Three-dimensional wake transition behind an elliptic cylinder near a moving wall

Abstract

Three-dimensional flow past an elliptic cylinder with an aspect ratio of 0.5 near a moving bottom wall is investigated numerically for gap ratios of G/D=0.1,0.2,0.3, and 0.4 (where G denotes the gap between the cylinder bottom and the moving wall, and D is the major-axis length of the cylinder) with Reynolds numbers (Re) ranging from 100 to 200 (based on a constant inlet velocity and the major-axis length of the cylinder); the transition between two- and three-dimensional flow regimes is described in detail. For G/D=0.4, the flow is first two-dimensional with a Kármán vortex street followed by a two-layered wake, then it evolves into a three-dimensional flow regime with near-wake and far-wake elliptic instabilities of vortex pairs; for Re≥180, the near-wake elliptic instability disappears (i.e., the near wake becomes two-dimensional) while the far-wake elliptic instability persists. For G/D=0.3, the flow is first two-dimensional without the development of the two-layered wake, then it evolves into a three-dimensional flow regime with streamwise vorticity pairs propagating periodically in the spanwise direction; this propagation becomes irregular for Re≥160. For G/D=0.2, the flow is first two-dimensional as for G/D=0.3, then it becomes three-dimensional, exhibiting a behavior of modified mode C instability; for Re≥140, this flow exhibits a chaotic behavior. For G/D=0.1, the flow is first three-dimensional and steady without vortex shedding, and then develops into an unsteady flow with a dominating upper shear layer in the near-wake and a chaotic wake structure farther downstream.