Free-Flight Experiments on Swept Laminar Separation Bubbles

Abstract

Wind tunnel and free-flight experiments were performed on a modified NACA 643-618 airfoil which is known to develop a laminar separation bubble (LSB) on the suction side over a range of angles of attack at low Reynolds numbers. Particle Image Velocimetry (PIV) measurements were taken in the Arizona Low Speed Wind Tunnel (ALSWT) at Re = 200k for a sweep of angles of attack, describing mean separation and transition behavior on an unswept wing section. Time-resolved surface pressure measurements near the region of transition confirmed strong periodicity of vortex shedding with a Strouhal number St = 16 for Re = 200k and St = 20 at Re = 300k. The Kelvin-Helmholtz (K-H) instability showed strong sensitivity to changes in Reynolds number and angle of attack as an increase in either led to mean transition location moving upstream. Free-flight experiments with 35-degree swept wings were conducted between Re = 360-513k. Free-stream turbulence was measured with a constant-voltage anemometer and a 1-D hotwire probe resulting in an average value of Tu = 0.31% throughout the course of a flight. Surface pressure measurements taken in flight demonstrated a shortened bubble compared with wind tunnel experiments at lower Reynolds numbers and separation occurred closer to peak suction. The flight experiments demonstrated the capability of predicting locations of separation, transition, and reattachment using IR thermography with less than 4%-chord error relative to pressure measurements. Despite the introduction of a 35-degree wing sweep angle, the flow features observed in IR images appear fully 2D. To further evaluate the influence of crossflow instability, experiments with low free-stream turbulence are recommended.