11.   The Reynolds stress arises from the correlation of two components of the velocity fluctuation at the same point.

12.   In principle, of course, correlations involving the pressure fluctuations as well as the velocity fluctuations may be formulated.

13.   Ignoring a small contribution from the non-linear effects of the velocity fluctuations,

14.   Turbulent flows have much higher entrainment rates than the corresponding laminar flows, a fact that may be related to the additional dissipation by the velocity fluctuations.

15.   Associated with the velocity fluctuations of any turbulent motion are intense vorticity fluctuations.

16.   The flow outside the interface, although called non-turbulent, does involve velocity fluctuations.

17.   However, these velocity fluctuations are entirely irrotational and are dynamically quite different from turbulent fluctuations.

18.   The investigation of the flow past obstacles or of boundary layers requires a uniform flow with minimal velocity fluctuations.