The present study focuses on an experimental investigation of flows undergoing periodic unsteady separation. The experimental setup consists of a flat plate and an oscillating airfoil. The oscillating airfoil induces periodically the unsteady separation of the boundary layer over the flat plate. A split film sensor is used to measure the velocity field. Streamlines obtained from the measurement data at each phase angle of oscillation clearly demonstrate flow patterns signifying generation, growth, and convection of the separation bubble. A saddle point of the unsteady separation is identified. It is also found that the vortical structure of the separation bubble is convected downstream at a speed of 28% of average free stream velocity.
In order to support the experimental finding, a numerical computaion of the same flow as in the experiment has been carried out by using a STAR-CD code. Computational results confirm the generation, growth, and convection of the separation vortex. It is also found that the saddle point, which is considered to be the separation point for the flow with upstream or downstream moving walls, exists in front of the separation vortex. Path lines are constructed from the computational results. Path lines clearly indicate the ejection of particles associated with the unsteady separation as can be observed in the experiment.