The instantaneous, quantitative patterns of vortices arising from sinusoidal oscillation of a cylinder in quiescent fluid are experimentally characterized for the first time using high-image-density particle image velocimetry. The near-wake does not indicate a separated layer of distributed vorticity leading to a single, large-scale vortex. Rather, for sufficiently high Reynolds number, a sequence of small-scale vorticity concentrations is formed. Agglomeration of only a fraction of the adjacent concentrations forms a larger-scale vortex. Simultaneously, vorticity concentrations of opposite sense are formed along the base (rear) of the cylinder. Streamline patterns typically indicate, however, only the larger-scale vortex; it has a circulation smaller than the total circulation of all vorticity concentrations that are not revealed by the streamlines. These observations are interpreted in the context of the effective resolution of the flow images.
Experiments in Fluids
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