Lift and Thrust Measurements on a Flapping Membrane Foil

Flapping compliant membrane wings demonstrate increased lift and thrust when compared to rigid wings, particularly when flapping close to the natural frequency of the wing. However, if the wing is too compliant, the forces are decreased. Here, we performed experiments to measure the hydrodynamic forces acting on a heaving compliant membrane in a water flume facility, at a Reynolds number of 20,000-60,000. By varying the membrane's elastic modulus, thickness and prestretch, we modify the natural frequency of the membrane. We observe linear and nonlinear behaviors of the forces and membrane deformations with respect to the Strouhal number and natural frequency. For cases with small deformations, we compare the experimental results with a linear, small amplitude potential flow based model. Finally, we discuss the flow structure and wake patterns obtained from Particle Image Velocimetry (2-D PIV) measurements, and their relation to propulsive performance.