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The flow over a backward-facing step under controlled perturbation: laminar separation

Published online by Cambridge University Press:  26 April 2006

M. A. Z. Hasan
M. A. Z. Hasan
Affiliation:
Mechanical Engineering Department, King Fahd University of Petroleum & Minerals. Dhahran 31261, Saudi Arabia
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Abstract

The flow over a backward-facing step with laminar separation was investigated experimentally under controlled perturbation for a Reynolds number of 11000, based on a step height h and a free-stream velocity UO. The reattaching shear layer was found to have two distinct modes of instability: the ‘shear layer mode’ of instability at Stθ ≈ 0.012 (Stθ ≡ fθ/UO, θ being the momentum thickness at separation and f the natural roll-up frequency of the shear layer); and the ‘step mode’ of instability at Sth ≈ 0.185 (Sth ≡ fh/U0). The shear layer instability frequency reduced to the step mode one via one or more stages of a vortex merging process. The perturbation increased the shear layer growth rate and the turbulence intensity and decreased the reattachment length compared to the unperturbed flow. Cross-stream measurements of the amplitudes of the perturbed frequency and its harmonics suggested the splitting of the shear layer. Flow visualization confirmed the shear layer splitting and showed the existence of a low-frequency flapping of the shear layer.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 238 , May 1992 , pp. 73 - 96
Copyright
© 1992 Cambridge University Press

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