Local isotropy and the decay of turbulence in a stratified fluid

Abstract

The validity of the assumption of local isotropy is investigated using measurements of three orthogonal components of the turbulent velocity fields associated with initially high-Reynolds-number geophysical turbulence. The turbulent fields, generated by various large-scale internal motions caused by tidal flows over an estuarine sill, decay under the influence of stable mean density gradients. With measurements from sensors mounted on a submersible, we examine the evolution of spectral shapes and of ratios of cross-stream to streamwise components, as well as the degree of high-wavenumber universality, for the observational range of the parameter I[identical with] ks/kb = lb/ls. This ratio is a measure of separation between the Kolmogoroff wavenumber ks[identical with] (3)¼ [identical with] 2), and the buoyancy wavenumber kb [identical with] (N3//lb typical of scales at which the ambient stratification parameter N [identical with] (z/22/ may safely be estimated from a single measured component of the dissipation tensor, and also explore questions of appropriate low-wavenumber normalization for buoyancy-modified turbulence.