Transport Of Large Particles Through The Transition To Turbulence Of A Swirling Flow

We investigate the behavior of large particles in a transitional swirling flow in a closed vessel, focusing on both their transport and flow sampling. We conduct a Lagrangian study of slightly buoyant particles, considering three particle diameters, keeping the fluid's density and velocity magnitude constant and varying the Reynolds number by changes of the fluid's viscosity. The literature lacks study of material particles in such flows, where chaos and bursts of turbulence can exist, despite applications in industrial and natural situations. One striking result is that particles beyond a certain size, between 10 and 18 mm, are subject to a strong trapping in the vicinity of the islands of the laminar flow, while large particles below this threshold sample the flow homogeneously, independently of the Reynolds number. The exploration of the flow by the large particles is widely different from the preferential sampling in fully developed turbulent conditions, in terms of characteristic times associated with motions escaping the regions and dimensions of the sampled regions. While the particle mean velocity is found to be independent of both the fluid's viscosity and particle size, the fluctuation magnitude strongly increases with decreasing flow viscosity and is marginally affected by size. We also characterize the intensity of the trapping through probability density functions of the particle positions, a measure of the dimensions of the sampled regions, and particle position autocorrelation functions as an attempt to quantify residence times. A qualitative origin for the trapping existence criterion based on a shear-induced lift force is proposed, as an argument for its sole dependence on particle size.