SEMINAR: Dynamics of a tidallyforced stratified shear flow on the continental slope


Dynamics of a tidallyforced stratified shear flow on the continental slope : SESE and Oceans Institute Seminar 
Other events...

The energy contained in largescale ocean flows is dissipated in smallscale turbulent motions and these control the rate at which heat, momentum, chemicals, nutrients, and biological matter are stirred in the ocean. On the global scale, a large proportion of the mechanical energy contained in the ocean’s tides is converted to internal wave energy that can propagate large distances before ultimately dissipating particularly near bottom boundaries where enhanced mixing occurs due to bottominduced friction. These complexities make both the study of turbulence in the ocean and the development of mixing models for the global ocean particularly challenging.
The first part of my presentation examines how to rigorously estimate turbulence properties, in particular, the rate of dissipation of turbulent kinetic energy, from moored field observations. I developed a methodology that takes into consideration the sampling program, the instruments’ capabilities, and the flow characteristics. Notably, the method considers both the effects of mean flow shear and density stratification on turbulence spectral properties in a systematic and robust way, making the method applicable for a vast range of environmental flows.
The second part of my presentation examines the results of applying this methodology to nearbottom observations from the continental slope on Australia’s NorthWest Shelf. Internal bores propagate up slope through the site, generating strong shear and intensified nearbed currents (>6 times background tidal currents) in a highly unsteady environment. These bores are associated with large isotherm displacements and enhanced turbulent dissipation. The observations of the mean and turbulent flows demonstrate that idealized laws, often used in ocean mixing models, cannot describe the vertical extent of turbulent overturns and the induced mixing.
In the final part of my presentation, I use the turbulence properties and mixing rates derived from the above field observations to assess various mixing models. Despite the high mixing rates observed (>200300 times molecular rates), the mixing efficiency was of the order of 1%. This analysis demonstrates that using a constant mixing efficiency of 20%, which is customary in the oceanic community, overpredicts the mixing rate by more than an order of magnitude. I demonstrate that mixing rate predictions are improved when the reduction in mixing efficiency with increasing turbulence intensity is taken into account, implying that oceanographic numerical models need to adopt a variable mixing efficiency to accurately predict flow dynamics in energetic regions.
Speaker(s) 
Cynthia Bluteau, PhD Thesis Defence, SESE and OI

Location 
Blakers Lecture Theatre, Mathematics Building


Contact 
Lorraine Dorn
<[email protected]>
: 3701

URL 
https://www.sese.uwa.edu.au

Start 
Thu, 22 Sep 2011 16:00

End 
Thu, 22 Sep 2011 17:00

Submitted by 
Lorraine Dorn <[email protected]>

Last Updated 
Fri, 23 Sep 2011 15:50

Included in the following Calendars: 

 Locations of venues on the Crawley and Nedlands campuses are
available via the Campus Maps website.
 Download this event as:
Text 
iCalendar

Mail this event:
