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Laminarisation of flow at low Reynolds number due to streamwise body force

He, S and He, K and Seddighi, M (2016) Laminarisation of flow at low Reynolds number due to streamwise body force. Journal of Fluid Mechanics, 809. pp. 31-37. ISSN 0022-1120

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Abstract

It is well established that when a turbulent flow is subjected to a non-uniform body force, the turbulence may be significantly suppressed in comparison with that of the flow of the same flow rate and hence the flow is said to be laminarised. This is the situation in buoyancy-aided mixed convection when severe heat transfer deterioration may occur. Here we report results of direct numerical simulations of flow with a linear or a step-change profile of body force. In contrast to the conventional view, we show that applying a body force to a turbulent flow while keeping the pressure force unchanged causes little changes to the key characteristics of the turbulence. In particular, the mixing characteristics of the turbulence represented by the turbulent viscosity remain largely unaffected. The so-called flow laminarisation due to a body force is in effect a reduction in the apparent Reynolds number of the flow, based on an apparent friction velocity associated with only the pressure force of the flow (i.e. excluding the contribution of the body force). The new understanding allows the level of the flow ‘laminarisation’ and when the full laminarisation occurs to be readily predicted. In terms of the near-wall turbulence structure, the numbers of ejections and sweeps are little influenced by the imposition of the body force, whereas the strength of each event may be enhanced if the coverage of the body force extends significantly away from the wall. The streamwise turbulent stress is usually increased in accordance with the observation of more and stronger elongated streaks, but the wall-normal and the circumferential turbulent stresses are largely unchanged.

Item Type: Article
Additional Information: © 2016 Cambridge University Press This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited
Uncontrolled Keywords: 01 Mathematical Sciences, 09 Engineering
Subjects: T Technology > TC Hydraulic engineering. Ocean engineering
T Technology > TJ Mechanical engineering and machinery
Divisions: Maritime and Mechanical Engineering
Publisher: Cambridge University Press
Date Deposited: 25 Nov 2016 11:21
Last Modified: 07 Sep 2017 15:09
DOI or Identification number: 10.1017/jfm.2016.653
URI: http://researchonline.ljmu.ac.uk/id/eprint/4868

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