  The rate of energy dissipation in solutions of the body-forced 3-d
incompressible Navier-Stokes equations is rigorously estimated with a focus on
its dependence on the nature of the driving force. For square integrable body
forces the high Reynolds number (low viscosity) upper bound on the dissipation
is independent of the viscosity, consistent with the existence of a
conventional turbulent energy cascade. On the other hand when the body force is
not square integrable, i.e., when the Fourier spectrum of the force decays
sufficiently slowly at high wavenumbers, there is significant direct driving at
a broad range of spatial scales. Then the upper limit for the dissipation rate
may diverge at high Reynolds numbers, consistent with recent experimental and
computational studies of "fractal-forced'' turbulence.
