Fluid Dynamic Forces on Plunging Spanwise-Flexible Elliptical Flat Plates at Low Reynolds Numbers

We consider the aerodynamic performance of flexible isotropic elliptical wings undergoing periodic plunge motions. Experiments were conducted in the Low Turbulence Water Channel at the University of Michigan using a pitch-plunge apparatus. Experimental results include dye flow visualization, laser Doppler vibrometer (LDV) wing deformation measurements, particle image velocimetry (PIV) flowfield quantification, and direct force measurements establishing a novel experimental framework for investigating pitchingplunging and flapping flexible wings. This investigation focuses on the effect of wing stiffness parameter, Π1, defined as the ratio of elastic to fluid dynamic forces. A parameter sweep is performed that spans four orders of magnitude from order 10 to 10. The Π1-parameter is varied by changing the plate thickness and material properties. The effects of structural density to fluid density, ρ�, and the thickness to chord ratio, h�s, are shown to be small. The wings have elliptical planform with aspect ratio 6.1. Sinusoidal plunging kinematics are used in forward fight at a low Reynolds number (5,300) with a neutral mean effective angle of attack. The plunging motion has large reduced frequency (1.82) and modest chordnormalized plunge amplitude (0.175). Deformation measurements show that for the present conditions the wings bend without twisting. PIV measurements at the 50%and 75%spanwise locations show that large deflections at the wing tip result in a stronger outboard leading edge vortex due to the increased effective angle of attack for increased flexibility. The force measurements proved that the prescribed parametric configuration is not thrust producing due to small wing cross-sectional thickness, motion kinematics, and lack of aerodynamic-twist. The lift (normal) force coefficient for moderate wing stiffness parameter (Π1 of order 10