文摘In the present paper, the unsteady, viscous, incompressible and 2-D flow around two side-by-side circular cylinders was simulated using a Cartesian-staggered grid finite volume based method. A great-source term technique was employed to identify the solid bodies （cylinders） located in the flow field and boundary conditions were enforced by applying the ghost-cell technique. Finally, the characteristics of the flow around two side-by-side cylinders were comprehensively obtained through several computational simulations. The computational simulations were performed for different transverse gap ratios （1.5≤T/D≤4） in laminar （Re=100,200） and turbulent （Re=104） regimes, where T and D are the distance between the centers of cylinders and the diameter of cylinders, respectively. The Reynolds number is based on the diameter of cylinders,D. The pressure field and vorticity distributions along with the associated streamlines and the time histories of hydrodynamic forces were also calculated and analyzed for different gap ratios. Generally, different flow patterns were observed as the gap ratio and Reynolds number varied. Accordingly, the hydrodynamic forces showed irregular variations for small gaps while they took a regular pattern at higher spacing ratios.
基金supported by the National Natural Science Foundation of China(Grant Nos.51206087,51306018,51179091 and 51376100)the National Basic Research Program of China("973"Project)(Grant No.2011BAF03B01)the Open Research Fund Program of State key Laboratory of Hydroscience and Engineering(Grant No.2014-KY-05 and 2015-E-03)
文摘The unsteady turbulent cloud cavitation around a NACA66 hydrofoil was simulated using the filter-based density corrected model(FBDCM). The cloud cavitation was treated as a homogeneous liquid-vapor mixture and the effects of turbulent eddy viscosity were reduced in cavitation regions near the hydrofoil and in the wake. The numerical results(in terms of the vapor shedding structure and transient pressure pulsation due to cavitation evolution) agree well with the available experimental data, showing the validity of the FBDCM method. Furthermore, the interaction of vortex and cavitation was analyzed based on the vorticity transport equation, revealing that the cavitation evolution has a strong connection with vortex dynamics. A detailed analysis shows that the cavitation could promote the vortex production and flow unsteadiness by the dilatation and baroclinic torque terms in the vorticity transport equation.
