  The turbulent boundary-layer is a region where both preferential dissipation
of energy and the production of significant vorticity arises as a consequence
of the strong velocity gradients. Previous work has shown that, following a
Reynolds decomposition of the enstrophy production, the purely fluctuating
contribution is the dominant term and that near the wall this varies in a
complex manner with height. In this study we additionally decompose the strain
rate and vorticity terms into normal and non-normal components using a Schur
decomposition and are able to explain all these features in terms of
contributions at different heights from constituents involving different
combinations of normal and non-normal quantities. What is surprising about our
results is that while the mean shear and the action of larger scale structures
should mean that non-normal effects are of over-riding importance, the most
important individual term involves the fluctuating, normal straining in the
transverse direction. Furthermore, the reason that the term that involves only
non-normal contributions is smaller on average than that involving normal
straining coupled to non-normal vorticity is that in the former case there are
individual constituents that are negative in the mean. Hence, we not only
explain the nature of near-wall enstrophy production in greater detail, but
highlight how local straining that is orthogonal to the direction of the
dominant mean and fluctuating shear plays a crucial role in amplifying
vorticity that is yet to have developed sufficiently to gain a solid body
rotational component.
