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Optimal suppression of a separation bubble in a laminar boundary layer

Published online by Cambridge University Press:  06 April 2020

Michael Karp Open the ORCID record for Michael Karp [Opens in a new window]  and
M. J. Philipp Hack Open the ORCID record for M. J. Philipp Hack [Opens in a new window]
Michael Karp
Affiliation:
Center for Turbulence Research, Stanford University, Stanford, CA94305, USA
M. J. Philipp Hack*
Affiliation:
Center for Turbulence Research, Stanford University, Stanford, CA94305, USA
*
†Email address for correspondence: mjph@stanford.edu
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Abstract

By means of nonlinear optimization, we seek the velocity disturbances at a given upstream position that suppress a laminar separation bubble as effectively as possible. Both steady and unsteady disturbances are examined and compared. For steady disturbances, an informed guess based on linear analysis of transient perturbation growth leads to significant delay of separation and serves as a starting point for the nonlinear optimization algorithm. It is found that the linear analysis largely captures the suppression of the separation bubble attained by the nonlinear optimal perturbations. The mechanism of separation delay is the generation of a mean flow distortion by nonlinear interactions during the perturbation growth. The mean flow distortion enhances the momentum close to the wall, counteracting the deceleration of the flow in that region. An examination of the effect of the disturbance spanwise wavenumber reveals that perturbations maximizing the mean flow distortion also approximately maximize the peak wall pressure, which is beneficial for lowering form drag. The optimal spanwise wavenumber leading to maximal peak wall pressure is significantly larger than the one maximizing the shift in separation onset. For unsteady disturbances, the mechanism of separation delay relies on enhancing wall-normal momentum transfer by triggering instabilities of the separated shear layer. It is found that Tollmien–Schlichting waves obtained from linear stability theory provide accurate estimates of the nonlinearly optimal disturbances. Comparison of optimal steady and unsteady perturbations reveals that the latter are able to obtain a higher time-averaged peak wall pressure.

JFM classification

Boundary Layers: Boundary layer separation Flow Control: Flow Control
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 892 , 10 June 2020 , A23
Copyright
© The Author(s), 2020. Published by Cambridge University Press

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Karp and Hack supplementary movie

Instantaneous streamwise component for the nonlinearly optimal case ω = 0.10.

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