Abstract
The large-amplitude to-and-fro swinging of the separation point of a flow on a convex wall, which was found in axisymmetric geometry under a certain flow condition, is found also in two-dimensional flow geometry. The swinging of separation point appears itself when the boundary layer upstream is nearly laminar, but the main stream contains considerable disturbances. In the present experiment, the main stream disturbance is given by a thin circular cylinder placed upstream, perpendicular to the flow but parallel with the wall. Through analysis of velocity fluctuations recorded simultaneously at different points, the followings are concluded : The boundary layer on the wall influenced by main stream disturbances undergoes transition to turbulence locally and intermittently. When and only when the turbulent region thus created reaches the convex part of the wall, the separation point is shifted far downstream. The turbulent region is initiated at the location or further downstream, where the wake disturbances of the cylinder starts to touch with the boundary layer. The turbulent region develops itself while convecting downstream. In its initial phase it has a laterally elongated plan-form, which remains virtually unchanged over a considerable distance. When the peak turbulence level inside reaches some threshold, however, its longitudinal dimension increases quickly, and its plan-form soon becomes longitudinally elongated.
