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Periodic forcing of a large turbulent separation bubble

Published online by Cambridge University Press:  11 March 2021

Abdelouahab Mohammed-Taifour  and
Julien Weiss Open the ORCID record for Julien Weiss [Opens in a new window]
Abdelouahab Mohammed-Taifour
Affiliation:
Laboratoire de thermo-fluide pour le transport, École de technologie supérieure, Montréal, QuébecH3C 1K3, Canada
Julien Weiss*
Affiliation:
Laboratoire de thermo-fluide pour le transport, École de technologie supérieure, Montréal, QuébecH3C 1K3, Canada Institute of Aeronautics and Astronautics, Technical University of Berlin, 10587Berlin, Germany
*
†Email address for correspondence: [email protected]
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Abstract

The response of a pressure-induced turbulent separation bubble (TSB) to periodic forcing by pulsed-jet actuators (PJAs) positioned in the upstream boundary layer is investigated experimentally in an attempt to elucidate the mechanism of low-frequency contraction and expansion (‘breathing’) already documented in this flow by Mohammed-Taifour & Weiss (J. Fluid Mech., vol. 799, 2016, pp. 383–412). The TSB is generated on a flat test surface by a combination of adverse and favourable pressure gradients and the free-stream velocity is $25\ \textrm {m}\,\textrm {s}^{-1}$. The results indicate that periodic forcing artificially reduces the size of the TSB by moving separation downstream and reattachment upstream. The smaller TSB is associated with narrower streamwise distributions of average pressure and forward-flow fraction, as well as smaller turbulent stresses in the shear layer bounding the recirculation region. Transient forcing experiments further demonstrate that the TSB responds to upstream forcing with a characteristic time scale that is of the same order of magnitude as that of the breathing motion. Overall, the results of this study support a mechanism whereby the low-frequency breathing motion is a response of the TSB to upstream perturbations that affect the position of separation first and, indirectly, the position of reattachment through a global redistribution of the pressure and velocity fields. The low-frequency behaviour of the TSB appears to be well illustrated by a first-order low-pass filter model that converts the broadband fluctuations of the incoming turbulent boundary layer into a low-frequency, large-scale oscillation of the separation and reattachment fronts, thus leading to a contraction and expansion of the TSB. The results of the continuous forcing experiments also offer new insights into active separation control with PJAs by showing that such actuators generate strong starting vortices that, when convected within an adverse pressure gradient, are associated with a downstream shift of the separation front.

JFM classification

Turbulent Flows: Turbulent boundary layers Boundary Layers: Boundary layer separation
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 915 , 25 May 2021 , A24
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Copyright
© The Author(s), 2021. Published by Cambridge University Press

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