The Uniform Distortion of Homogeneous Turbulence

By passing the turbulence found downstream of a uniform grid through a constant-area distorting duct, the effect on isotropic turbulence of a soddenly imposed uniform mean rate of strain has been studied. A complete theory exists for the limiting case of an infinitely rapid strain, and the relevance of this theory to the experimental conditions is discussed. It is concluded that the rate of strain is too small to justify application of the theory, but that, for small strains, the structure of the energy-containing eddies is similar to that produced by an instantaneous strain. For larger total strains the turbulence develops an equilibrium structure in which the intensities are still changing, and the similarity of this structure to the structure of ordinary shear flows is considered. Finally it is found that, during the distortion, the dissipating eddies are not in a state of local isotropy, although the return to isotropy is rapid when the distortion ceases. This is interpreted as meaning that the direct influence of the mean rate of strain is appreciable at these Reynolds numbers, but that even highly anisotropic turbulence dissipates its energy through nearly isotropic eddies.