Strong alignment of prolate ellipsoids in Taylor???Couette flow

We report on the mobility and orientation of finite-size, neutrally buoyant, prolate ellipsoids (of aspect ratio Lambda = 4) in Taylor-Couette flow, using interface-resolved numerical simulations. The set-up consists of a particle-laden flow between a rotating inner and a stationary outer cylinder. The flow regimes explored are the well-known Taylor vortex, wavy vortex and turbulent Taylor vortex flow regimes. We simulate two particle sizes l/d = 0.1 and l/d = 0.2, l denoting the particle major axis and d the gap width between the cylinders. The volume fractions are 0.01% and 0.07 %, respectively. The particles, which are initially randomly positioned, ultimately display characteristic spatial distributions which can be categorised into four modes. Modes (i) to (iii) are observed in the Taylor vortex flow regime, while mode (iv) encompasses both the wavy vortex and turbulent Taylor vortex flow regimes. Mode (i) corresponds to stable orbits away from the vortex cores. Remarkably, in a narrow Ta range, particles get trapped in the Taylor vortex cores (mode (ii)). Mode (iii) is the transition when both modes (i) and (ii) are observed. For mode (iv), particles distribute throughout the domain due to flow instabilities. All four modes show characteristic orientational statistics. The focus of the present study is on mode (ii). We find the particle clustering for this mode to be size-dependent, with two main observations. Firstly, particle agglomeration at the core is much higher for l/d = 0.2 compared with l/d = 0.1. Secondly, the Ta range for which clustering is observed depends on the particle size. For this mode (ii) we observe particles to align strongly with the local cylinder tangent. The most pronounced particle alignment is observed for l/d = 0.2 at around Ta = 4.2 x 10(5). This observation is found to closely correspond to a minimum of axial vorticity at the Taylor vortex core (Ta = 6 x 10(5)) and we explain why.