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From CFD-Wiki

The turbulence length scale, , is a physical quantity describing the size of the large energy-containing eddiesin a turbulent flow.

The turbulent length scale is often used to estimate the turbulent properties on the inlets of a CFDsimulation. Since the turbulent length scale is a quantity which is intuitively easy to relate to the physical sizeof the problem it is easy to guess a reasonable value of the turbulent length scale. The turbulent length scaleshould normally not be larger than the dimension of the problem, since that would mean that the turbulenteddies are larger than the problem size.

In the k-epsilon model the turbulent length scale can be computed as:

Please note that some CFD codes (Fluent, Phoenics and CFD-ACE for example), use a different length scaledefinition based on the mixing-length. Then the following formula should be used instead:

This means that the turbulence length-scale variable used in these codes is about two times larger than thelength-scale variable used in other codes.

is a model constant which in the standard version of the k-epsilon model has a value of 0.09.

Turbulence length scale -- CFD-Wiki, the free CFD reference http://www.cfd-online.com/Wiki/Turbulence_length_scale

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It is common to set the turbulence length scale to a certain percentage of a typical dimension of the problem.For example, at the inlet to a turbine stage a typical turbulence length scale could be say 5% of the channelheight. In grid-generated turbulence the turbulence length scale is often set to something close to the size ofthe grid bars.

Fully developed pipe flow

In pipe flows the turbulence length scale can be estimated from the hydraulic diameter. In fully developedpipe flow the turbulence length scale is ~3.8% of the hydraulic diameter (in the case of a circular pipe thehydraulic diameter is the same as the diameter of the pipe). Hence:

Where is the hydraulic diameter. For codes using a turbulence length-scale based on the mixing-length(Fluent, Phoenics and CFD-ACE for example) replace 0.038 and 3.8% with 0.07 and 7%.

Wall-bounded inlet flows

When the inlet flow is bounded by walls with turbulent boundary layers, the turbulence length scale can beestimated (approximately) from the inlet boundary layer thickness. Set to 0.22 of the inlet boundary layerthickness. For codes using a turbulence length-scale based on the mixing-length (Fluent, Phoenics andCFD-ACE for example) use 0.4 of the inlet boundary layer thickness.

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This page was last modified on 15 June 2012, at 19:56.Content is available under GNU Free Documentation License 1.2.

Turbulence length scale -- CFD-Wiki, the free CFD reference http://www.cfd-online.com/Wiki/Turbulence_length_scale

2 sur 2 07/02/2014 11:09