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Three-dimensional structural characteristics of flow separation induced by a forward-facing step in a turbulent channel flow

Published online by Cambridge University Press:  26 May 2021

Xingjun Fang Open the ORCID record for Xingjun Fang [Opens in a new window] ,
Mark F. Tachie Open the ORCID record for Mark F. Tachie [Opens in a new window] ,
Donald J. Bergstrom ,
Zixuan Yang Open the ORCID record for Zixuan Yang [Opens in a new window]  and
Bing-Chen Wang Open the ORCID record for Bing-Chen Wang [Opens in a new window]
Xingjun Fang*
Affiliation:
Department of Mechanical Engineering, University of Manitoba, Winnipeg, MBR3T 5V6, Canada
Mark F. Tachie
Affiliation:
Department of Mechanical Engineering, University of Manitoba, Winnipeg, MBR3T 5V6, Canada
Donald J. Bergstrom
Affiliation:
Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SKS7N 5A9, Canada
Zixuan Yang
Affiliation:
Institute of Mechanics, Chinese Academy of Sciences, Beijing100190, PR China
Bing-Chen Wang
Affiliation:
Department of Mechanical Engineering, University of Manitoba, Winnipeg, MBR3T 5V6, Canada
*
†Email address for correspondence: fangx@myumanitoba.ca
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Abstract

Separating and reattaching turbulent flows induced by a forward-facing step subjected to an incoming fully developed turbulent channel flow are studied using direct numerical simulation. The step height is one quarter of the channel height, and the Reynolds number based on friction velocity and half-channel height at the inlet is 180. The three-dimensional spatio-temporal characteristics of separation bubbles upstream and downstream of the step are analysed with particular attention to the effects of impinging hairpin structures and the topology of principal stretching. Immediately upstream of the step, the fluctuating vorticity parallel to the mean streamlines is significant. On the frontal surface of the step, strong spanwise skin friction appears in the form of alternating positive and negative values in vertical strips. Over the step, the principal stretching switches orientation along a curve emanating from the leading edge, which is termed the principal stretching line (PSL). The reverse flows upstream and downstream of the step possess dominant and harmonic frequencies that mirror those of the incoming flow. As a hairpin structure leans over the step, the associated vorticity is deformed by the principal stretching. Specifically, PSL marks the lower bound of the deformed hairpin legs, and an opposite-signed pair of counter-rotating quasi-streamwise vortices are induced near the top surface of the step. Consequently, the separation bubbles upstream of and over the step are enlarged and suppressed, respectively. For a sufficiently strong hairpin structure interacting with the step, an open-type separation occurs upstream of the step, while dual separation bubbles appear over the step.

JFM classification

Boundary Layers: Boundary layer separation Turbulent Flows: Turbulent boundary layers
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 919 , 25 July 2021 , A24
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Copyright
© The Author(s), 2021. Published by Cambridge University Press

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References

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