Self-organized criticality of turbulence in strongly stratified mixing layers
Journal of Fluid Mechanics
Cambridge University Press
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Salehipour, H., Peltier, W., & Caulfield, C. (2018). Self-organized criticality of turbulence in strongly stratified mixing layers. Journal of Fluid Mechanics, 856 228-256. https://doi.org/10.1017/jfm.2018.695
Motivated by the importance of stratified shear flows in geophysical and environmental circumstances, we characterize their energetics, mixing and spectral behavior through a series of direct numerical simulations of turbulence generated by Holmboe wave instability (HWI) under various initial conditions. We focus on circumstances where the stratification is sufficiently `strong' so that HWI is the dominant primary instability of the flow. Our numerical findings demonstrate the emergence of self-organised criticality (SOC) that is manifest as an adjustment of an appropriately defined gradient Richardson number, $Ri_g$, associated with the horizontally-averaged mean flow, in such a way that it is continuously attracted towards a critical value of $Ri_g \sim 1/4$. This self-organization occurs through a continuously reinforced localisation of the `scouring' motions (i.e. `avalanches') that are characteristic of the turbulence induced by the break down of Holmboe wave instabilities and are developed on the upper and lower flanks of the sharply localized density interface, embedded within a much more diffuse shear layer. These localised `avalanches' are also found to exhibit the expected scale invariant characteristics. From an energetics perspective, the emergence of SOC is expressed in the form of a long-lived turbulent flow that remains in a `quasi-equilibrium' state for an extended period of time. Most importantly, the irreversible mixing that results from such self-organised behavior appears to be characterized generically by a universal cumulative turbulent flux coefficient of $\Gamma_c \sim 0.2$ only for turbulent flows engendered by Holmboe wave instability. The existence of this self-organised critical state corroborates the original physical arguments associated with self-regulation of stratified turbulent flows as involving a `kind of equilibrium' as described by Turner (1973).
H.S. acknowledges the SOSCIP TalentEdge post doctoral fellowship and is grateful to the David Crighton Fellowship from D.A.M.T.P., University of Cambridge. All the computations were performed on the BG/Q supercomputer at the University of Toronto which is operated by SciNet for the Southern Ontario Smart Computing Innovation Platform. SciNet is funded by: the Canada Foundation for Innovation under the auspices of Compute Canada; the Government of Ontario; Ontario Research Fund - Research Excellence; and the University of Toronto. The research of W.R.P. at the University of Toronto is sponsored by NSERC Discovery Grant A9627. The research activity of C.P.C. is supported by EPSRC Programme Grant EP/K034529/1 entitled `Mathematical Underpinning of Stratified Turbulence'.
External DOI: https://doi.org/10.1017/jfm.2018.695
This record's URL: https://www.repository.cam.ac.uk/handle/1810/286216