The Disturbance of a High Reynolds Number Turbulent Boundary Layer by Small Forward Steps

Experiments have been performed on the disturbance of a high Reynolds number turbulent boundary layer by three forward steps with sizes close to 3.8%, 15% and 60 % of the boundary layer thickness. Particular attention is focused on the impact of the steps on the fluctuating surface pressure field. Measurements were made from 5 boundary layer thicknesses upstream to 22 boundary layer thicknesses downstream of the step, a distance equivalent to over 600 step heights for the smallest step size. Flow speeds of 30 and 60m/s were studied, corresponding to boundary layer momentum thickness Reynolds numbers of 15500 and 26600 and step size Reynolds numbers from 6640 to 213000. The steps produce a disturbance to the boundary layer pressure spectrum that scales on step size and decays remarkably slowly with distance downstream. The disturbance is still clearly visible at 150 step heights downstream of the mid-size step. Pressure correlations show that organized quasi periodic motions that become visible well downstream of reattachment are responsible. The coherence and scale of these motions, as seen in the wall pressure correlations, are a strong increasing function of step height relative to the boundary layer thickness.