The theme of this chapter will be a more detailed discussion of various kinds of turbulent shear flows, which are (at least at the beginning of the period of interest) well past the state of marginal stability. Some knowledge of the properties of a turbulent shear flow in a homogeneous fluid must be assumed (see, for example, Townsend 1956), and we consider here the additional effects introduced by the presence of density gradients. Turbulent flows in which gravity plays an essential role in driving the mean motion (e.g. turbulent gravity currents) will be treated separately in chapter 6.

These flows will be discussed against the background of the classification introduced in §4.3.1, emphasizing the turbulent features, but also referring to the wave aspects when necessary. We consider first a shear flow near a horizontal boundary, and the effect of a vertical density gradient on the velocity and density profiles, and on the rates of transport. Next we discuss the few theoretical results which are available to describe ‘boundary’ and ‘interior’ turbulence in stratified shear flows. Finally, we present and discuss laboratory and larger scale observations which can be used to test these ideas.

Velocity and density profiles near a horizontal boundary

The most important example of a shear flow near a boundary is the wind near the ground, in a shallow enough layer for the effects of the earth's rotation to be ignored.