  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.
