Forced vortex merging and splitting events in ferrofluidic Couette flow

Time-dependent boundary conditions are ubiquitous in numerous natural and industrial flows. However, to date, the influence of such temporal modulations has been given minor attention. This research addresses ferrofluidic Couette flow in between co-rotating cylinders in a spatially homogeneous magnetic field subject to time-periodic modulation. Using a modified Niklas approximation, we study the effect of amplitude and frequency modulation onto the transition scenarios between different toroidal flow structures, non-propagating vortex (nV) states, via vortex merging and splitting. Thereby, the system response appears to be quite sensitive/dependent on the driving frequency ΩH, which can cause a notable “delay” in the system response. Further, as a result of the inertia of the ferrofluid, resisting the fast-changing accelerating Kelvin force, new, temporal nV states appear within an alternating field. These states are unstable under static fields. Finally, we show that within the same nV state, while keeping similar flow dynamics, large discrepancies in angular momentum and torque can be observed.