Asymmetry in Wake of Oscillating Foils with Combined Pitching and Heaving Motion

Abstract

This is a numerical study of the dynamics of primary and secondary vortex structures in high performance wakes of oscillating foils with combined heaving and pitching motion at Reynolds number of 8000. The peak performance is associated with either high efficiency or large thrust production, which are achieved at specific motion settings. Using three-dimensional direct numerical simulations, we characterized and compared the dominant instability features for two peak propulsive wakes of a teardrop foil. Spanwise vortex dislocations on paired rollers for high efficiency setting revealed qualitative features of elliptic instability of primary vortex that led to tongue-like, and crescent shaped valley and bulge formations. However, asymmetric shedding of vortex dipoles in large thrust production setting depicted large scale dislocations in the form of conjoint hairpin-horseshoe formations that emerged out of vortex cores. These hinted at the dominance of both core and centrifugal instability. Here, we propose a hypothesis that large scale dislocations have greater influence on the spatial topology of secondary vortex structures compared to their temporal topology, which were observed to be similar in both wakes.