Re: Is the momentum being transfered constantly at the universality domain?

Hi Lena,

In turbulent motion, kinetic energy is transferred from large to small scales, and eventually dissipated. This doesn't mean, however, that we have discovered non-conservation of momentum. Turbulent motions are, almost by definition, random, and so the cascade that transfers energy to smaller scales does not transfer any net momentum (remember, momentum is a vector quantity). When the kinetic is dissipated at the smallest scales, no net momentum is lost, and the overall momentum of the flow is conserved.

Turbulence acts to increase the random motions of a fluid, and so acts like molecular viscosity to increase the viscous stress. It does, therefore, enter into the momentum equation in a similar fashion. There is a pretty good introduction to all of this in the book "A First Course in Turbulence," by Tennekes and Lumley.