Local versus volume-integrated turbulence and mixing in breaking internal waves on slopes. (17th February 2017)

• Record Type: Journal Article
• Authors: Arthur, Robert S.; Koseff, Jeffrey R.; Fringer, Oliver B.
• Is Part Of: Journal of fluid mechanics. Volume 815(2017)
• Page Start: 169
• Page End: 198
• Publication Date: 2017-02-17
• Subjects: internal waves, -- topographic effects, -- turbulent mixing; Fluid mechanics -- Periodicals; 532.005
• DOI: 10.1017/jfm.2017.36
• Languages: English
• ISSNs: 0022-1120

Abstract

Abstract : Using direct numerical simulations (DNS), we explore local and volume-integrated measures of turbulence and mixing in breaking internal waves on slopes. We consider eight breaking wave cases with a range of normalized pycnocline thicknesses $k\unicode[STIX]{x1D6FF}$, where $k$ is the horizontal wavenumber and $\unicode[STIX]{x1D6FF}$ is the pycnocline thickness, but with similar incoming wave properties. The energetics of wave breaking is quantified in terms of local turbulent dissipation and irreversible mixing using the method of Scotti & White ( J. Fluid Mech., vol. 740, 2014, pp. 114–135). Local turbulent mixing efficiencies are calculated using the irreversible flux Richardson number $R_{f}^{\ast }$ and are found to be a function of the turbulent Froude number $Fr_{k}$ . Volume-integrated measures of the turbulent mixing efficiency during wave breaking are also made, and are found to be functions of $k\unicode[STIX]{x1D6FF}$ . The bulk turbulent mixing efficiency ranges from 0.25 to 0.37 and is maximized when $k\unicode[STIX]{x1D6FF}\approx 1$ . In order to connect local and bulk mixing efficiency measures, the variation in the bulk turbulent mixing efficiency with $k\unicode[STIX]{x1D6FF}$ is related to the turbulent Froude number at which the maximum total mixing occurs over the course of the breaking event, $Fr_{k}^{max}$ . We find that physically, $Fr_{k}^{max}$ is controlled by the vertical length scale of billows at the interface during wave breaking.