  A temporal study of energy transfer across length scales is performed in 3D
numerical simulations of homogeneous shear flow and isotropic turbulence. The
average time taken by perturbations in the energy flux to travel between scales
is measured and shown to be additive. Our data suggests that the propagation of
disturbances in the energy flux is independent of the forcing and that it
defines a `velocity' that determines the energy flux itself. These results
support that the cascade is, on average, a scale-local process where energy is
continuously transmitted from one scale to the next in order of decreasing
size.
