Transitions in Taylor-Couette flow of concentrated non-colloidal suspensions.

Taylor-Couette flow of concentrated non-colloidal suspensions with a rotating inner cylinder and a stationary outer one is numerically investigated. We consider suspensions of the bulk particle volume fraction  = 0.2, 0.3 with the ratio of annular gap to the particle radius  = 60 confined in a cylindrical annulus of the radius ratio (i.e. ratio of inner and outer radii)  = 0.877. Numerical simulations are performed by applying suspension-balance model and rheological constitutive laws. To observe flow patterns caused by suspended particles, the Reynolds number of the suspension, based on the bulk particle volume fraction and the rotating velocity of the inner cylinder, is varied up to 180. At high Reynolds number, modulated patterns undiscovered in the flow of a semi-dilute suspension emerge beyond a wavy vortex flow. Thus, a transition occurs from the circular Couette flow via ribbons, spiral vortex flow, wavy spiral vortex flow, wavy vortex flow and modulated wavy vortex flow for the concentrated suspensions. Moreover, friction and torque coefficients for suspensions are estimated. It turns out that suspended particles significantly enhance the torque on the inner cylinder while reducing friction coefficient and the pseudo-Nusselt number. In particular, the coefficients are reduced in the flow of more dense suspensions. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal paper (Part 2)'.