细长椭球体在水中自由下落的运动特性实验研究

EXPERIMENTAL STUDY ON THE KINEMATICS OF FREELY FALLING PROLATE SPHERE IN THE WATER

固体颗粒在液体中的运动现象在日常生活和工业应用领域广泛存在,其中因蕴含着丰富的流体力学现象而受到学者们的广泛关注.本文通过实验研究了细长椭球体在水中受浮力影响的下落特性.实验中采用带有两台相互垂直的高速摄像机和光源组成的运动跟踪平台并结合荧光染色技术对细长椭球体下落过程中的运动轨迹和尾涡结构进行研究.文中选用的细长椭球体与环境流体的密度比为1.2,其长短轴比范围为2~10,相应的阿基米德数范围为400~1400,对应实现的终态雷诺数范围为120~1350.实验过程中我们观察到细长椭球体在水中下落过程中产生的5种典型路径,分别为:小振幅不规则运动、小振幅高频振荡运动、大振幅低频振荡运动、高度非线性运动以及直线运动,并得到了对应的速度振荡以及倾斜角的演化规律.进一步地,分析了细长椭球体运动过程中受到的阻力系数与雷诺数之间的关系.随后采用荧光可视化技术清晰获得了颗粒下落过程中的尾涡结构特性,并结合颗粒的运动状态详细分析了涡脱落过程对颗粒运动状态转捩的影响.最后,通过对比前人关于圆柱体下落的运动特性的相关结果,获得了细长椭球体和细长圆柱体运动特性之间的异同点以及其潜在的物理机理.

The motion of solid particles in the liquid is frequently encountered in everyday life and engineering applications. It attracts the devotement of many researchers due to the abundant fluid dynamic phenomena behind the motion of solid particles in the liquid. In the present paper, we experimentally study the falling characteristics of a single prolate sphere particle descending in water under the influence of buoyancy force. The motion tracking platform which consists of two orthogonal high-speed cameras and light sources accompanied by the fluorescence visualization technology is adopted to obtain the paths and vortex structures of the falling prolate sphere particle. The density ratio between the selected prolate sphere and surrounding fluid is 1.2, while the aspect ratio of the selected prolate sphere varies from 2 to 10, the corresponding Archimedes number changes from 400 to 1400,and finally,the terminal Reynolds number is limited in the range from 120 to 1350.During the experiments,we observe five typical types of paths during the falling process of the prolate spheres,which corresponds to small amplitude irregular motion,small-amplitude high-frequency oscillation motion,large-amplitude low-frequency oscillation motion,highly nonlinear motion and rectilinear motion,respectively.And the evolution of the oscillation of velocity and the inclination angle is obtained.Furthermore,we analyze the relationships between the drag coefficient of the freely falling prolate sphere and the Reynolds number during the falling process.Then,by using the fluorescence visualization technique,we identify the different vortex shedding modes,which are responsible for different falling trajectories,and study the influence of vortex shedding on the path of free falling prolate sphere.Finally,comparing with some previous results on the falling characteristics of the slender cylinder,we find the similarities and differences of the motion characteristics between the prolate sphere and the slender cylinder and the potential physical mechanism.