PIV Measurement of Separation Bubble on an Airfoil at Low Reynolds Numbers

Abstract

A wind tunnel experiment was conducted to measure the flow over a suction side of a DAE51 airfoil at low Reynolds numbers using particle image velocimetry (PIV). Five chord Reynolds numbers ranging from 39,000 to 118,000 were investigated at angles of attack ranging from 0° to 10° at an interval of 1°. The locations of laminar separation, transition onset, transition end, and reattachment were identified by examining the measured averaged and RMS velocity data. The locations determined from the PIV data were in very good agreement with the features of measured surface pressure distribution. A large amplification of the RMS velocity leading to transition of a separated shear layer was observed centered at the inflection point of the averaged velocity profile, which indicates the important role of inflectional instability on the transition onset. The behaviors of those locations with respect to the Reynolds number and angle of attack were analyzed. For a fixed Reynolds number, the size of the separation bubble decreases quickly, and the bubble moves upstream as the angle of attack increases. The transition onset location moves upstream with increasing Reynolds number and angle of attack. The transition length becomes shorter as the Reynolds number increases. The lowest angle of attack at which a separation bubble is formed on the airfoil surface depends significantly on the Reynolds number. The movement of the separation point with respect to the Reynolds number indicates a different tendency depending on the presence of the separation bubble.