Reynolds averaging
From CFD-Wiki
Reynolds averaging refers to the process of averaging a variable or an equation in time. Let 
be any dependent variable that varies in time. This variable can be decomposed into a fluctuating part, 
and an average part 
in the following way:
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(1) |
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Where 
is a long enough time to average out the typical fluctuations in .
Reynolds averaging is often used in fluid dynamics to separate turbulent fluctuations from the mean-flow. The term "Reynolds averaging" originates from Osborne Reynolds, who was the first to use this type of averaging in fluid dynamics.
