  The tendency of turbulent flows to produce fine-scale motions from
large-scale energy injection is often viewed as a scale-wise cascade of kinetic
energy driven by vorticity stretching. This has been recently evaluated by an
exact, spatially-local relationship [Johnson, P. L. 2020, Phys. Rev. Lett.,
\textbf{124}, 104501], which also highlights the contribution of strain
self-amplification. In this paper, the role of these two mechanisms is explored
in more detail. Vorticity stretching and strain amplification interactions
between velocity gradients filtered at the same scale account for about half of
the energy cascade rate, directly connecting restricted Euler dynamics to the
energy cascade. Multiscale strain amplification and vorticity stretching are
equally important, however, and more closely resemble eddy viscosity physics.
Moreover, ensuing evidence of a power-law decay of energy transfer
contributions from disparate scales supports the notion of an energy cascade,
albeit a `leaky' one. Besides vorticity stretching and strain
self-amplification, a third mechanism of energy transfer is introduced and
related to the vortex thinning mechanism important for the inverse cascade in
two dimensions. Simulation results indicate this mechanism also provides a net
source of backscatter in three-dimensional turbulence, in the range of scales
associated with the bottleneck effect. Taken together, these results provide a
rich set of implications for large-eddy simulation modeling.
