Instability and kinematic for sand-water mixture or dry sand flowing past a circular cylinder

The present analysis refers on experiments with a mixture of grains and water and dry grains around a circular cylinder. The flow is driven by the internal cylinder of a Taylor-Couette cell. Velocity measurements of the mixture at the external surface are carried out using a commercial PIV and a new microPIV. The flow field is quite similar to the flow field generated by a Newtonian fluid around a circular cylinder, with a wake and a re-circulation zone. Near the body, the velocity scale of the wake decays linearly if the asymptotic velocity is high, decays logarithmically for low asymptotic velocity. The transverse length scale increases as x′n (x′ is measured along the axis of symmetry with the origin at the boundary of the body) with n < 0.5 (or nearly zero in some flow conditions). For dry grains flow, microPIV reveals strong instabilities, with a fast movement en-block of the grains. © 2006 Taylor & Francis Group.