Instabilities and particle-induced patterns in co-rotating suspension Taylor–Couette flow

The first experimental results on pattern transitions in the co-rotation regime (i.e. the rotation ratio $\varOmega = \omega _o/\omega _i > 0$ , where $\omega _i$ and $\omega _o$ are the angular speeds of the inner and outer cylinders, respectively) of the Taylor–Couette flow (TCF) are reported for a neutrally buoyant suspension of non-colloidal particles, up to a particle volume fraction of $\phi = 0.3$ . While the stationary Taylor vortex flow (TVF) is the primary bifurcating state in dilute suspensions ( $\phi \leq ~0.05$ ), the non-axisymmetric oscillatory states, such as the spiral vortex flow (SVF) and the ribbon (RIB), appear as primary bifurcations with increasing particle loading, with an overall de-stabilization of the primary bifurcating states (TVF/SVF/RIB) being found with increasing $\phi$ for all $\varOmega \geq ~0$ . At small co-rotations ( $\varOmega \sim 0$ ), the particles play the dual role of stabilization ( $\phi < 0.1$ ) and destabilization ( $\phi \geq ~0.1$ ) on the secondary/tertiary oscillatory states. The distinctive features of the ‘particle-induced’ spiral vortices are identified and contrasted with those of the ‘fluid-induced’ spirals that operate in the counter-rotation regime.