Comparison of experiments on bio-inspired hover kinematics with the unsteady vortex model and CFD

In this paper we compare the force and Particle Image Velocimetry (PIV) measurements of biologically inspired hover kinematics to results of an unsteady aerodynamic vortex model (UAVM) and a Navier-Stokes (NS) solver. The baseline Reynolds number and the reduced frequency are 4.8 ×10³ and 0:38, respectively. We consider three versions of the hovering kinematics measured from an Agrius Convolvuli: i) without elevation angle, ii) elevation angle accounted in the pitch angle; and iii) pure sinusoidal pitch-plunge neglecting higher harmonics. The NS computations show good qualitative agreement with experiments with consistent underprediction. The time-averaged thrust coeffcient obtained using NS computations are 15% to 18% of the corresponding force measurements. The standard deviation of time history of thrust coeffcients, also normalized by the measured time-averaged values, is 13% to 20%. The underprediction is possibly due to blockage effects in the experiments or excessive dissipation, also reflected in lower values of the vorticity compared to the PIV measurements. The UAVM 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 UAVM due to which the vorticity in the leading-edge vortex is overpredicted and significant differences in the wing-wake interaction are also noted.