Efficient enhancement of turbulent entrainment by small-scale shear instability

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...

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Bibliographic Details
Published in:Journal of fluid mechanics 2024-05, Vol.988, Article A20
Main Author: Watanabe, Tomoaki
Format: Article
Language:English
Subjects:
Amplification
Direct numerical simulation
Entrainment
Fluid flow
High Reynolds number
Instability
Interface stability
JFM Papers
Kelvin-Helmholtz instability
Perturbation
Perturbations
Propagation velocity
Reynolds number
Shear
Shear layers
Turbulent entrainment
Turbulent flow
Velocity
Vortices
Wavelength
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Summary: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.
ISSN:0022-1120
1469-7645
DOI:10.1017/jfm.2024.427