Effects of Axial Slots on Airfoils - First Results

Most two-dimensional airfoils typically increase lift and delay stall by use of trailing edge flaps or leading edge slats, or a combination of both. These devices actively open slots along the span of the airfoil allowing flow from the pressure side to the suction side which energizes the boundary layer and prevents or delays separation. In this study we investigate an alternate passive approach which uses axial slots beginning at the sharp trailing edge and extending forward through the airfoil, while repeating along the span. Such slots allow flow from the pressure side to the suction side as angle of attack increases, with each slot producing a pair of counter-rotating vortices on the suction side, thus affecting overall flow separation. Results are obtained by use of STAR-CCM+ computations on a NACA 0012 airfoil at Reynolds numbers of 1.8 million. The code first reproduces existing experimental results (NACA, 1955) on unslotted airfoils from 0 degrees angle of attack (rounded leading edge, sharp trailing edge) to 180 degrees (sharp leading edge, rounded trailing edge), showing the two stall behaviors seen experimentally, then produces new results on axially slotted airfoils. We report the changes to the lift and drag over the same range of angle of attack, the flow separation characteristics, and the effects of varying the length, width and spacing of the axial slots.