Low Reynolds Number Laminar Airfoil with Active Flow Control

Abstract

Wind tunnel tests were performed on a NACA 643-618 airfoil at Re ≈ 64,000 using particle image velocimetry, hot film anemometry, surface-oil flow visualization, infrared imaging and surface pressure measurements via pressure taps as the tools of this investigation. Preliminary testing determined that four distinct flow regimes exist with respect to angle of attack. Four angles of attack were tested which encompass weak laminar separation, strong laminar separation, laminar separation bubble, and turbulent separation. Laminar separation occurs because the pressure gradient at moderate angles of attack (-3-8 degrees) becomes too severe for the low momentum flow to follow the airfoil suction surface and thus separates. However, as the angle of attack is further increased to around 10 degrees, a significant increase in lift is realized due to the formation of a laminar separation bubble near the leading edge which acts as a natural boundary layer trip to stabilize the flow. Flow control is employed to act similarly to the laminar separation bubble such that a similar lift increase can be realized over a broader range of angle of attack. Normal blowing, suction and zigzag tape are used which are all well-characterized devices and have the potential to produce the same result as a laminar separation bubble. Lift is increased significantly and separation is delayed in all regimes, except for the 10-14 degree regime where the laminar separation bubble was already effective at reducing separation. It is observed that the optimal flow control device changes between regimes.