KINEMATIC ALIGNMENT EFFECTS IN TURBULENT FLOWS

It is shown that, in the neighborhood of a vortex whose maximum vorticity is large with respect to that in the surrounding flow, the two principal strains with the largest absolute values lie in the equatorial plane, so that the vorticity is automatically aligned to the intermediate eigenvector. This purely kinematic effect is offered as an explanation for the alignment properties recently reported for the velocity and pressure derivatives in turbulent flows. The model is compared with experimental evidence from numerical simulations. The observed ratio between the principal strains is also related to the properties of a two-dimensional Burgers' vortex.