The effect of streamwise vortices generated from passive vortex generators was investigated to manipulate the separation bubble behind the vertical fence. The experiments were carried out in a circulating water channe...The effect of streamwise vortices generated from passive vortex generators was investigated to manipulate the separation bubble behind the vertical fence. The experiments were carried out in a circulating water channel and the velocity fields were measured using 2D and stereoscopic PIV method.The distance between the vortex generator and fence and the effect of the Reynolds number were investigated. In addition,the effect of boundary layer thickness was also investigated. The averaged recirculation lengths were compared with that of uncontrolled fence flow. The results showed the oscillatory variation of recirculation region appeared under the existence of vortex generators. The reduction of the separation bubble became larger when the fence was submerged in the thick boundary layer with increasing the distance between the generator and fence. When the boundary layer is thin,vortex generator can only suppress the separation bubble under the specific condition.展开更多
A new type of propeller that is optimized for low Reynolds numbers is required to propel a small object in a medium where the flow is dominated by viscous rather than inertial forces. A propeller in the shape of a bac...A new type of propeller that is optimized for low Reynolds numbers is required to propel a small object in a medium where the flow is dominated by viscous rather than inertial forces. A propeller in the shape of a bacterial flagellum seems an appro- priate choice for driving a small object. Accordingly, in this study, we visualized the velocity field induced by a spring-like propeller inspired by the Escherichia coli flagellum, using a macroscopic model and applying stereoscopic particle image velocimetry. We also experimentally evaluated the effect of pitch and rotational speed on the performance of this flagellar propeller. Silicone oil, which has a kinematic viscosity 100,000 times that of water, was used as the working fluid to generate a low Reynolds number for the macroscopic model. Thrust, torque, and velocity were measured as functions of pitch and rota- tional speed, and the efficiency of the propeller was calculated from the measured results. We found that the flagellar propeller reached a max!mum efficiency when the pitch angle was approximately 53°. Compared to pitch, rotational speed had a relatively small effect on the efficiency, and the pitch altered the flow pattern behind the rotating propeller.展开更多
基金supported from the Priority Research Centers Program(2012-048078)Basic Science Research Program(2012-008918)+1 种基金through the National Research Foundation(NRF)a grant(08GASPLANTC04) from the program of KAIA funded by the Ministry of Land,Infrastructure and Transport of the Republic of Korea
文摘The effect of streamwise vortices generated from passive vortex generators was investigated to manipulate the separation bubble behind the vertical fence. The experiments were carried out in a circulating water channel and the velocity fields were measured using 2D and stereoscopic PIV method.The distance between the vortex generator and fence and the effect of the Reynolds number were investigated. In addition,the effect of boundary layer thickness was also investigated. The averaged recirculation lengths were compared with that of uncontrolled fence flow. The results showed the oscillatory variation of recirculation region appeared under the existence of vortex generators. The reduction of the separation bubble became larger when the fence was submerged in the thick boundary layer with increasing the distance between the generator and fence. When the boundary layer is thin,vortex generator can only suppress the separation bubble under the specific condition.
文摘A new type of propeller that is optimized for low Reynolds numbers is required to propel a small object in a medium where the flow is dominated by viscous rather than inertial forces. A propeller in the shape of a bacterial flagellum seems an appro- priate choice for driving a small object. Accordingly, in this study, we visualized the velocity field induced by a spring-like propeller inspired by the Escherichia coli flagellum, using a macroscopic model and applying stereoscopic particle image velocimetry. We also experimentally evaluated the effect of pitch and rotational speed on the performance of this flagellar propeller. Silicone oil, which has a kinematic viscosity 100,000 times that of water, was used as the working fluid to generate a low Reynolds number for the macroscopic model. Thrust, torque, and velocity were measured as functions of pitch and rota- tional speed, and the efficiency of the propeller was calculated from the measured results. We found that the flagellar propeller reached a max!mum efficiency when the pitch angle was approximately 53°. Compared to pitch, rotational speed had a relatively small effect on the efficiency, and the pitch altered the flow pattern behind the rotating propeller.
