A Schur decomposition reveals the richness of structure in homogeneous, isotropic turbulence as a consequence of localised shear

Abstract: An improved understanding of turbulence is essential for the effective modelling and control of industrial and geophysical processes. Homogeneous, isotropic turbulence (HIT) is the archetypal field for developing turbulence physics theory. Based on the Schur transform, we introduce an additive decomposition of the velocity gradient tensor into a normal part (containing the eigenvalues) and a non-normal or shear-related tensor. We re-interrogate some key properties of HIT and show that the the tendency of the flow to form disc-like structures is not a property of the normal tensor; it emerges from an interaction with the non-normality. Also, the alignment between the vorticity vector and the second eigenvector of the strain tensor is another consequence of local shear processes.