1.   Simultaneously, they interact with the mean flow, so that the velocity profile also has three-dimensional variations.

2.   This term represents the action of the velocity fluctuations on the mean flow arising from the non-linearity of the Navier-Stokes equation.

3.   Writing the last two terms of as shows that the velocity fluctuations produce a stress on the mean flow.

4.   It is important to realize that T is a representation of the action of the turbulence on the mean flow and not a property of the fluid.

5.   Further understanding of the interaction between the mean flow and the fluctuations is obtained from the equation for the kinetic energy of the turbulence.

6.   The equation for the energy of the mean flow contains a corresponding term of opposite sign.

7.   The term thus represents a transfer of energy from the mean flow to the turbulence.