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James J. Riley
University of Washington, Seattle Washington

June 10, 2010
Foothills Laboratory 2, Room 1022
Lecture 10:30am

On Pathways to Turbulence in Strongly-Stratified Flows

Important issues in stable, strongly-stratified flows, often occurring in the atmosphere and oceans, are when and how "classical" 3D turbulence appears, and the properties of the resulting turbulence. In the oceans, at horizontal scales above a few meters, for example, buoyancy often exerts a dominant influence on the flows, and hence how turbulence is generated. One pathway to classical 3D turbulence in a strongly stratified flow is through the generation and breakdown of propagating internal waves. In this seminar another, possibly more general, pathway is proposed, that of "stratified turbulence"; these are motions at horizontal scales large enough to be dominated by buoyancy.
To address these flows very high resolution direct numerical simulations are utilized. The flows are initiated at low Froude number (buoyancy dominated) but with the Reynolds number as large as possible in order that smaller-scale, 3D turbulence can occur. It is found that strong, vertical shearing of the horizontal motions develops, resulting in intermittent, smaller-scale turbulence, and in a strong cascade of kinetic and potential energy to small scales. At length scales larger than the overturning scales, this appears to result in an inertial subrange in the horizontal, but not in the vertical, energy spectra. The subrange, distinct from the classical inertial subranges of Kolmogorov, Oboukov, and Corrsin, and strongly affected by the stable density stratification, is characterized by the dissipation rates of kinetic and potential energy. The results are shown to be consistent with some previously unexplained oceanographic and atmospheric field data.