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Derek D. Stretch James W. Rottman S.Karan Venayagamoorthy Keiko K. Nomura Chris R. Rehmann 《Dynamics of Atmospheres and Oceans》2010
Mixing efficiency in stratified flows is a measure of the proportion of turbulent kinetic energy that goes into increasing the potential energy of the fluid by irreversible mixing. In this research direct numerical simulations (DNS) and rapid distortion theory (RDT) calculations of transient turbulent mixing events are carried out in order to study this aspect of mixing. In particular, DNS and RDT of decaying, homogeneous, stably-stratified turbulence are used to determine the mixing efficiency as a function of the initial turbulence Richardson number Rit0=(NL0/u0)2, where N is the buoyancy frequency and L0 and u0 are initial length and velocity scales of the turbulence. The results show that the mixing efficiency increases with increasing Rit0 for small Rit0, but for larger Rit0 the mixing efficiency becomes approximately constant. These results are compared with data from towed grid experiments. There is qualitative agreement between the DNS results and the available experimental data, but significant quantitative discrepancies. The grid turbulence experiments suggest a maximum mixing efficiency (at large Rit0) of about 6%, while the DNS and RDT results give about 30%. We consider two possible reasons for this discrepancy: Prandtl number effects and non-matching initial conditions. We conclude that the main source of the disagreement probably is due to inaccuracy in determining the initial turbulence energy input in the case of the grid turbulence experiments. 相似文献
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Peter C. Smith 《Dynamics of Atmospheres and Oceans》1977,1(3):241-272
The dynamics of oceanic bottom currents are examined both theoretically and in the laboratory. A class of similarity solutions for steady flow indicates that the geostrophic current is drained by Ekman flux at its downslope edge and ultimately extinguished at a downstream distance of order magnified by .Laboratory source flows are found to be consistently wave-like. Nevertheless, certain gross features of the steady Ekman flux mechanism are observed. Instabilities are classified according to the magnitudes of Rossby (?) and Ekman (E) numbers for the steady flow scaling. Characteristic were forms include: (1) a meandering jet (E < ?, 10?2); (2) transverse waves on broad contour current (? < E, 10?2); and (3) transverse waves on a viscous flow (?, E > 10?2). The dispersion relation for source flow waves resembles that of baroclinic instabilities for a uniform two-layer channel flow, and an empirically determined stability boundary is in rough agreement with the inviscid channel flow criterion.An interpretation of field measurements from the Denmark Strait Overflow in terms of the laboratory results is presented. 相似文献