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

均匀流运动圆柱上非定常边界层的分离机理

• 批准号: 62550126
• 负责人: NAGATA Hiroshi
• 金额: $ 1.22万
• 依托单位: Gifu University
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for General Scientific Research (C)
• 财政年份: 1987
• 资助国家: 日本
• 起止时间: 1987 至 1988
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-62550126/
• 关键词: Flow around a Circular Cylinder; Unsteady Boundary Layer; Separation; Separation Vortex; Singular Point of the Boundary Layer Equations; Flow Visualization; 表面せん断応力

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.

通过对均匀流中振荡、静止和静水中脉冲起动的圆柱表面附近流动的详细观察，明确了分离点附近边界层特性（位移厚度、形状因子、边界层外缘速度和表面摩擦力）的变化。主要研究结果如下。(1)当雷诺数为10^3量级时，在圆柱静止、振荡或冲动起动的任何情况下，都没有观察到边界层的奇异行为，如分离点处边界层厚度的突然增加。(2)当振动圆柱体向上止点运动时，下止点侧表面的边界层与静止圆柱体的边界层具有几乎相同的特性，而上止点侧表面的边界层与静止圆柱体的边界层具有明显不同的特性。在正常边界层中发展了一个新的边界层，表面摩擦力消失点与圆柱表面流体粒子对流脱落的分离点不重合。(3)在静水中以脉冲方式起动的圆柱绕流中，当雷诺数Re达到10^4量级时，第一涡在极靠近圆柱表面处形成，不仅诱导出二次涡，而且诱导出三次涡。随着Re的增大，第一涡诱导二次或三次涡的时间间隔变短，当Re>>10^4时，第一涡诱导二次或三次涡的时间间隔近似为零，第一涡出现的位置和时间与最近理论研究计算的边界层方程奇点出现的时间和位置一致。