PIV Study of Flow Separations at a Forward-Facing Step using Long-Range Microscope

The flow separation taking place over the forward-facing step is measured by high-resolution PIV using long-range microscope. The step has height of h=10 mm and the free stream velocity is 20 m/s, resulting in a Reynolds number of Reh=13,700. Two separation bubbles are produced in the immediate upstream and downstream of the step respectively. The upstream separation has a length of 0.6h, while the downstream one is longer and has length of 1.0h. A shear layer is formed in the region between the reversed flow and free stream. The streamwise velocity fluctuation downstream the step is significantly stronger than that in the upstream separation, the wall-normal velocity, however, exhibits less intensity in fluctuation magnitude, although its strength is higher in the downstream portion. Focused Reynolds shear stress is found to overlap with the velocity shear downstream of the step, suggesting the prominent vertical activity, which is revealed in the instantaneous flow field. It is these vortices that produce the surface pressure fluctuations. Statistical analysis is finally carried out and reveals the correlation between the separation area and the total vorticity magnitude generated in the flow over the step.