Numerical investigation of free-stream turbulence effects on the transition-in-wake state of flow past a circular cylinder

Abstract

This work investigates how a turbulent free-stream (TFS) affects the transition-inwake state of flow past a cylinder. Direct numerical simulations have been performed, where the intensity of a decaying TFS varied from 3.6% to 12.2% (as it reaches the cylinder), with integral scales 0.5, 0.75 and 1.0 times the cylinder diameter for the high intensity case. Distinct effects of the TFS are observed for Reynolds numbers in the lower (Re < 190) and upper (Re ≥ 250) ranges of the transitional regime. For a flow with a laminar free-stream (LFS) the onset of the transition-in-wake is observed at a critical Reynolds number Rec ≈ 190, where the first three-dimensional effects develop in the wake (mode A instabilities). A TFS perturbs the flow, forcing the onset of the transition at a lower Reynolds number and inhibiting a sharply defined critical Reynolds number. Quasi-stable states, where the wake alternates between two-and three-dimensional vortex shedding, are observed for Re < 190. These states are closely related to intermittent vortex dislocations in the wake. In the upper Re part of the transition-in-wake regime, mode B instabilities dominate the wake of the LFS flow. A TFS stimulates the existence of mode A instabilities, resulting in mixed A-B instabilities in the wake for Re ≥ 250. This effect correlates with the turbulence intensity, and is strongest for the high intensity TFS.

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