Growth of Turbulent Spots in Plane Couette Flow.

Abstract

In plane Couette flow, the structure and growth of turbulent spots are investigated by means of flow visualization. The turbulent spot is artificially produced from a point disturbance. Under the minimum critical Reynolds number, the produced spot has the coherent structure of longitudinal vortices, which is relaminarized upon moving downstream. At greater than the minimum critical Reynolds number, the same structure of the longitudinal vortices remains in the downstream and upstream regions of the spots. In the central region of the spots, the longitudinal vortices break down to small-scale turbulence, which constitutes a turbulent region. Therefore the turbulent spots have a weak turbulent region of longitudinal vortices and a central turbulent region at greater than the minimum critical Reynolds number. For the incipient spots, the turbulent region has an elliptic leading edge. At low Reynolds number, the leading edge is partially relaminarized upon moving downstream, and the reproducibility of the shape is lost. At high Reynolds number, the turbulent region shows a wedge-shaped leading edge upon moving downstream. The turbulent region has constant propagation velocity for the trailing edge. The leading edge of the turbulent region reduces the velocity after the turbulent region expands to the lateral width of 13 H and the streamwise length of 20 H. This reduction decreases the streamwise growth velocity of the spots. The spreading angle of the spots is nearly constant along the streamwise direction, but increases with Reynolds number.