The cavitation incipience and development of water flow over a thin hydrofoil placed in the test section of high-speed cavitation tunnel were investigated.Hydrofoils with smooth and rough leading edge were tested for ...The cavitation incipience and development of water flow over a thin hydrofoil placed in the test section of high-speed cavitation tunnel were investigated.Hydrofoils with smooth and rough leading edge were tested for different upstream velocities and incidence angles.The observations clearly revealed that cavitation incipience is enhanced by roughness at incidence angle below 2°.This is in line with the former reports,according to whose roughness element decreases the wettability and traps a larger amount of gas.As a result,surface nucleation is enhanced with an increased risk of cavitation.Surprisingly,for higher incidence angles(>3°),it was found that cavitation incipience is significantly delayed by roughness while developed cavitation is almost the same for both smooth and rough hydrofoils.This unexpected incipience delay is related to the change in the boundary layer structure due to roughness.It was also reported a significant influence of roughness on the dynamic of developed cavitation and shedding of transient cavities.展开更多
Nowadays,most researchers focus on the cavity shedding mechanisms of unsteady cavitating flows over different objects,such as 2D/3D hydrofoils,venturi-type section,axisymmetric bodies with different headforms,and so o...Nowadays,most researchers focus on the cavity shedding mechanisms of unsteady cavitating flows over different objects,such as 2D/3D hydrofoils,venturi-type section,axisymmetric bodies with different headforms,and so on.But few of them pay attention to the differences of cavity shedding modality under different cavitation numbers in unsteady cavitating flows over the same object.In the present study,two kinds of shedding patterns are investigated experimentally.A high speed camera system is used to observe the cavitating flows over an axisymmetric blunt body and the velocity fields are measured by a particle image velocimetry（PIV）technique in a water tunnel for different cavitation conditions.The U-type cavitating vortex shedding is observed in unsteady cavitating flows.When the cavitation number is 0.7,there is a large scale cavity rolling up and shedding,which cause the instability and dramatic fluctuation of the flows,while at cavitation number of 0.6,the detached cavities can be conjunct with the attached part to induce the break-off behavior again at the tail of the attached cavity,as a result,the final shedding is in the form of small scale cavity and keeps a relatively steady flow field.It is also found that the interaction between the re-entrant flow and the attached cavity plays an important role in the unsteady cavity shedding modality.When the attached cavity scale is insufficient to overcome the re-entrant flow,it deserves the large cavity rolling up and shedding just as that at cavitation number of 0.7.Otherwise,the re-entrant flow is defeated by large enough cavity to induce the cavity-combined process and small scale cavity vortexes shedding just as that of the cavitation number of0.6.This research shows the details of two different cavity shedding modalities which is worthful and meaningful for the further study of unsteady cavitation.展开更多
研究高速射弹小角度入水过程中的空化现象和弹体运动规律.计算采用VOF多相流模型捕捉空泡界面,采用Schnerr and Sauer空化模型模拟空化现象,弹体运动通过6自由度方程与重叠网格技术进行求解.计算分析了弹体小角度入水过程中的弹道特性...研究高速射弹小角度入水过程中的空化现象和弹体运动规律.计算采用VOF多相流模型捕捉空泡界面,采用Schnerr and Sauer空化模型模拟空化现象,弹体运动通过6自由度方程与重叠网格技术进行求解.计算分析了弹体小角度入水过程中的弹道特性、空泡的演化规律及射弹的流体动力特性,初步探索了高速射弹的小角度入水时产生的弹跳现象.研究表明:此高速运动的旋转射弹在小角度入水时,弹体轨迹和姿态容易发生很大的变化,空泡形态不对称,弹体大面积沾湿导致弹体受到很大的流体动力和力矩,弹体运动失稳产生弹跳翻转现象.分析表明,弹体小角度入水容易沾湿,沾湿对流体动力及其运动姿态有着极大的影响,弹体的不对称沾湿是超空泡射弹失稳的重要原因.展开更多
基金National Natural Science Foundation of China(51139007)National “Twelfth Five-Year” Plan for Science&Technology Support(2015BAD20B01)China Scholarship Council(201506350088)
文摘The cavitation incipience and development of water flow over a thin hydrofoil placed in the test section of high-speed cavitation tunnel were investigated.Hydrofoils with smooth and rough leading edge were tested for different upstream velocities and incidence angles.The observations clearly revealed that cavitation incipience is enhanced by roughness at incidence angle below 2°.This is in line with the former reports,according to whose roughness element decreases the wettability and traps a larger amount of gas.As a result,surface nucleation is enhanced with an increased risk of cavitation.Surprisingly,for higher incidence angles(>3°),it was found that cavitation incipience is significantly delayed by roughness while developed cavitation is almost the same for both smooth and rough hydrofoils.This unexpected incipience delay is related to the change in the boundary layer structure due to roughness.It was also reported a significant influence of roughness on the dynamic of developed cavitation and shedding of transient cavities.
基金Supported by National Natural Science Foundation of China(Grant Nos.51209004,11172040)Beijing Municipal Natural Science Foundation of China(Grant No.3144043)
文摘Nowadays,most researchers focus on the cavity shedding mechanisms of unsteady cavitating flows over different objects,such as 2D/3D hydrofoils,venturi-type section,axisymmetric bodies with different headforms,and so on.But few of them pay attention to the differences of cavity shedding modality under different cavitation numbers in unsteady cavitating flows over the same object.In the present study,two kinds of shedding patterns are investigated experimentally.A high speed camera system is used to observe the cavitating flows over an axisymmetric blunt body and the velocity fields are measured by a particle image velocimetry（PIV）technique in a water tunnel for different cavitation conditions.The U-type cavitating vortex shedding is observed in unsteady cavitating flows.When the cavitation number is 0.7,there is a large scale cavity rolling up and shedding,which cause the instability and dramatic fluctuation of the flows,while at cavitation number of 0.6,the detached cavities can be conjunct with the attached part to induce the break-off behavior again at the tail of the attached cavity,as a result,the final shedding is in the form of small scale cavity and keeps a relatively steady flow field.It is also found that the interaction between the re-entrant flow and the attached cavity plays an important role in the unsteady cavity shedding modality.When the attached cavity scale is insufficient to overcome the re-entrant flow,it deserves the large cavity rolling up and shedding just as that at cavitation number of 0.7.Otherwise,the re-entrant flow is defeated by large enough cavity to induce the cavity-combined process and small scale cavity vortexes shedding just as that of the cavitation number of0.6.This research shows the details of two different cavity shedding modalities which is worthful and meaningful for the further study of unsteady cavitation.
文摘研究高速射弹小角度入水过程中的空化现象和弹体运动规律.计算采用VOF多相流模型捕捉空泡界面,采用Schnerr and Sauer空化模型模拟空化现象,弹体运动通过6自由度方程与重叠网格技术进行求解.计算分析了弹体小角度入水过程中的弹道特性、空泡的演化规律及射弹的流体动力特性,初步探索了高速射弹的小角度入水时产生的弹跳现象.研究表明:此高速运动的旋转射弹在小角度入水时,弹体轨迹和姿态容易发生很大的变化,空泡形态不对称,弹体大面积沾湿导致弹体受到很大的流体动力和力矩,弹体运动失稳产生弹跳翻转现象.分析表明,弹体小角度入水容易沾湿,沾湿对流体动力及其运动姿态有着极大的影响,弹体的不对称沾湿是超空泡射弹失稳的重要原因.
