Reynolds Number Effects on Unsteady Boundary Layer of an Oscillating Airfoil

Abstract

An experimental study was conducted in order to investigate the influence of Reynolds number on an oscillating airfoil. A NACA 0012 airfoil was sinusoidally pitched at quarter chord, and the oscillation amplitude was between ±6°. Surface-mounted probes and smokewire visualization were used for the examination of the boundary layers. The free-stream velocities were 1.98, 2.83, and 4.03 m/s, and the corresponding chord Reynolds numbers were Rec=2.3×10, 3.3×10, and 4.8×10, respectively. In each case, the frequency of the airfoil oscillation was adjusted to fix the reduced frequency of 0.1. It was observed in the presented low Reynolds number range that for a pitching airfoil, the separation bubble is formed within the stall angle as reattachment occurs. Also, unsteady laminar separation was observed at a saddle point, which is composed by the wall, external flow, and two vortices that is independent of reverse flow. It stands for that laminar separation, reattachment, and unsteady laminar separation occur simultaneously over the airfoil, which means that the unsteady laminar separation occurred in the process of transition after the reattachment of separated boundary layer in unsteady flow. The reattachment and the unsteady laminar separation are promoted (delayed) by the increase of Reynolds number in upstroke (downstroke).