One of the most common strategies to model a turbulent fluid flow is to attempt to model the average, or mean flow field, by expanding the velocity field into an average and a fluctuating component. This video introduces this idea of "Reynolds averaging" and shows how to obtain a mass continuity equation for the mean flow.
Citable link for this video: https://doi.org/10.52843/cassyni.tcxvxy
Check out the excellent notes by Lex Smits: http://profs.sci.univr.it/~zuccher/downloads/FD-MAE553-Smits.pdf
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