  The transport equations for velocity variances are investigated using data
from DNS of incompressible channel flows at $Re_\tau$ up to 5200. Each term in
the transport equation has been spectrally decomposed to expose the
contribution of turbulence at different length scales to the processes
governing the flow of energy in the wall-normal direction, in scale and among
components.
  The outer-layer turbulence is dominated by very large-scale streamwise
elongated modes. Away from the wall, production occurs primarily in these
large-scale streamwise-elongated modes in the streamwise velocity, but
dissipation occurs nearly isotropically in both velocity components and scale.
For this to happen, the energy is transferred from the streamwise elongated
modes to modes with a range of orientations through non-linear interactions,
and then transferred to other velocity components. This allows energy to be
transferred more-or-less isotropically from these large scales to the small
scales at which dissipation occurs. The VLSMs also transfer energy to the
wall-region resulting in a modulation of the autonomous near-wall dynamics.
  The near-wall energy flows are consistent with the well-known autonomous
near-wall dynamics. Through the overlap region between outer and inner layer
turbulence, there is a self-similar structure to the energy flows. The VLSM
production occurs at spanwise scales that grow with $y$. There is transport of
energy away from the wall over a range of scales that grows with $y$. And,
there is transfer of energy to small dissipative scales which grow like
$y^{1/4}$. Finally, the small-scale near-wall processes characterised by
wavelengths less than 1000 wall units are largely Reynolds number independent,
while the larger-scale outer layer process are strongly Reynolds number
dependent. The interaction between them appears to be relatively simple.
