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The structure and dynamics of the laminar separation bubble

Published online by Cambridge University Press:  05 November 2024

Eltayeb M. Eljack Open the ORCID record for Eltayeb M. Eljack [Opens in a new window]
Eltayeb M. Eljack*
Affiliation:
Mechanical Engineering Department, University of Khartoum, P.O. Box 321, Khartoum 11115, Sudan
*
†Email address for correspondence: eltayebeljack1@gmail.com
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Abstract

A novel selective mode decomposition, proper orthogonal decomposition and dynamic mode decomposition methods are used to analyse large-eddy simulation data of the flow field about a NACA0012 airfoil at low Reynolds numbers of $5\times 10^4$ and $9\times 10^4$, and at near-stall conditions. The objective of the analysis is to investigate the structure of the laminar separation bubble (LSB) and its associated low-frequency flow oscillation (LFO). It is shown that the flow field can be decomposed into three dominant flow modes: two low-frequency modes (LFO-Mode-1 and LFO-Mode-2) that govern an interplay of a triad of vortices and sustain the LFO phenomenon, and a high-frequency oscillating (HFO) mode featuring travelling Kelvin–Helmholtz waves along the wake of the airfoil. The structure and dynamics of the LSB depend on the energy content of these three dominant flow modes. At angles of attack lower than the stall angle of attack and above the angle of a full stall, the flow is dominated by the HFO mode. At angles of attack above the stall angle of attack the LFO-Mode-2 overtakes the HFO mode, triggers instability in the LSB and initiates the LFO phenomenon. Previous studies peg the structure, stability and bursting conditions of the separation bubble to local flow parameters. However, the amplitude of these local flow parameters is dependent on the energy content of the three dominant flow modes. Thus, the present work proposes a more robust bursting criterion that is based on global eigenmodes.

JFM classification

Boundary Layers: Boundary layer separation Instability: Transition to turbulence
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 998 , 10 November 2024 , A56
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Copyright
© The Author(s), 2024. Published by Cambridge University Press

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Supplementary material: File

Eljack supplementary movie

“Streamline patterns of the POD reconstruction of the fluctuating flow superimposed on colour maps of the spanwise vorticity (main panel), streamline patterns of the POD reconstruction of the instantaneous flow superimposed on the instantaneous streamwise velocity (top-left inset) and plot of time histories of the fluctuating lift and drag coefficients (bottom-right inset).”
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