Experimental and Computational Study on Flapping Wings with Bio-Inspired Hover Kinematics

Abstract

In this paper, force and particle-image-velocimetry vorticity measurements of biologically inspired hover kinematics are compared to corresponding results of an unsteady aerodynamic vortex model and a Navier–Stokes (NS) solver. The Reynolds number and the reduced frequency are and 0.38, respectively. Three kinematics derived from the measured hovering kinematics of an Agrius convolvuli are considered: 1) without elevation angle, 2) elevation angle accounted in the pitch angle, and 3) pure sinusoidal pitch–plunge neglecting higher harmonics. The Navier–Stokes computations show good qualitative agreement with experiments with consistent underprediction. The time-averaged thrust coefficients obtained using Navier–Stokes computations are 82 to 87% of the corresponding force measurements. The standard deviation of time history of thrust coefficients, also normalized by the measured time-averaged values, is 13 to 20%. The underprediction is possibly due to blockage effects in the experiments, also reflected in lower values of the vorticity compared to particle-image-velocimetry measurements. The unsteady aerodynamic vortex model captures some of the peaks in a qualitative manner. The relative difference in the time-averaged forces and standard deviation are 8 to 18% and 66 to 93%, respectively. The differences in prediction of time histories are not reflected in the estimation of time-averaged forces due to cancellation effects, wherein the forces are underpredicted in the first half of the stroke and overpredicted in the second half. The discrepancies are attributed to the simplifying assumptions in the unsteady aerodynamic vortex model, which overpredicts the vorticity in the leading-edge vortex and results in significant differences in the wing–wake interaction process.