Vortex shedding from a circular cylinder near a moving ground

Abstract

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 ͑PIV͒ measurements were carried out at two upper-subcritical Reynolds numbers of 0.4 and 1.0ϫ 10 5 ͑based on the cylinder diameter d͒ 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.

Drag and lift forces on a counter-rotating cylinder in rotating flow

“…Strongly increasing region: Comparing with Nishino et al (2007), we have more data for very small gap ratios, as shown in Fig. 16 (b).…”

“…Zdravkovich (1985) found that the lift coefficient is governed by the gap to diameter ratio for a cylinder near a plane wall. Nishino et al (2007) found a rapid increase of the lift coefficient as the gap to diameter ratio decreases to less than about 0.5.…”

“…The characteristics of the flow are determined by Re and the gap ratio G/2a, which is the ratio of the the gap distance (G) and the cylinder diameter (2a) (Nishino et al 2007). Nishino et al (2007) Fig. 13 for various cylinders with radius a = 7.75 mm (open circles); 12.75 mm (triangles); 15.5 mm (squares); 20 mm (diamonds) and 30 mm (stars).…”

“…16(b), the lift coefficient changes from ∼ 0 to ∼ 0.5 when decreasing the gap to diameter ratio from 1 to 0.1, and the results for all cylinders follow this trend. The solid triangles represent the measurements by Nishino et al (2007) of Re = 10 5 . In their experiments, a uniform flow is established near the wall using a moving ground to eliminate shear effects.…”

On the unsteady motion and stability of a heaving airfoil in ground effect

“…In stationary bluff bodies, there is a critical gap between the body and the ground below which vortex shedding is suppressed [23]. While a similar effect can be expected of airfoils, the change in ride height with time will undoubtedly introduce an additional constraint to the flow.…”