Taylor-Couette flow instabilities in neutrally-buoyant suspensions

Experimentally-determined instabilities and flow states of a neutrally-buoyant suspension are described. The flow is studied in a concentric-cylinder device with inner-to-outer cylinder ratio of 0.877 with inner cylinder rotating and outer stationary. The cylinder length to annular gap ratio is 20, while the gap to particle size ratio is approximately 30, for spherical particles of 250 $\mu$m diameter. Using a slowly increasing or decreasing $Re$ ramp, the flow agrees with all expectations for the pure fluid, while a slowly decreasing (quasi-static) ramp is used for the suspension flow, which is found to be unstable at lower Reynolds number $Re$ (based on the effective viscosity) than pure fluid, and exhibits spiraling and ribbon states not found for a pure fluid with only inner cylinder rotating. Strikingly, the suspension at solid fraction $\phi\ge0.05$ goes unstable first to a nonaxisymetric state rather than axisymmetric Taylor vortices. At $0.1\le \phi \le 0,2$, the flow exhibits numerous states during quais-static ramping of $Re$, while for $\phi = 0.3$, the base state Couette flow gives way to wavy spirals (WS) at $Re \approx 80$ and exhibits only the WS state up to $Re = 150$. Transient behavior on sudden change of $Re$ and particle tracking will also be presented.