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The influence of buoyancy upon pollution trapping and dispersal in the wake of a backward-facing step

Published version
Peer-reviewed

Repository DOI


Type

Article

Change log

Authors

Davies Wykes, M 

Abstract

jats:pThe importance of buoyancy relative to free-stream flow is described using an adapted Froude number jats:inline-formula jats:alternatives <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S2633425923000144_inline1.png" /> jats:tex-mathFr′=U/f01/3</jats:tex-math> </jats:alternatives> </jats:inline-formula>, where jats:inline-formula jats:alternatives <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S2633425923000144_inline2.png" /> jats:tex-mathU</jats:tex-math> </jats:alternatives> </jats:inline-formula> is the flow speed and jats:inline-formula jats:alternatives <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S2633425923000144_inline3.png" /> jats:tex-mathf0</jats:tex-math> </jats:alternatives> </jats:inline-formula> is the exhaust buoyancy flux per unit length. We varied jats:inline-formula jats:alternatives <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S2633425923000144_inline4.png" /> jats:tex-mathFr</jats:tex-math> </jats:alternatives> </jats:inline-formula> by changing the free-stream flow rate, the exhaust flow rate and the buoyancy of the exhaust. We have experimentally identified two flow regimes, depending on the value of jats:inline-formula jats:alternatives <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S2633425923000144_inline5.png" /> jats:tex-mathFr</jats:tex-math> </jats:alternatives> </jats:inline-formula>. For high jats:inline-formula jats:alternatives <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S2633425923000144_inline6.png" /> jats:tex-mathFr</jats:tex-math> </jats:alternatives> </jats:inline-formula> (low buoyancy), dispersion is driven by inertial forces in the wake and the amount of a pollutant in the wake is independent of jats:inline-formula jats:alternatives <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S2633425923000144_inline7.png" /> jats:tex-mathFr</jats:tex-math> </jats:alternatives> </jats:inline-formula>. For moderate jats:inline-formula jats:alternatives <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S2633425923000144_inline8.png" /> jats:tex-mathFr</jats:tex-math> </jats:alternatives> </jats:inline-formula>, a wall plume develops up the back of the step, directly feeding the pollutant into the shear layer, but without altering the shape of the wake. This wall plume reduces the amount of pollutant trapped behind the step. We developed an analytic model to describe the quantity of pollutant trapped behind the step. The model predicts the transition from buoyancy being negligible to being the dominant transport mechanism within the wake. We have hypothesised and observed some evidence of a third regime at low jats:inline-formula jats:alternatives <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S2633425923000144_inline9.png" /> jats:tex-mathFr</jats:tex-math> </jats:alternatives> </jats:inline-formula>, when the buoyancy is sufficient to distort the macrostructure of the shear layer and wake.</jats:p>

Description

Keywords

Buoyancy, Pollution, Wakes, Wall plumes, Convection

Journal Title

Flow

Conference Name

Journal ISSN

2633-4259
2633-4259

Volume Title

3

Publisher

Cambridge University Press (CUP)
Sponsorship
Engineering and Physical Sciences Research Council (EP/N509620/1)
EPSRC (P/V036238/1)