Chapter Ten - Turbulence in stably stratified fluids

Abstract

In this chapter, the effects of stable density stratification on turbulence are discussed, using the examples of the wake of a sphere and the decay of homogeneous turbulence. To better understand the dynamics, elements of the theory of internal gravity waves, and of the stability of stratified shear flows, are presented. It is found that the flows could be described as being in one of three distinct flow regimes: a three-dimensional turbulence regime, a quasi-equilibrium or stratified turbulence regime, and a quasi-two-dimensional regime. It is suggested that these regimes could be understood by considering two local parameters: a Froude number based upon a horizontal integral scale, and Gn, the Buoyancy Reynolds number. Various features of turbulent wakes are presented, including the generation of internal waves by the wake and the formation of quasi-horizontal vortices at later times into the quasi-two-dimensional regime.