# General CFD FAQ

### From CFD-Wiki

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== Physics == | == Physics == | ||

=== What is the Reynolds decomposition? === | === What is the Reynolds decomposition? === | ||

- | The Reynolds decomposition is used to separate the scales in a turublent flow and resolve the velocity field as the sum of an average component and a fluctuating component. | + | 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? === | === What is the difference between the Reynolds decomposition and LES filtering? === |

## Revision as of 08:16, 5 December 2005

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## Contents |

## 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...