A micro-layer model is proposed to account for the lubrication effect of liquid layer near collisions of immersed particles at moderate particle Reynolds number.This new model is to allow determination of the pressure...A micro-layer model is proposed to account for the lubrication effect of liquid layer near collisions of immersed particles at moderate particle Reynolds number.This new model is to allow determination of the pressure profile within the micro-layer including the fluid inertia and viscosity.Then a correction based on the micro-layer model is applied to unsteady 3-D direct simulation of a particle approaching another one.The simulation is based on a modified immersed boundary method with direct force scheme.The quantitative agreement between numerical and experimental results validates the model presented in the study.The simulation results show that the fluid is squeezed prior to contact.When a particle approaches a flat wall or another particle,the lubrication force,obtained by integrating the pressure profile over the particle surface,is increased and prevents the particle from approaching.The model predicts that the velocity of approaching particle starts to decrease when separation distance of particles is less than 0.1dp,where dp is the particle diameter.展开更多
A 1-dimensional K-model parameterized by the local similarity theory is used to study the structure of the stably stratified boundary layer. The computed results are quite consistent with those of the second-order mom...A 1-dimensional K-model parameterized by the local similarity theory is used to study the structure of the stably stratified boundary layer. The computed results are quite consistent with those of the second-order moments model. Thus, the model can be used to improve the simulated results and the parameterized accuracy.展开更多
A quasi three–dimensional, intermediate planetary boundary layer (PBL) model is developed by including inertial acceleration with the Ekman momentum approximation, but a nonlinear eddy viscosity based on Blackadar’s...A quasi three–dimensional, intermediate planetary boundary layer (PBL) model is developed by including inertial acceleration with the Ekman momentum approximation, but a nonlinear eddy viscosity based on Blackadar’s scheme was included to improve the theoretical model proposed by Tan and Wu (1993). The model could keep the same complexity as the classical Ekman model in numerical, but extends the conventional Ekman model to include the horizontal accelerated flow with the Ekman momentum approximation. A comparison between this modified Ekman model and other simplified accelerating PBL models is made. Results show that the Ekman model overestimates (underestimates) the wind speed and pumping velocity in the cyclonic (anticyclonic) shear flow due to the neglect of the acceleration flow, however, the semi–geostrophic Ekman model overestimates the acceleration effects resulting from the underestimating (overestimating) of the wind speed and pumping velocity in the cyclonic (anticyclonic) shear flow. The Ekman momentum approximation boundary layer model could be applied to the baroclinic atmosphere. The baroclinic Ekman momentum approximation boundary layer solution has both features of classical baroclinic Ekman layer and the Ekman momentum approximate boundary lager.展开更多
An urban boundary layer model （UBLM） is improved by incorporating the effect of buildings with a sectional drag coefficient and a height-distributed canopy drag length scale. The improved UBLM is applied to simulate...An urban boundary layer model （UBLM） is improved by incorporating the effect of buildings with a sectional drag coefficient and a height-distributed canopy drag length scale. The improved UBLM is applied to simulate the wind fields over three typical urban blocks over the Beijing area with different height-towidth ratios. For comparisons, the wind fields over the same blocks are simulated by an urban sub-domain scale model resolving the buildings explicitly. The wind fields simulated from the two different methods are in good agreement. Then, two-dimensional building morphological characteristics and urban canopy parameters for Beijing are derived from detailed building height data. Finally, experiements are conducted to investigate the effect of buildings on the wind field in Beijing using the improved UBLM.展开更多
文摘A micro-layer model is proposed to account for the lubrication effect of liquid layer near collisions of immersed particles at moderate particle Reynolds number.This new model is to allow determination of the pressure profile within the micro-layer including the fluid inertia and viscosity.Then a correction based on the micro-layer model is applied to unsteady 3-D direct simulation of a particle approaching another one.The simulation is based on a modified immersed boundary method with direct force scheme.The quantitative agreement between numerical and experimental results validates the model presented in the study.The simulation results show that the fluid is squeezed prior to contact.When a particle approaches a flat wall or another particle,the lubrication force,obtained by integrating the pressure profile over the particle surface,is increased and prevents the particle from approaching.The model predicts that the velocity of approaching particle starts to decrease when separation distance of particles is less than 0.1dp,where dp is the particle diameter.
文摘A 1-dimensional K-model parameterized by the local similarity theory is used to study the structure of the stably stratified boundary layer. The computed results are quite consistent with those of the second-order moments model. Thus, the model can be used to improve the simulated results and the parameterized accuracy.
基金National Key Basic Research Project: Research on the FormationMechanism and Predication Theory of severe synoptic Disasters in
文摘A quasi three–dimensional, intermediate planetary boundary layer (PBL) model is developed by including inertial acceleration with the Ekman momentum approximation, but a nonlinear eddy viscosity based on Blackadar’s scheme was included to improve the theoretical model proposed by Tan and Wu (1993). The model could keep the same complexity as the classical Ekman model in numerical, but extends the conventional Ekman model to include the horizontal accelerated flow with the Ekman momentum approximation. A comparison between this modified Ekman model and other simplified accelerating PBL models is made. Results show that the Ekman model overestimates (underestimates) the wind speed and pumping velocity in the cyclonic (anticyclonic) shear flow due to the neglect of the acceleration flow, however, the semi–geostrophic Ekman model overestimates the acceleration effects resulting from the underestimating (overestimating) of the wind speed and pumping velocity in the cyclonic (anticyclonic) shear flow. The Ekman momentum approximation boundary layer model could be applied to the baroclinic atmosphere. The baroclinic Ekman momentum approximation boundary layer solution has both features of classical baroclinic Ekman layer and the Ekman momentum approximate boundary lager.
基金funded by National Nat-ural Science Foundation of China(Grants Nos.40505002,40652001,and 40775015)Beijing Natural Science Foun-dation(Grant No.8051002)+1 种基金Beijing New Star Project of Science and Technology(Grant No.2005A03)the Ministry of Science and Technology of China(Grant Nos.2008BAC37B04,2006BAJ02A01,and GYHY200906035)
文摘An urban boundary layer model （UBLM） is improved by incorporating the effect of buildings with a sectional drag coefficient and a height-distributed canopy drag length scale. The improved UBLM is applied to simulate the wind fields over three typical urban blocks over the Beijing area with different height-towidth ratios. For comparisons, the wind fields over the same blocks are simulated by an urban sub-domain scale model resolving the buildings explicitly. The wind fields simulated from the two different methods are in good agreement. Then, two-dimensional building morphological characteristics and urban canopy parameters for Beijing are derived from detailed building height data. Finally, experiements are conducted to investigate the effect of buildings on the wind field in Beijing using the improved UBLM.
