Boundary-Layer Transition by Roughness Element

Abstract

Measurements were made on the boundary layer along a flat plate in a two-dimensional low turbulence wind tunnel. A fine rod was attached to the plate as a two-dimensional roughness element, and the resulting transition from laminar to turbulent flow was investigated by hot-wire equipment.
The transition was found to differ in marked contrast according as the height of roughness element k is small or large as compared with the thickness of boundary layer δ_k at the element. When k⁄δ_k is small, the boundary-layer flow separates at the element, but reattaches to the plate at some distance producing scarcely any disturbance. Transition occurs in the reattached boundary layer. A sinusoidal velocity fluctuation, characteristic of the laminar-boundary-layer oscillation, is observed at a distance downstream from the roughness element. When k⁄δ_k is large, transition occurs in the separated layer before it reattaches to the plate. Within the separated layer, a sinusoidal fluctuation is observed having the frequency pertinent to the separated layer from a sharp corner.
For a given roughness element at a given position on the plate, the transition moves forward and the transition Reynolds number continues to decrease as the free-stream velocity is increased. At a certain value of k⁄δ_k the transition reaches the element, and the transition Reynolds number begins to increase as the free-stream velocity is further increased. The minimum of the transition Reynolds number was found to correspond approximately to the demarcation of the two types of transition mentioned above.