Low aspect-ratio membrane wing undergoing transient pitching motion at a low Reynolds number

High-speed Digital Image Correlation (DIC) and Particle Image Velocimetry (PIV) measurements were performed on a rectangular, perimeter-reinforced membrane wing with no pre-tension or excess length, with an aspect-ratio of two and at a chord Reynolds number of 46,000. The wing was subjected to a transient sinusoidal pitching manoeuvre in order to understand how the wing might react to an unsteady gust. Three values for the reduced frequency of the gust were tested: k=0.022, k=0.044 and k=0.110. The motion had a pitch amplitude A=10° and was initiated from four starting incidence angles: αs=0°, αs=5°, αs=10° and αs=15°. The membrane deformation characteristics were seen to vary considerably with starting angle; both in terms of time-averaged and instantaneous quantities. The αs=10° case demonstrated significant time-lag in the membrane behaviour, with transient effects extending thirty-seven convective timescales beyond the end of the motion. The PIV flow field measurements for this case showed signs of hysteresis between the pitch-up and pitchdown parts of the wing motion, which was not the case for a rigid wing with identical planform. The fluid-structure interaction was insensitive to changes in the reduced frequency for the range of values tested.