Diffusion and heat conductivity within stars are shown to be substantially affected by fluid shear motions, even when these motions do not mix fluid elements (i.e., even when no fluid element wanders far from its equilibrium position). Internal gravity waves (g-modes) are one example of such a nonmixing flow. A formalism, based on Lagrangian flow coordinates, is developed to describe the effect in Boussinesq approximation. The diffusion of passive contaminants and of specific entropy is found to be generally enhanced, while the conduction of heat in stably stratified layers is suppressed (the fluid motions power a thermodynamic refrigerator). Explicit formulae are derived for flows which can be approximated as having spatially constant velocity gradients, and also for interval gravity waves. In the latter case the mechanical dissipation and thermodynamic efficiencies are calculated.
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Press et al. (1981) studied this question.