MEASUREMENTS IN A SEPARATION BUBBLE ON AN AIRFOIL USING LASER VELOCIMETRY

An experimental investigation was conducted to measure the reverse flow within the transitional separation bubble that forms on an airfoil at low Reynolds numbers. Measurements were used to determine the effect of the reverse flow on integrated boundary-layer parameters often used to model the bubble. Velocity profile data were obtained on an NACA 663-018 airfoil at angle of attack of 12 deg and a chord Reynolds number of 1.4 × 105 using laser Doppler and single-sensor hot-wire anemometry. Trends in displacement, momentum, and energy thickness growth as well as shape-factor variation with chordwise position were unaffected by reverse flow; however, significant errors in the magnitudes of the thicknesses and shape factors were realized by not accounting for local flow direction. The dividing streamline correlation proposed by Schmidt was evaluated and modified. A new correlation was proposed based on zero velocity position since the Schmidt correlations fail in the turbulent portion of the bubble. A two-parameter profile model gave better agreement than the Stewartson reverse-flow solutions with measured profiles. The poor agreement of the Stewartson profiles was attributed to the assumption of a power law external velocity distribution used to derive the Falkner-Skan equation. © 1990 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.