Experimental measurements of streamwise and lateral forces acting on bio-inspired pitching panels

The influence of planform and Strouhal number (St) on the propulsive performance of bio-inspired pitching panels is investigated for 0.09≤St≤0.66. Aspect ratio (AR) is varied in the range 3.0≤AR≤6.6 by changing the trailing edge sweep of a trapezoidal planform, resulting in panels of forked, straight, and pointed trailing edges. Positive mean thrust only develops for panels of reduced AR with straight or pointed trailing edges, contrary to expectations from biology. Planform and St have significant impacts on force amplitudes and improved performance is associated with larger force amplitudes. All panels experience positive thrust during significant portions of a cycle, but positive mean thrust only occurs when thrust maxima exceed drag maxima. Strong lateral forces develop for panels with the best mean performance, showing that low AR pointed panels may provide better maneuverability, as well as high thrust and efficiency. Decomposition of the thrust force into its normal and axial forces components in the frame of the panel reveal the relative impact of these components on mean performance. When mean axial forces are small or negative, a thrust-producing panel has relatively large propulsive efficiency and thrust is due primarily to normal forces. Once a thrust-producing panel reaches a local maximum in efficiency, further increases in St result in increases in mean thrust due to larger axial forces. Additionally, thrust-producing panels experience time-varying axial forces that contribute more effectively to thrust in a way not observed in forked planforms.