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|a The flow and force characteristics have been experimentally investigated of a circular cylinder with an aspect ratio of 8.33, with and without end-plates, placed near and parallel to a moving ground, on which substantially no boundary layer developed to interfere with the cylinder. Mean drag and lift measurements, surface oil flow visualization, and particle image velocimetry (PIV) measurements were carried out at two upper-subcritical Reynolds numbers of 0.4 and 1.0×105 (based on the cylinder diameter d) to investigate the mechanisms of the ground effect, i.e., the effect of the gap-to-diameter ratio h/d, where h is the gap between the cylinder and the ground. For the cylinder with end-plates, on which the oil flow patterns were observed to be essentially two-dimensional, the drag rapidly decreased as h/d decreased to less than 0.5 but became constant for h/d of less than 0.35, unlike that usually observed near a fixed ground. This critical drag behavior was found to be directly related to a global change in the near wake structure of the cylinder; as h/d decreased the Kármán-type vortex shedding became intermittent at h/d=0.4, and then totally ceased and instead two nearly parallel shear layers were formed behind the cylinder at h/d=0.3 and below. For the cylinder without end-plates, however, no such critical change in drag was observed as the Kármán-type vortices were not generated in the near wake region at all h/d investigated. Based on the experimental results obtained, further discussions are also given to the essential cause of the cessation of the Kármán vortex shedding in the ground effect.
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