The velocity gradient tensor for homogeneous, isotropic turbulence (HIT), with explicit consideration of local and non-local effects using a Schur decomposition

Abstract

A Schur decomposition of the velocity gradient tensor (VGT) for homogeneous, isotropic turbulence (HIT) is undertaken and its physical consequences examined. This decomposition permits the normal parts of the tensor (represented by the eigenvalues) to be separated explicitly from the non-normal effects. Given the restricted {E}uler approximation to the VGT dynamics is written in terms of the isotropic part of the pressure Hessian and the invariants of the characteristic equation of the VGT (in turn expressed in terms of the eigenvalues), the non-normal terms are related to the non-local aspects of the dynamics and the anisotropic part of the pressure Hessian. Using a direct numerical simulation of HIT, we show that the norm of the non-normal part of the tensor is of a similar order to the normal part, highlighting the importance of non-local effects. In fact, beneath the discriminant function in a Q-R plot, all enstrophy arises from the non-normal term, meaning that vorticity and intermediate strain eigenvector alignment in this region is an immediate consequence of non-normality. A non-normal term appears in the expressions for both enstrophy and total strain and cancels when calculating the second invariant of the VGT, while the self-amplification of non-normality and the normal straining of non-normality appear in the strain production and enstrophy production equations and cancel when calculating the third invariant. However, these terms are significant for understanding the full VGT dynamics, explaining how flow structures evolve to a disc-like state despite the strain eigenvalues sometimes indicating opposite (rod-like) behaviour, as well as explaining vorticity and strain alignments in HIT.