The preferential accumulation and the settling velocity of small heavy particles in Taylor–Couette flows

In a gap between two concentric cylinders, we discover that the small heavy particle settling velocity (V_s) is greater than its associated terminal velocity (V_t) in featureless turbulence that is generated by counter-rotating inner and outer cylinders simultaneously. The peak enhancement of V_s occurs at St× (V_t/u) ∼ 1, where u and Stare, respectively, the r.m.s. turbulence fluctuation velocity and the Stokes number defined as the ratio between the particle response time to the Kolmogorov turbulence time. The present (V_s − V_t)/u data as a function of St measured by the high-speed particle imaging velocimetry are compared with previous experimental results obtained from different turbulent flow configurations in an attempt to gain further understandings of the complicated interactions between small heavy particles and featureless turbulence with negligible mean velocities. Moreover, it is found that small heavy particles are preferentially accumulated around the edges of an array of counter-rotating turbulent Taylor vortices with the inner cylinder rotation alone when a time ratio between particle response and Taylor vortices is near unity. These experimental results are interesting that may deserve to disseminate in our fluid mechanics community.