Direct numerical simulations of stablystratified sheared turbulence: Implications for oceanic mixing
Abstract
Direct numerical simulations of the time evolution of homogeneous stably stratified turbulent shear flows have been performed for several Richardson numbers Ri and Reynolds numbers R(sub lambda) in earlier works. The results show excellent agreement with length scale models developed from laboratory experiments to characterize oceanic turbulence. When the Richardson number Ri is less than the stationary value Ri(sub s), the turbulence intensity grows at all scales, and the growth rate appears to be a function of Ri. The size of the vertical density inversions also increases. On the other hand, when Ri is greater than or equal to Ri(sub s) the largest turbulent eddies become vertically constrained by buoyancy when the Ellison (turbulence) scale L(sub E) and the Ozmidov (buoyancy) scale L(sub O) are equal. At this point, the mixing efficiency is maximal and corresponds to a flux Richardson number R(sub f) = 0.20. The vertical mass flux becomes countergradient when epsilon = 19(nu)N(exp 2) and vertical density overturns are suppressed in less than half a BruntVaisala period. The results of the simulations were also recast in terms of the Hydrodynamic Phase Diagram introduced in fossil turbulence models. The socalled point of fossilization occurs when epsilon = 4DCN(exp 2); Gibson proposed 13DCN(exp 2). This value is in agreement with indirect laboratory observations and field observations. Finally, the validity of the steadystate models to estimate vertical eddy diffusivities in the oceanic thermocline is discussed.
 Publication:

Studying Turbulence Using Numerical Simulation Databases. 3: Proceedings of the 1990 Summer Program
 Pub Date:
 December 1990
 Bibcode:
 1990stun.proc..163I
 Keywords:

 Reynolds Number;
 Richardson Number;
 Scale Models;
 Shear Flow;
 Turbulence;
 Turbulent Flow;
 Vortices;
 BruntVaisala Frequency;
 Buoyancy;
 Hydrodynamics;
 Thermoclines;
 Turbulence Models;
 Fluid Mechanics and Heat Transfer