New Reynolds' mean momentum equations including both turbulent viscosity and dispersion are used to analyze atmospheric balance motions of the planetary boundary layer. It is pointed out that turbulent dispersion ...New Reynolds' mean momentum equations including both turbulent viscosity and dispersion are used to analyze atmospheric balance motions of the planetary boundary layer. It is pointed out that turbulent dispersion with r 0 will increase depth of Ekman layer, reduce wind velocity in Ekman layer and produce a more satisfactory Ekman spiral lines fit the observed wind hodograph. The wind profile in the surface layer including tur-bulent dispersion is still logarithmic but the von Karman constant k is replaced by k1 = 1 -2/k, the wind increasesa little more rapidly with height.展开更多
The effect of mechanical properties of erythrocytes on the near-wall motion of platelets was numerically studied with the immersed boundary method. Cells were modeled as viscous-fluid-filled capsules surrounded by hyp...The effect of mechanical properties of erythrocytes on the near-wall motion of platelets was numerically studied with the immersed boundary method. Cells were modeled as viscous-fluid-filled capsules surrounded by hyper-elastic membranes with negligible thickness. The numerical results show that with the increase of hematocrit, the near-wall approaching of platelets is enhanced, with which platelets exhibit larger deformation and orientation angle of its near-wall tank-treading motion, and the lateral force pushing platelets to the wall is increased with larger fluctuation amplitude. Meanwhile the near-wall approaching is reduced by increasing the stiffness of erythrocytes.展开更多
Flying and marine animals often use flapping wings or tails to generate thrust. In this paper, we will use the simplest flapping model with a sinusoidal pitching mo- tion over a range of frequency and amplitude to inv...Flying and marine animals often use flapping wings or tails to generate thrust. In this paper, we will use the simplest flapping model with a sinusoidal pitching mo- tion over a range of frequency and amplitude to investigate the mechanism of thrust generation. Previous work focuses on the Karman vortex street and the reversed Karman vor- tex street but the transition between two states remains un- known. The present numerical simulation provides a com- plete scenario of flow patterns from the Karman vortex street to reversed Karman vortex street via aligned vortices and the ultimate state is the deflected Karman vortex street, as the parameters of flapping motions change. The results are in agreement with the previous experiment. We make further discussion on the relationship of the observed states with drag and thrust coefficients and explore the mechanism of enhanced thrust generation using flapping motions.展开更多
By use of geostrophic momentum approximation,the analytical expressions of the wind distribution within the planetary boundary layer and the vertical velocity at the top of the boundary layer are obtained when the dis...By use of geostrophic momentum approximation,the analytical expressions of the wind distribution within the planetary boundary layer and the vertical velocity at the top of the boundary layer are obtained when the distribution of eddy transfer coefficient k is divided into three sections:k_1z(z_0≤z<h_1),k_2(h_1≤z<h_2), and k_3(h_2≤z).The results are in agreement with the observations.In particular,the wind profile in the surface layer(z_0≤z<h_1)coincides with the logarithmic distribution.The maximum angle between winds near the surface and at the bottom of the free atmosphere is only about 30°.This work improves the work of Wu and Blumen(1982)who introduced the geostrophic momentum approximation to the boundary layer.The solutions in barotropic and neutral conditions have been also extended to the baroclinic and stratified atmosphere.展开更多
A simplified and efficient procedure, based on the viscous-spring artificial boundary and the modal superposition method, is developed to analyze the dynamic soil-structure interaction system in the time domain. The v...A simplified and efficient procedure, based on the viscous-spring artificial boundary and the modal superposition method, is developed to analyze the dynamic soil-structure interaction system in the time domain. The viscous-spring artificial boundary introduced in this procedure transforms the infinite soil-structure interaction system to an approximately finite system. A seismic wave input method is used to transform the wave scattering problem into the wave source problem. The modal superposition method is then applied to this approximate finite system. The results show that this method with only a few modes can significantly reduce the computational time with almost the same precision as the traditional direct integration method. Comparison of results from different loading times demonstrates that the advantages of this method are evident in computing with long loading time.展开更多
文摘New Reynolds' mean momentum equations including both turbulent viscosity and dispersion are used to analyze atmospheric balance motions of the planetary boundary layer. It is pointed out that turbulent dispersion with r 0 will increase depth of Ekman layer, reduce wind velocity in Ekman layer and produce a more satisfactory Ekman spiral lines fit the observed wind hodograph. The wind profile in the surface layer including tur-bulent dispersion is still logarithmic but the von Karman constant k is replaced by k1 = 1 -2/k, the wind increasesa little more rapidly with height.
基金supported by the National Natural Science Foundation of China(11072155 and 11232010)Doctoral Fund of Ministry of Education of China(20100073120009)
文摘The effect of mechanical properties of erythrocytes on the near-wall motion of platelets was numerically studied with the immersed boundary method. Cells were modeled as viscous-fluid-filled capsules surrounded by hyper-elastic membranes with negligible thickness. The numerical results show that with the increase of hematocrit, the near-wall approaching of platelets is enhanced, with which platelets exhibit larger deformation and orientation angle of its near-wall tank-treading motion, and the lateral force pushing platelets to the wall is increased with larger fluctuation amplitude. Meanwhile the near-wall approaching is reduced by increasing the stiffness of erythrocytes.
基金supported by the Natural Science Foundation of Jiangxi Province(2010GZC0162)
文摘Flying and marine animals often use flapping wings or tails to generate thrust. In this paper, we will use the simplest flapping model with a sinusoidal pitching mo- tion over a range of frequency and amplitude to investigate the mechanism of thrust generation. Previous work focuses on the Karman vortex street and the reversed Karman vor- tex street but the transition between two states remains un- known. The present numerical simulation provides a com- plete scenario of flow patterns from the Karman vortex street to reversed Karman vortex street via aligned vortices and the ultimate state is the deflected Karman vortex street, as the parameters of flapping motions change. The results are in agreement with the previous experiment. We make further discussion on the relationship of the observed states with drag and thrust coefficients and explore the mechanism of enhanced thrust generation using flapping motions.
文摘By use of geostrophic momentum approximation,the analytical expressions of the wind distribution within the planetary boundary layer and the vertical velocity at the top of the boundary layer are obtained when the distribution of eddy transfer coefficient k is divided into three sections:k_1z(z_0≤z<h_1),k_2(h_1≤z<h_2), and k_3(h_2≤z).The results are in agreement with the observations.In particular,the wind profile in the surface layer(z_0≤z<h_1)coincides with the logarithmic distribution.The maximum angle between winds near the surface and at the bottom of the free atmosphere is only about 30°.This work improves the work of Wu and Blumen(1982)who introduced the geostrophic momentum approximation to the boundary layer.The solutions in barotropic and neutral conditions have been also extended to the baroclinic and stratified atmosphere.
基金Supported by the National Key Basic Research and Development (973) Program of China (No. 2002CB412706), the National Natu-ral Science Foundation of China (No. 50478014), and the Beijing Natural Science Foundation (No. 8061003)
文摘A simplified and efficient procedure, based on the viscous-spring artificial boundary and the modal superposition method, is developed to analyze the dynamic soil-structure interaction system in the time domain. The viscous-spring artificial boundary introduced in this procedure transforms the infinite soil-structure interaction system to an approximately finite system. A seismic wave input method is used to transform the wave scattering problem into the wave source problem. The modal superposition method is then applied to this approximate finite system. The results show that this method with only a few modes can significantly reduce the computational time with almost the same precision as the traditional direct integration method. Comparison of results from different loading times demonstrates that the advantages of this method are evident in computing with long loading time.