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Three-dimensional transition and force characteristics of low-Reynolds-number flows past a plunging airfoil

Published online by Cambridge University Press:  23 October 2023

An-Kang Gao
Affiliation:
Department of Modern Mechanics, University of Science and Technology of China, Hefei, Anhui 230026, PR China Department of Aeronautics, Imperial College London, London SW7 2AZ, UK
Chris D. Cantwell
Affiliation:
Department of Aeronautics, Imperial College London, London SW7 2AZ, UK
Onur Son
Affiliation:
Department of Mechanical Engineering, University of Bath, Bath BA2 7AY, UK Department of Aeronautical Engineering, Gebze Technical University, Kocaeli 41400, Turkey
Spencer J. Sherwin*
Affiliation:
Department of Aeronautics, Imperial College London, London SW7 2AZ, UK
*
Email address for correspondence: jfluidmech@imperial.ac.uk

Abstract

The three-dimensional (3-D) transition of the leading-edge vortex (LEV) and the force characteristics of the plunging airfoil are investigated in the chord-based Strouhal number $St_c$ range of 0.10 to 1.0 by means of experimental measurements, numerical simulations and linear stability analysis in order to understand the spanwise instabilities and the effects on the force. We find that the interaction pattern of the LEV, the LEV from a previous cycle (pLEV) and the trailing-edge vortex (TEV) is the primary mechanism that affects the 3-D transition and associated force characteristics. For $St_c \leq 0.16$, the 3-D transition is dominated by the LEV–TEV interaction. For $0.16 < St_c \leq 0.44$, the TEV lies in the middle of the LEV and the pLEV and therefore vortex interaction between them is relatively weak; as a result, the LEV remains two-dimensional up to a relatively high Reynolds number of $Re = 4000$ at $St_c = 0.32$. For $0.44 < St_{c} \leq 0.54$, and at relatively low Reynolds numbers, the pLEV and the TEV tend to form a clockwise vortex pair, which is beneficial for the high lift and stability of the LEV. For $0.49 \leq St_c$, the pLEV and TEV tend to form an anticlockwise vortex pair, which is detrimental to the lift and flow stability. In the last $St_c$ range, vortex interaction involving the LEV, the TEV and the pLEV results in an unstable period-doubling mode which has a wavelength of about two chord-lengths and the 3-D transition enhances the lift.

Type
JFM Papers
Copyright
© The Author(s), 2023. Published by Cambridge University Press

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