Bifurcations beneath the bluff body instability modes

A new family of Hopf bifurcations is detected in a cylinder wake. Besides widely known streamwise bifurcations, our study reveals a new route to wake transition via cross-stream flow undulations and a class of previously unknown spanwise bifurcations along von Karman vortex cores; leading to an improved understanding of wake transformation and the transitional flow physics. It shows, alternate vortex shedding generates significant cross-stream momentum transfer, which facilitates self-sustained spanwise wake oscillation and growth of sequence of bifurcations along Karman corelines. The study shows how spanwise oscillation of pressure/velocity/KE keep growing with \textit{Re}, and influence onset of ``Mode A'' and ``Mode B'' instabilities. It reports two distinct stages of wake undulation for 125$\le $\textit{Re}$\le $240. While weakly subcritical periodic-oscillation of pressure/velocity/vorticity along Karman corelines and the uniform/wider length-scale bifurcations dominate during ``Mode A'' instability, the transition to ``Mode B' is prompted following eruption/swapping of significantly smaller variable length-scale bifurcations, and the spanwise flow irregularity. Onset of a secondary frequency in the flow appeared crucial for transition to ``Mode B.''