Mechanisum of the Separation of the Unsteady Boundary Layer on the Moving Cylinder in a Uniform Flow

对均匀流动中的振荡流动、静止流动以及在静水中脉动启动的圆柱体表面附近的流动进行了详细的可视化，以了解分离点附近边界层的特征(位移厚度、形状因子、边界层外缘速度和壁面摩擦)的变化。主要研究结果如下：(1)当雷诺数为10^3时，无论是静止、振荡还是脉冲式启动，都没有观察到分离点处边界层厚度突然增加等奇异现象。(2)当振荡圆柱向上死点运动时，面向下死点的表面边界层与静止圆柱的边界层特征基本相同，而面向上死点的边界层与静止圆柱的边界层特征有很大的不同。(3)在静止水中脉冲启动的圆柱绕流中，雷诺数Re约为10^4时，第一涡在圆柱表面附近形成，不仅产生二次涡，还会产生三次涡。随着Re的增大，第一个涡诱导二次涡或三次涡的时间间隔变短，当Re&gt；&gt；10^4时，该时间间隔近似为零，第一个涡出现的位置和时间被认为与最近理论研究计算的边界层方程奇点出现的位置和时间一致。

The flows near the surface of the cylinder which is oscillating or stationary in a uniform flow, or which is started impulsively in a still water were visualized in detail to make clear the changes of the characteristics (displacement thickness, shape factor, velocity at the outer edge of the boundary layer, and skin friction) of the boundary layer near the separation point. The main results are summarized as follows.(1) When the Reynolds number is of the order of 10^3 ,no singular behavior of the boundary layer, such as abrupt increase of the thickness of the boundary layer at the separation point is observed at any cases when the cylinder is stationary, oscillating, or starting impulsively.(2) When the oscillating cylinder moves toward the upper dead point, the boundary layer on the surface facing lower dead point has almost the same characteristics as the ones in the case of stationary cylinder and the ones facing the upper dead point has considerably different characteristics from the case of the stationary cylinder. A new boundary layer is developed in the ordinary boundary layer and the point of vanishing skin friction does not coincide with the separation point where the fluid particles on the surface of the cylinder are shed covectively.(3) In the flows around the cylinder which is started impulsively in a still water, when the Reynolds number, Re becomes of the order of 10^4, the first vortex is formed extremely near the surface of the cylinder and induces not only the secondary vortices but also tertiary vortices . As Re is larger, the time intervals to be induced the secondary or tertiary vortices by the first vortex become shorter, and when Re>>10^4, the time interval is inferred to become approximately zero and the location and time of the appearance of the first vortex is considered to be coincide with that of the appearance of the singular point of the boundary layer equations which is calculated by the recent theoretical studies.