A simulation of a periodic cylindrical cloud of concentrated, neutrally buoyant, suspended particles, is used to investigate the dispersion of the particles in an oscillating Couette flow. In experiments by Metzger & Butler (2012) with spherical clouds of non-Brownian particles, the clouds are shown to elongate at volume fraction φ = 0.4 but form “galaxies” where the cloud rotates as a single body with extended arms when φ > 0.4 and the ratio of the cloud radius to particle radius, R/a, is sufficiently large. In simulations, rotating galaxies form at high volume fraction φ = 0.55, and are characterized by a particle-induced flow in the wall-normal direction. Here, particle locations are shown relative to the averaged wall-normal velocity ⟨v⟩. The particle arms form in the locations where the wall- normal velocity field induced by the cloud rotation switches direction.

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