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Measurements of pressure and velocity fluctuations in a family of turbulent separation bubbles

Published online by Cambridge University Press:  07 September 2020

Arnaud Le Floc'h ,
Julien Weiss Open the ORCID record for Julien Weiss [Opens in a new window] ,
Abdelouahab Mohammed-Taifour  and
Louis Dufresne
Arnaud Le Floc'h
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
Abdelouahab Mohammed-Taifour
Affiliation:
Laboratoire de thermo-fluide pour le transport, École de technologie supérieure, Montréal, QuébecH3C 1K3, Canada
Louis Dufresne
Affiliation:
Laboratoire de thermo-fluide pour le transport, École de technologie supérieure, Montréal, QuébecH3C 1K3, Canada
*
†Email address for correspondence: [email protected]
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Abstract

Measurements of wall-pressure and velocity fluctuations are performed in a family of three incompressible, pressure-induced, turbulent separation bubbles (TSBs) of varying sizes, with an emphasis on the energetic low and medium frequencies. In all three cases the streamwise distribution of wall-pressure fluctuations shows a bi-modal character, with a first local maximum close to the position of maximum adverse pressure gradient and a second local maximum at the very end of the region of intermittent back flow. The first maximum is shown to be caused by the superposition of two separate phenomena occurring at approximately the same streamwise position: first, the pressure signature of a low-frequency contraction and expansion (breathing) of the TSBs, whose amplitude is shown to increase with the size of the separation bubble, and second, the effect of the adverse pressure gradient on the turbulent structures responsible for the pressure fluctuations in the attached boundary layer. The second maximum of the wall-pressure fluctuation coefficient also increases with the size of the TSB and is associated with the convection of large structures within the shear layer. Possible scaling laws are examined to show that both the local maximum Reynolds shear stress ${-\rho \overline {u'v'}_{max}}$ and the local maximum wall-normal stress ${\rho \overline {v'v'}_{max}}$ are adequate to scale the pressure fluctuations along the TSBs, with a better match when low frequencies are removed. Furthermore, a comparison with existing data from the literature illustrates the effects of Reynolds number and TSB size on the wall-pressure and velocity fluctuations. Finally, measurements in the spanwise direction demonstrate that, although corner effects strongly distort the average flow, the scaling of wall-pressure fluctuations with the turbulent stresses remains relatively unaffected. The present results provide new insights into the unsteady character of pressure-induced turbulent separation bubbles and their associated wall-pressure fluctuations.

JFM classification

Boundary Layers: Boundary layer separation Turbulent Flows: Turbulent boundary layers
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 902 , 10 November 2020 , A13
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Copyright
© The Author(s), 2020. Published by Cambridge University Press

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