Airfoil boundary layer measurements at low Re in an accelerating flow from a nonzero velocity

The effects of an accelerating freestream from a nonzero velocity on the transitional separation bubble characteristics were investigated quantitatively. Hot wire anemometry was used to determine the boundary layer velocity profile repsonse to the acceleration at selected chordwise locations on a Wortmann FX 63-137 airfoil at 7 ° angle of attack. Both positive and negative accelerations were studied from base chord Reynolds number of 100,000 and 150,000, respectively. The purpose of this experiment was to verify the trends witnessed in previous research concerning a sinusoidally oscillating freestream velocity by uncoupling the accelerating and decelerating boundary layer effects. The experimental results indicate that as a result of a freestream acceleration, the separation bubble position shifts in the direction opposite to the chordwise direction it would move for a quasi-steady velocity change. The transition location was more responsive to the acceleration than was the separation position. This supports the oscillating freestream experiment conclusions.This research was supported by the U.S. Navy Office of Naval Research under contract N00014-83-K-0239