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Absolute and convective instabilities of two-dimensional bluff body wakes in ground effect

Absolute and convective instabilities of two-dimensional bluff body wakes in ground effect
Absolute and convective instabilities of two-dimensional bluff body wakes in ground effect
Local and global instabilities are investigated of wakes of general two-dimensional bluff bodies placed near and parallel to a plane boundary or ground. A spatio-temporal linear stability analysis is first applied to a four-parameter family of local wake profiles to investigate the fundamental local stability characteristics of the wake in ground effect. The analysis shows significant dependencies of the stability characteristics of the wake on the distance from the wake centreline to the ground (normalised by the wake width), and also on the velocity ratio of the near- and far-ground sides of the wake. The analysis is then compared with earlier experiments on a circular cylinder to examine, according to the transition scenario of the steep global modes, the streamwise variation of the local stability characteristics of the wake in ground effect. The comparison indicates that the near wake region of the cylinder changes from being absolutely unstable to being convectively unstable when the cylinder comes down into the near-ground range in which the von Kármán-type vortex shedding from the cylinder is suppressed, being qualitatively consistent with the transition scenario for general wake-type flows. A possible explanation is also given for the counter-intuitive relation between the thickness of the boundary layer on the ground and the critical gap distance for the cessation of the von Kármán-type vortex shedding in ground effect.
absolute instability, bluff body wake, circular cylinder, vortex shedding, ground effect
0997-7546
13pp
Nishino, Takafumi
dc3e5827-0bf7-431d-871b-b4bdeff169c1
Roberts, Graham T.
deaf59ac-e4ee-4fc2-accf-df0639d39368
Nishino, Takafumi
dc3e5827-0bf7-431d-871b-b4bdeff169c1
Roberts, Graham T.
deaf59ac-e4ee-4fc2-accf-df0639d39368

Nishino, Takafumi and Roberts, Graham T. (2007) Absolute and convective instabilities of two-dimensional bluff body wakes in ground effect. European Journal of Mechanics - B/Fluids, 13pp. (doi:10.1016/j.euromechflu.2007.09.006).

Record type: Article

Abstract

Local and global instabilities are investigated of wakes of general two-dimensional bluff bodies placed near and parallel to a plane boundary or ground. A spatio-temporal linear stability analysis is first applied to a four-parameter family of local wake profiles to investigate the fundamental local stability characteristics of the wake in ground effect. The analysis shows significant dependencies of the stability characteristics of the wake on the distance from the wake centreline to the ground (normalised by the wake width), and also on the velocity ratio of the near- and far-ground sides of the wake. The analysis is then compared with earlier experiments on a circular cylinder to examine, according to the transition scenario of the steep global modes, the streamwise variation of the local stability characteristics of the wake in ground effect. The comparison indicates that the near wake region of the cylinder changes from being absolutely unstable to being convectively unstable when the cylinder comes down into the near-ground range in which the von Kármán-type vortex shedding from the cylinder is suppressed, being qualitatively consistent with the transition scenario for general wake-type flows. A possible explanation is also given for the counter-intuitive relation between the thickness of the boundary layer on the ground and the critical gap distance for the cessation of the von Kármán-type vortex shedding in ground effect.

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More information

Submitted date: 19 December 2006
Published date: 18 October 2007
Keywords: absolute instability, bluff body wake, circular cylinder, vortex shedding, ground effect
Organisations: Aerodynamics & Flight Mechanics

Identifiers

Local EPrints ID: 50296
URI: http://eprints.soton.ac.uk/id/eprint/50296
ISSN: 0997-7546
PURE UUID: 4ac6c879-22d9-4ff2-874f-4adbb32c2f7e

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Date deposited: 12 Feb 2008
Last modified: 15 Mar 2024 10:05

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Author: Takafumi Nishino

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