Abstract
The direct‐interaction approximation for turbulence is extended to shear flows and Boussinesq convection contained in regions of arbitrary size and shape. It is pointed out that the approximation provides a consistent generalization of mixing‐length concepts which continues to be applicable when the scale of inhomogeneity is comparable to the effective mixing length. The degree of faithfulness to be expected of the approximation is discussed on the basis of inferences from numerical results on isotropic turbulence.
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