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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Where is a long enough time to average out the 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.