The Role of Scale in the Development and Evolution of Stratified Shear Turbulence, Entrainment and Mixing
Abstract
Project Summary: The Role of Scale in the Development and Evolution of Stratified Shear Turbulence, Entrainment and Mixing PIs: D. MacDonald/M. Raessi, UMass Dartmouth 15 October 2014 Much of the recent theoretical and mechanistic work regarding turbulence in stratified-shear flows has been performed at the laboratory scale (e.g. Ellison and Turner 1959; Thorpe 1973; Yuan and Horner-Devine 2013), or using direct numerical simulations (DNS) which are confined by computational limits to low Reynolds number flows, similar in scale to laboratory experiments (e.g. Smyth et al 2001, Bartello and Tobias 2013). These efforts have resulted in well-tested relationships for entrainment relative to velocity shear as a function of a bulk Richardson number ( , where is a reduced gravity, h is the thickness of the shear layer, and the velocity difference across the layer). The study of turbulence in the ocean, or at other geophysical scales, where Reynolds numbers ( ) are orders of magnitude higher than in the laboratory, presents measurement and observational challenges which prohibit the fine scale evaluation of turbulent structure and dynamics possible in the laboratory or with DNS. As such, the results of laboratory and DNS modeling are often used to inform studies of geophysical scale turbulence, without adequately characterizing the vast scale differences. Recent comparisons with data collected in strong stratified-shear regions at geophysical scales suggest that geophysical turbulence is several orders of magnitude lower in intensity than laboratory scale turbulence at similar RiB values. These comparisons can be difficult to make, however, as laboratory studies often measure entrainment, while field studies typically measure energetic quantities, such as TKE dissipation, or buoyancy flux. The objective of this proposal is to develop better methods to relate fundamental turbulent quantities such as entrainment and buoyancy flux, and to better understand what processes affect turbulent structure and entrainment across several orders of magnitude of Re parameter space. We hypothesize that RiB alone cannot adequately describe stratified-shear turbulence, but that energetic and physical scales, represented by Re, are critically important. The rationale for the proposed research is that once we understand the linkages of turbulence across scales, laboratory and DNS models can be better utilized to inform studies of geophysical scale turbulence. The project is focused around two specific objectives: 1. Develop relationships between entrainment and buoyancy flux/TKE dissipation rate Although seemingly straightforward, the relationship between entrainment and buoyancy flux relies on a number of assumptions and/or empirical coefficients relating entrainment velocities to turbulent velocities. Analytical relationships will be developed and evaluated using both field and laboratory data, and DNS simulations. 2. Determine the scale dependence of turbulent properties As values of Re in a stratified-shear flow increase, the relative size of the characteristic turbulent lengthscale may decrease, potentially resulting in reduced entrainment and/or turbulent buoyancy flux. Once effective relationships are developed in (1) above, the nature of these scale-dependent processes can be effectively explored, again using both field/laboratory data and numerical models. We will pursue these objectives using existing data (both published and unpublished) from various oceanographic environments and laboratory studies. We will also utilize targeted DNS modeling efforts.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 12, 2016
- Source ID
- N000141512456
Entities
People
- Daniel Macdonald
Organizations
- Office of Naval Research
- United States Navy
- University of Massachusetts