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Three-dimensional low-frequency dynamics of a turbulent separation bubble

Published online by Cambridge University Press:  19 September 2025

Ben Steinfurth Open the ORCID record for Ben Steinfurth [Opens in a new window] ,
Mogeng Li Open the ORCID record for Mogeng Li [Opens in a new window] ,
Fulvio Scarano Open the ORCID record for Fulvio Scarano [Opens in a new window]  and
Julien Weiss Open the ORCID record for Julien Weiss [Opens in a new window]
Ben Steinfurth*
Affiliation:
Chair of Aerodynamics, Technische Universität Berlin, Berlin 10587, Germany
Mogeng Li
Affiliation:
Aerodynamics Section, Delft University of Technology, HS Delft 2629, The Netherlands School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney, Darlington 2008, NSW, Australia
Fulvio Scarano
Affiliation:
Aerodynamics Section, Delft University of Technology, HS Delft 2629, The Netherlands
Julien Weiss
Affiliation:
Chair of Aerodynamics, Technische Universität Berlin, Berlin 10587, Germany
*
Corresponding author: Ben Steinfurth, ben.steinfurth@tu-berlin.de
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Abstract

Turbulent separating and reattaching flows are known to exhibit low-frequency fluctuations manifested in a large-scale contraction and expansion of the reverse-flow region. Previous experimental investigations have been restricted to planar measurements, while the computational cost to resolve the low-frequency spectrum with high-fidelity simulations currently appears to be unaffordable. In this article, we make use of volumetric measurements to reveal the low-frequency dynamics of a turbulent separation bubble (TSB) formed in the fully turbulent flow past a smooth backward-facing ramp. The volumetric velocity field measurements cover the entire separated flow region over a domain with a spanwise extent of $S=0.6\, {\textrm{m}}$. Spectral proper orthogonal decomposition (SPOD) of the velocity fluctuations reveals low-rank low-frequency behaviour at Strouhal numbers ${\textit{St}}\lt 0.05$, which was also observed in previous planar measurements. However, in contrast with the interpretation of a two-dimensional contraction/expansion motion, the low-frequency dynamics is shown to be inherently three-dimensional, and governed by large elongated structures with a spanwise wavelength of approximately $S/2$. A low-order model constructed with the leading SPOD mode confirms substantial changes of the TSB extent in the centre plane, linking it to the modal pattern that is strongly non-uniform in the spanwise direction. The findings presented in this study promote a more complete understanding of the low-frequency dynamics in turbulent separated flows, thereby enabling novel modelling and control approaches.

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Type
JFM Rapids
Information
Journal of Fluid Mechanics , Volume 1019 , 25 September 2025 , R4
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Copyright
© The Author(s), 2025. Published by Cambridge University Press

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Supplementary material: File

Steinfurth et al. supplementary movie

Low-order model of low-frequency dynamics, (a) streamwise velocity component inside the symmetry plane, (b) three-dimensional view with selected slices and iso-surface indicating $U^\unicode{x221E} = 0 m/s$.
Download Steinfurth et al. supplementary movie(File)
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