Unsteady fluid–structure interactions of a pitching membrane wing

Experiments were conducted on rectangular wings with an aspect ratio of two, at a chord Reynolds number Re c = 46 , 000 , in order to understand how membrane wings behave in a prescribed transient motion. As a reference, high-speed Digital Image Correlation (DIC) and Particle Image Velocimetry (PIV) measurements were performed on stationary membrane wings in the range 0 ° < α < 25 ° . Four distinct incidence regions were identified based on the mode shape, frequency and amplitude of the surface vibrations. Flow-field measurements indicated that the different regions were caused by the interactions of the leading-edge separated shear layer with the membrane. Then the membrane wing was subjected to transient sinusoidal pitching manoeuvres at starting incidences in each of these regions. The membrane deformation characteristics were seen to vary considerably with starting angle; both in terms of time-averaged and instantaneous quantities. In some cases there was significant time-lag in the response of the membrane. The PIV flow field measurements for this case showed signs of hysteresis between the pitch-up and pitch-down parts of the wing motion, which was not the case for a rigid wing with identical planform.