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Efficient enhancement of turbulent entrainment by small-scale shear instability

Published online by Cambridge University Press:  31 May 2024

Tomoaki Watanabe Open the ORCID record for Tomoaki Watanabe [Opens in a new window]
Tomoaki Watanabe*
Affiliation:
Department of Mechanical Engineering and Science, Kyoto University, Kyoto 615-8540, Japan Education and Research Center for Flight Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan
*
†Email address for correspondence: watanabe.tomoaki.8x@kyoto-u.ac.jp
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Abstract

Turbulent entrainment is a process by which a locally turbulent region draws in an outer irrotational fluid. A large number of small-scale vortices and shear layers exist near the turbulent/non-turbulent interface; these features influence the local entrainment process. Direct numerical simulations of a turbulent front evolving into a quiescent flow without mean shear show that the entrainment rate is amplified by triggering the instability of small-scale shear layers via weak perturbations with a wavelength matching that of the unstable mode of the shear layers. Imposing artificial perturbations with a length scale approximately 30 times the Kolmogorov scale leads to the rapid collapse of small-scale shear layers due to instability, generating vortices near the turbulent/non-turbulent interface. Amplification of the entrainment rate is linked to the enlarged area and increased propagation velocity of the interface. The impact of perturbations on the entrainment rate becomes most pronounced when the flow evolves over approximately 7 times the Kolmogorov time scale, after which their influence diminishes over time. Additionally, the increase in entrainment rate is dictated by the ratio of the perturbation amplitude to the Kolmogorov velocity scale. The entrainment enhancement process is governed by Kolmogorov scales, suggesting that even weak perturbations can amplify the entrainment rate in high Reynolds number flows.

JFM classification

Wakes/Jets: Jets Wakes/Jets: Shear layers
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 988 , 10 June 2024 , A20
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Copyright
© The Author(s), 2024. Published by Cambridge University Press

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