Energy dissipation in body-forced turbulence

Bounds on the bulk rate of energy dissipation in body-force-driven steady-state turbulence are derived directly from the incompressible Navier-Stokes equations. We consider flows in three spatial dimensions in the absence of boundaries and derive rigorous a priori estimates for the time-averaged energy dissipation rate per unit mass, epsilon, without making any further assumptions on the flows or turbulent fluctuations. We prove epsilon less than or equal to c(1)nu U-2/l(2) + c(2) U-3/l, where nu is the kinematic viscosity, U is the root-mean-square (space and time averaged) velocity, and l is the longest length scale in the applied forcing function. The prefactors c(1) and c(2) depend only on the functional shape of the body force and not on its magnitude or any other length scales in the force, the domain or the flow. We also derive a new lower bound on epsilon in terms of the magnitude of the driving force F. For large Grashof number Gr = Fl(3)/nu(2), we find c(3) nuFl/lambda(2) less than or equal to epsilon, where lambda = rootnuU(2)/epsilon is the Taylor microscale in the flow and the coefficient c(3) depends only on the shape of the body force. This estimate is seen to be sharp for particular forcing functions producing steady flows with lambda/l similar to O(1) as Gr --> infinity. We interpret both the upper and lower bounds on epsilon in terms of the conventional scaling theory of turbulence- where they are seen to be saturated-and discuss them in the context of experiments and direct numerical simulations.