Separation behaviour in front of a two-dimensional fence

Abstract

The wall-bounded turbulent shear flow in front of a two-dimensional fence was investigated experimentally. In this prototype of a rapidly separating flow the assumptions for a first-order boundary-layer theory cease to apply. This is caused by the streamline curvature, the ensuing pressure gradient normal to the wall and the large vertical velocity component v in front of the fence. For the present experiment, where the ratio of the boundary layer thickness δ 0 measured without the fence and the fence height h is 0.82, the time mean separation length upstream of the fence l f is 0.65 h. However, instantaneous reverse flow events can be detected up to 4 mean separation lengths l f upstream of the fence. The maximum value of the reverse flow factor χ w is 95% indicating a strong reverse flow region. The experiments were performed by LDA and a wall pulsed-wire skin-friction meter. They show the limits of first-order boundary-layer theory and provide the first comprehensive data set of mean and fluctuating velocities and of wall shear-stress for this type of separating flow.