General CFD FAQ

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Physics

What is the Reynolds decomposition?

The Reynolds decomposition is used to separate the scales in a turublent flow and resolve the velocity field (or any other scalar field) as the sum of an average component and a fluctuating component. The time average of the fluctuating field is identically zero.

What is the difference between the Reynolds decomposition and LES filtering?

The Reynolds decomposition separates the velocity field into an average component and a fluctuating component. The time average of the fluctuating component is identically zero. In LES, a filtering operation is performed to decompose the velocity field into a filtered velocity field and a filtered residual field. The crucial difference between both methods is

• The filtered velocity field is a random variable while the average field (Reynolds) is not
• The time average of the residual field is generally nonzero while it is identically zero for fluctuating component in the Reynolds decomposition.

Numerics

What is the difference between FEM, FVM and FDM?

Describe differences between the finite volume method, the finite element method and the finite difference method...