dc.contributor.author Salehipour, H en dc.contributor.author Peltier, WR en dc.contributor.author Caulfield, Colm-cille en dc.date.accessioned 2018-12-01T00:31:14Z dc.date.available 2018-12-01T00:31:14Z dc.date.issued 2018-12-10 en dc.identifier.issn 0022-1120 dc.identifier.uri https://www.repository.cam.ac.uk/handle/1810/286216 dc.description.abstract 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). dc.description.sponsorship 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'. dc.publisher Cambridge University Press dc.title Self-organized criticality of turbulence in strongly stratified mixing layers en dc.type Article prism.endingPage 256 prism.publicationDate 2018 en prism.publicationName Journal of Fluid Mechanics en prism.startingPage 228 prism.volume 856 en dc.identifier.doi 10.17863/CAM.33527 dcterms.dateAccepted 2018-08-22 en rioxxterms.versionofrecord 10.1017/jfm.2018.695 en rioxxterms.licenseref.uri http://www.rioxx.net/licenses/all-rights-reserved en rioxxterms.licenseref.startdate 2018-12-10 en dc.contributor.orcid Caulfield, Colm-cille [0000-0002-3170-9480] dc.identifier.eissn 1469-7645 rioxxterms.type Journal Article/Review en pubs.funder-project-id EPSRC (EP/K034529/1) rioxxterms.freetoread.startdate 2019-04-02
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