Lift reduction by counter flowing wall jets

Abstract

Counter flowing wall jets are proposed to reduce extreme loads, enabling lighter and more efficient aircraft. Upstream blowing on the upper surface of a NACA0012 airfoil at a Reynolds number of 660,000 was investigated by means of force, pressure and particle image velocimetry measurements, and was found to be more effective than blowing normal to the surface. Blowing locations near the trailing edge are more effective for low angles of attack; locations near the leading edge are more effective for higher angles of attack. At the stall angle of attack, a maximum reduction in the lift coefficient was observed as ΔCL=−0.33 when the blowing location was at xJ/c=0.08 for the largest flow rate coefficient (CQ=0.44%) tested. Even for completely separated flows, upstream blowing near the trailing-edge can modify the recirculation region and cause the shear layer to deflect upwards, resulting in a reduction of lift. This is a significant advantage over mechanical flaps placed near the trailing-edge, which remain in the separated flow and lose their effectiveness.