In (computational) fluid dynamics there is a notion of mean flow that I don't quite understand:
E.g. in the Wikipedia entry for turbulence modeling it says:
Averaging the equations gives the Reynolds-averaged Navier–Stokes (RANS) equations, which govern the mean flow.
What is this mean flow?
Is it precisely the kind of flow you simulate by Reynolds-averaging? That is after all a type of "mean". Is there an abstract notion of mean flow in some simplification of the Navier-Stokes equations?
Is there either of those two: some very useful intuition behind this idea of mean flow and or a clear definition?