Abstract
The purpose of this study is to investigate the influence of periodic flow on the laminar boundary layer of a pitching NACA 0012 airfoil at a low Reynolds number. The periodic flow is the freestream oscillating sinusoidally in the streamwise direction. The velocity field is measured by particle image velocimetry (PIV) under the conditions of a Reynolds number of 3700 and a reduced frequency of 2.01. In the processing, we introduce an overlapping technique and a rectangular interrogation window to evaluate the boundary layer profile with high spatial resolution. However, the velocity on the airfoil surface cannot be obtained because of the angle of view, so a numerical simulation is performed under the same conditions to obtain the velocity data on the surface. The flow reversal related to dynamic stall is evaluated with the wall shear stress calculated by numerical simulation, which is confirmed to agree with the boundary layer profile obtained by PIV. The periodic flow case is compared with the steady case to determine the effect of freestream oscillation on the flow reversal. It is found that the reversed flow vanishes during freestream acceleration, whereas the reversed flow region increases during freestream deceleration.
