Near-wake characteristics of rigid and membrane wings in ground effect

Wind tunnel measurements are conducted at a Reynolds numbers of Re=56,000 to examine the characteristics of near-wake vortices of rigid flat-plates and membrane wings from free-flight into ground-effect conditions. Synchronised high-speed load cell measurements, digital image correlation and particle image velocimetry are performed to resolve lift, drag and pitch oscillations simultaneously with membrane deformation and flow dynamics. Flow measurements are acquired in a crossflow-plane, one chord downstream of the trailing-edge, allowing the examination of time evolution of the wake and its relationship to the forces and membrane deformation. Membrane wings are found to delay ground-effect or high angles-of-attack induced tip-vortex break-down and result in larger tip-vortex push-out (beyond the wing span) compared to rigid flat-plate wings. The leading-edge vortex appears to shed with streamwise vorticity close to the root of the wing and this frequency of this shedding is found to match with the dominant frequency observed in membrane fluctuations. In specific resonance conditions, membrane and flow fluctuations are found to correlate well to the fluctuations in loads and moments.