Leading-edge vortex structure of nonslender delta wings at low Reynolds number

The velocity field near the apex region of moderately swept delta wings was measured in a water tunnel, using a version of stereoscopic digital particle imaging velocimetry. Flow visualization was also used to verify these results. In contrast to most recent studies, low angles of attack were emphasized, with most data in the range of 5-20 deg. Delta wings of 50- and 65-deg leading-edge sweep and 30-deg windward-side bevels were tested at Reynolds numbers of 6 x 10(3)-1.5 x 10(4). At these low Reynolds numbers, secondary leading-edge vortices were weak, giving way to essentially stagnant How outboard of the primary leading-edge vortices at the higher angles of attack. Otherwise, velocity data for the 65-deg wing were consistent with well-known observations for slender delta wings. The 50-deg wing exhibited unexpectedly strong primary leading-edge vortices at low angles of attack, with a generally conical velocity field. Upstream progression of vortex breakdown with increasing angle of attack exhibited extensive regions of streamwise undulation. Leading-edge shear-layer rollup was observed in crossflow planes well downstream of the breakdown region, but with an increased occurrence of paired vortical structures of opposite sign inside the shear layer itself.