The effects of boundary proximity on Kelvin–Helmholtz instability and turbulence
Journal
Journal of Fluid Mechanics
Series/Report No.
Journal of Fluid Mechanics
Journal Volume
966
Start Page
A2
ISSN
0022-1120
1469-7645
Date Issued
2023-06-26
Author(s)
Abstract
Studies of Kelvin-Helmholtz (KH) instability have typically modelled the initial flow as an isolated shear layer. In geophysical cases, however, the instability often occurs near boundaries and may therefore be influenced by boundary proximity effects. Ensembles of direct numerical simulations are conducted to understand the effect of boundary proximity on the evolution of the instability and the resulting turbulence. Ensemble averages are used to reduce sensitivity to small variations in initial conditions. Both the transition to turbulence and the resulting turbulent mixing are modified when the shear layer is near a boundary: the time scales for the onset of instability and turbulence are longer, and the height of the KH billow is reduced. Subharmonic instability is suppressed by the boundary because phase lock is prevented due to the diverging phase speeds of the KH and subharmonic modes. In addition, the disruptive influence of three-dimensional secondary instabilities on pairing is more profound as the two events coincide more closely. When the shear layer is far from the boundary, the shear-aligned convective instability is dominant; however, secondary central-core instability takes over when the shear layer is close to the boundary, providing an alternate route for the transition to turbulence. Both the efficiency of the resulting mixing and the turbulent diffusivity are dramatically reduced by boundary proximity effects.
Subjects
shear layer turbulence
shear-flow instability
turbulent mixing
Publisher
Cambridge University Press (CUP)
Type
journal article