  The three dimensional optimal energy growth mechanism, in plane parallel
shear flows, is reexamined in terms of the role of vortex stretching and the
interplay between the span-wise vorticity and the planar divergent components.
For high Reynolds numbers the structure of the optimal perturbations in
Couette, Poiseuille, and mixing layer shear profiles is robust and resembles
localized plane-waves in regions where the background shear is large. The waves
are tilted with the shear when the span-wise vorticity and the planar
divergence fields are in (out of) phase when the background shear is positive
(negative). A minimal model is derived to explain how this configuration
enables simultaneous growth of the two fields, and how this mutual
amplification reflects on the optimal energy growth. This perspective provides
an understanding of the three dimensional growth solely from the two
dimensional dynamics on the shear plane.
