Dynamics and stability of flow past a circular cylinder in ground effect

Abstract

A combined experimental, computational and theoretical study is presented on the dynamics and stability characteristics of turbulent flow past a circular cylinder placed near and parallel to a moving ground. The study consists of four main parts: (i) wind tunnel experiment, (ii) numerical simulation, (iii) linear stability analysis, and (iv) proper orthogonal decomposition (POD) analysis. The main focus of the study is on the cessation of large-scale, von Karman-type vortex shedding in 'ground effect', i.e., the cessation observed when the cylinder comes close to the ground. The experiments, performed at upper-subcritical Reynolds numbers of 0.4 and 1.0 x 105, show that the cessation of von Karman-type vortex shedding and an attendant critical drag reduction of the cylinder (equipped with end-plates) occurs at the gap-to-diameter ratio h/d of around 0.35, at which point the flow through the gap between the cylinder and the ground is till not blocked at all due to the ground moving at the same speed as the free stream. It is subsequently shown that detached-eddy simulations (DES) can correctly reproduce these critical phenomena, whereas unsteady RANS simulations predict them at much smaller h/d of between 0.1 and 0.2, despite the fact that the unsteady RANS simulations are 'overly dissipative' compared with the DES. The linear stability analysis of analytical wake profiles then provides a possible explanation for the above experimental and computational results; that is, the cessation of the von Karman-type vortex shedding in ground effect may also be largely explained by the change of inviscid instability characteristics in the near wake region from 'absolutely unstable' to 'convectively unstable', in analogy with the case for a cylinder equipped with a backward splitter plate in a free stream. Finally, the near wake structure of the cylinder in ground effect is further investigated with the POD analysis. The results show that about 60% of the total kinetic energy in the near wake region (in the time-averaged sense) is contained only in the first three POD modes even when the energetically dominant, von Karman-type vortex shedding becomes intermittent at h/d = 0.4. It is also shown that both shedding and non-shedding states at this gap ratio can roughly be reproduced from the combination of these three POD modes.

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