Comparison of Fundamental Flapping Kinematics of an Airfoil

Abstract

In this paper, basic flapping mechanisms of a symmetric airfoil is studied. The question that is addressed here is, if the fundamental flapping mechanisms are indeed equivalent as is commonly assumed in engineering practices, especially at high flapping amplitudes and frequencies. In this regard, pure plunging oscillation and its kinematically equivalent pitching are compared. Though kinematically equivalent, earlier experimental studies report that the wake pattern obtained from both look very different. Further, different computational models based on grid based solvers failed to match experimental observations. These discrepancies are addressed in the present study using a Lagrangian vortex particle based solver. We also simulate these cases using commercial software Fluent for comparison with the Lagrangian solver and the earlier results. Lagrangian Navier-Stokes solver has been able to capture the wake deflection in pure pitch, also found in experimental observations but not captured by the grid based CFD models. The mode of wake deflection can be altered by altering the starting conditions. This has been demonstrated with different mean angles of attack. The wake deflection increases with the total angle of attack, that is mean and the amplitude together. At zero mean angle of attack there is no deflection, indicating qualitative similarity between pure heave and pure pitch. For non-zero mean angle of attack wake deflection is observed which switches with the change in starting conditions. This observation is also similar to that of a plunging airfoil, pointing to qualitative similarity between both the fundamental kinematics.