Wake vorticity measurements for low aspect ratio wings at low Reynolds number

Trailing vortex structure of low aspect ratio wings was studied in a water tunnel at Reynolds numbers of 8000 and 24,000 using dye injection and digital particle image velocimetry in cross-flow planes in the near wake, for rectangular, semi-elliptical, and delta-wing planforms. The velocity data were used to calculate lift via circulation and effective span, and the results were compared with force balance measurements and classical inviscid theory. The objectives of the study were to assess how low-Reynolds number effects might affect the measurement of lift coefficient from tip-vortex circulation, how well the measurements fit the various theoretical models of lift curve slope for low aspect ratio wings, and the extent to which planform shape affects lift coefficient while aspect ratio and planform area are kept constant. All models were thin flat plates with square edges. For the conditions studied, apparent viscous effects on vorticity integrations in the wake were small, resulting in good agreement between lift coefficient values inferred from the measurements, the force balance data, and the classical inviscid formulas. Whereas the three wings exhibited distinctly different wake structures, lift curve slopes were similar.