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On the unsteady characteristics of turbulent separations over a forward–backward-facing step

Published online by Cambridge University Press:  29 January 2019

Xingjun Fang Open the ORCID record for Xingjun Fang [Opens in a new window]  and
Mark F. Tachie
Xingjun Fang*
Affiliation:
Department of Mechanical Engineering, University of Manitoba, Winnipeg, MB, R3T 5V6, Canada
Mark F. Tachie
Affiliation:
Department of Mechanical Engineering, University of Manitoba, Winnipeg, MB, R3T 5V6, Canada
*
†Email address for correspondence: fangx@myumanitoba.ca
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Abstract

Turbulent separation bubbles over and behind a two-dimensional forward–backward-facing step submerged in a deep turbulent boundary layer are investigated using a time-resolved particle image velocimetry. The Reynolds number based on the step height and free-stream velocity is 12 300, and the ratio of the streamwise length to the height of the step is 2.36. The upstream turbulent boundary layer thickness is 4.8 times the step height to ensure a strong interaction of the upstream turbulence structures with the separated shear layers over and behind the step. The velocity measurements were performed in streamwise–vertical planes at the channel mid-span and streamwise–spanwise planes at various vertical distances from the wall. The unsteady characteristics of the separation bubbles and their associated turbulence structures are studied using a variety of techniques including linear stochastic estimation, proper orthogonal decomposition and variable-interval time averaging. The results indicate that the low-frequency flapping motion of the separation bubble over the step is induced by the oncoming large-scale alternating low- and high-velocity streaky structures. Dual separation bubbles appear periodically over the step at a higher frequency than the flapping motion, and are attributed to the inherent instability in the rear part of the mean separation bubble. The separation bubble behind the step exhibits a flapping motion at the same frequency as the separation bubble over the step, but with a distinct phase delay. At instances when an enlarged separation bubble is formed in front of the step, a pair of vertical counter-rotating vortices is formed in the immediate vicinity of the leading edge.

JFM classification

Boundary Layers: Boundary layer separation Turbulent Flows: Turbulent boundary layers

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Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 863 , 25 March 2019 , pp. 994 - 1030
Copyright
© 2019 Cambridge University Press 

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