UNSTEADY VORTICAL FLOW AROUND 3-DIMENSIONAL LIFTING SURFACES

Flow visualization experiments were conducted in a low Reynolds number towing tank to study the time-dependent flow around two generic classes of wings. Delta and swept (including zero sweep) wings were sting mounted to a four-bar mechanism, which generated a large-amplitude, harmonic pitching motion around the one-quarter chord location at a reduced frequency that varied in the range of 0. 2-03. 0. Fluorescent dye layers were placed in the weakly stratified water channel prior to towing the wing. The horizontal dye sheets were excited using a vertical sheet of laser light parallel to or perpendicular to the flow direction. The dye marked the flow in the separation region around the wing, the flow in the wake region, and the potential flow away from the lifting surface. The complex, time-dependent flowfield around the different wings can be explained primarily from the mutual induction between the leading-edge separation vortex and the trailing-edge shedding vortex. The reduced frequency, the shape of the leading edge, and the wing's planform play important roles in determining the flow patterns. The effects of the finite aspect ratio on the flow around the wing are explored.