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Large structure in the far wakes of two-dimensional bluff bodies

Cimbala, John M. and Nagib, Hassan M. and Roshko, Anatol (1988) Large structure in the far wakes of two-dimensional bluff bodies. Journal of Fluid Mechanics, 190 . pp. 265-298. ISSN 0022-1120. doi:10.1017/S0022112088001314.

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Smoke-wire flow visualization and hot-wire anemometry have been used to study near and far wakes of two-dimensional bluff bodies. For the case of a circular cylinder at 70 < Re < 2000, a very rapid (exponential) decay of velocity fluctuations at the Kármán-vortex-street frequency is observed. Beyond this region of decay, larger-scale (lower wavenumber) structure can be seen. In the far wake (beyond one hundred diameters) a broad band of frequencies is selectively amplified and then damped, the centre of the band shifting to lower frequencies as downstream distance is increased. The far-wake structure does not depend directly on the scale or frequency of Kármán vortices shed from the cylinder; i.e. it does not result from amalgamation of shed vortices. The growth of this structure is due to hydrodynamic instability of the developing mean wake profile. Under certain conditions amalgamation can take place, but is purely incidental, and is not the driving mechanism responsible for the growth of larger-scale structure. Similar large structure is observed downstream of porous flat plates (Re [approximate] 6000), which do not initially shed Kármán-type vortices into the wake. Measured prominent frequencies in the far cylinder wake are in good agreement with those estimated by two-dimensional locally parallel inviscid linear stability theory, when streamwise growth of wake width is taken into account. Finally, three-dimensionality in the far wake of a circular cylinder is briefly discussed and a mechanism for its development is suggested based on a secondary parametric instability of the subharmonic type.

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Additional Information:Copyright © 1988 Cambridge University Press. Reprinted with permission. (Received 19 April 1985 and in revised form 28 September 1987) The authors are grateful for the financial support of the Office of Naval Research, US Navy, under Contract No. N00014-7642-0260. We also thank Toshi Kubota for discussions of the wake stability problem and for generously allowing us to use his calculations of wake stability
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Deposited On:14 Apr 2008
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