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Coherent structures in oscillatory boundary layers
Published online by Cambridge University Press: 26 April 2006
Abstract
An experimental investigation of the circumstances leading to the creation and subsequent evolution of the low-speed streaks and other quasi-coherent structures on a long cylindrical body immersed in a sinusoidally oscillating flow (Stokes flow) is described. The wall shear stress and the phase lead of the maximum wall shear over the maximum free-stream velocity have been measured to characterize the unsteady boundary layer. The evolution of a sinuous streak, from its inception to its ultimate demise, and the generation of multiple streaks, arches, hairpins, and other vortical structures have been traced through flow visualization. The results have shown that the strong pressure gradients, inflexion points in the velocity profile, and the reversal of the shear stress have profound effects on the stability of the flow. The Reynolds number (Reδ = Umaxδ/ν) delineates the boundaries of the laminar stable flow, transitional flow, and turbulent flow at the start of which the phase angle decreases sharply, the friction coefficient increases rapidly, and the turbulent motion prevails over larger fractions of the flow cycle. The transitional and turbulent states are rich with vortical motions which burst themselves into existence most intensely during the later stages of the deceleration phase. The effect of the manipulation of the viscosity of the wall-layer fluid on the creation and bifurcation of the low-speed streaks is discussed in some detail.
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- © 1993 Cambridge University Press
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