The recirculating wake behind the obstacle at moderate Reynolds numbers was devoid of particles, this was discovered by Haddadi et al. (J. Fluid Mech., 2014). However, only one obstacle and narrow Reynolds numbers were considered in their work. In this work, we constructed more confined environment, where the suspensions of solid fraction 0.25% with different particle diameters of 1, 5 and 10 μm flow past the critical confined low-aspect-ratio cylinder arrays (H/D = 0.3) with different arrangements were experimentally carried out. Reynolds numbers performed in the experiments ranged from 14∼550, and different flow patterns were observed. It was found that particles could flow into the region behind cylinder at low flow rate. Then, particle-depleted wake zone was formed behind the cylinder when increasing Re, which is similar with reported literature. It was interesting to find that when increasing Re further, the particles could flow into the recirculating wake zone behind cylinder. We generalized the particle behavior behind cylinder as from “entry” to “particle-free” and to “re-entry”. Additionally, the influence of different layout modes with inline and staggered cylinder arrays were also investigated. We found the particle-depleted wake zones behind the in-line cylinder array were larger than the one of the staggered cylinder array as the velocity were the same at “particle-free” stage. The in-line cylinder array possessed higher ability of allowing 1 μm particles to fill with the recirculating wake, whereas there were always existing particle-free zones in the core of recirculating wake of staggered cylinder array at “re-entry” stage. In order to understand particle-free mechanism better, microparticle image velocimetry (μPIV) technique was utilized to quantitatively measure the flow fields of in-line and staggered arranged cylinders. The obtained fluorescence pictures demonstrated that few particles flow into the zone behind cylinder at the “particle-free” stage, and fluorescence particles can flow into the wake at “re-entry” stage. That is, fluorescence particles also experienced the stages from “entry” to “particle-free” and to “re-entry”.

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