Effect of instability on boundary layer detachment

Abstract

When fluid or gas flows around a body, a thin boundary layers forms near its surface. The behavior of this boundary layer is determined by hydrodynamic resistance. If the boundary layer is detached from the surface, the resistance increases sharply [i, 2]. In order to reduce this resistance, the detachment must be stretched out; that is the boundary layer line detachment must be shifted as far as possible to the aft critical point, so that the region of stagnant flow (the wake) behind the body is narrowed. In this regard, investigations of nonstationary fluid around a body are of current interest. The acceleration of a cylindrical body into a quiescent fluid has been examined [2]. Undetached flow around the body was observed immediately after the acceleration started. Then, after the cylinder traveled a distance s = 0.351 R (where R is the cylinder radius), the flow detached near the aft critical point of the body. The detachment gradually moved forward along the flow and increased the wake behind the body. After a certain time, a pair of vortices appeared behind the body, which grew and continually broke off to form a vortical wake. As measurements show [3], the hydrodynamic resistance coefficient is minimized in the case of undetached flow. This leads to the importance of investigating nonstationary flow around bodies.