On the Association of Kinematics, Spanwise Instability and Growth of Secondary Vortex Structures in the Wake of Oscillating Foils

TL;DR

This study numerically investigates how oscillating foil wake dynamics, especially spanwise instabilities and secondary vortex growth, vary with motion parameters, revealing new transition routes linked to changes in heave and pitch dominance.

Contribution

It identifies new mechanisms and transition routes for spanwise instability and secondary vortex growth in oscillating foil wakes based on combined kinematic parameters.

Findings

Spanwise instability origin shifts from secondary LEV to TEV with increased heave.

Growth of secondary structures correlates with primary LEV circulation strength.

Transition routes mapped on phase-space show decreased heave leads to less secondary vortex activity.

Abstract

Three-dimensional wake of an oscillating foil with combined heaving and pitching motion is numerically evaluated at a range of chord-based Strouhal number (0.32 \le Stc \le 0.56) and phase offset (90 deg \le \phi \le 70 deg) at Re = 8000. The changes in \phi and Stc reflect a unique route of transition in mechanisms that govern the origin of spanwise instabilities and growth of secondary wake structures. At lower Stc, heave dominated kinematics demonstrates a strong secondary leading edge vortex (LEV ) as the source of growing spanwise instability on the primary LEV , followed by an outflux of streamwise vorticity filaments from the secondary LEV . With increasing heave domination, the origin of stronger spanwise instability is governed by a counter-rotating trailing edge vortex (TEV ) and LEV that leads to growth of streamwise secondary structures. A decreasing heave domination ultimately coincides with an absence of strong LEV undulations and secondary structures. The consistent transition routes are represented on a phase-space map, where a progression of spanwise instability and growth of secondary structures becomes evident within regimes of decreased heave domination. The increasing strength of circulation for the primary LEV , with increasing Stc, provides a crucial reasoning for this newly identified progression.