# Turbulence length scale

(Difference between revisions)
 Revision as of 21:08, 23 March 2006 (view source)Jola (Talk | contribs)m (Turbulent lenght-scale moved to Turbulent length-scale)← Older edit Revision as of 10:53, 24 March 2006 (view source)Jola (Talk | contribs) m (spelling corrections)Newer edit → Line 1: Line 1: - The turbulence length scale, $l$ , is a physical quantity describing the size of the large energy containing eddies in a turbulent flow. + The turbulence length-scale, $l$ , is a physical quantity describing the size of the large energy containing eddies in a turbulent flow. - The turbulent lenght-scale is often used to estimate the turbulent properties on the inlets of a CFD simulation. Since the turbulent lenght-scale is a quantity which is intuitively easy to relate to the physical size of the problem it is easy to guess a reasonable value of the turbulent lenght-scale. The turbulent lenght-scale should normally not be larger than the dimension of the problem, since that would mean that the turbulent eddies are larger than the problem size. + The turbulent length-scale is often used to estimate the turbulent properties on the inlets of a CFD simulation. Since the turbulent length-scale is a quantity which is intuitively easy to relate to the physical size of the problem it is easy to guess a reasonable value of the turbulent length-scale. The turbulent length-scale should normally not be larger than the dimension of the problem, since that would mean that the turbulent eddies are larger than the problem size. - In the [[Standard k-epsilon model|k-epsilon model]] the turbulent lenghts-scale can be computed as: + In the [[Standard k-epsilon model|k-epsilon model]] the turbulent length-scale can be computed as: :$l = C_\mu \, \frac{k^\frac{3}{2}}{\epsilon}$ :$l = C_\mu \, \frac{k^\frac{3}{2}}{\epsilon}$ Line 9: Line 9: $C_\mu$ is a model constant which in the standard version of the k-epsilon model has a value of 0.09. $C_\mu$ is a model constant which in the standard version of the k-epsilon model has a value of 0.09. - ==Estimating the turbulent lenght-scale== + ==Estimating the turbulent length-scale== - It is common to set the turbulent lenght-scale to a certain percentage of a typical dimension of the problem. For example, at the inlet to a turbine stage a typical turbulent lenght-scale could be say 5% of the channel height. In grid-generated turbulence the turbulent length-scale is often set to something close to the size of the grid bars. In pipe-flows the turbulent lenght-scale can be estimated from the [[hydraulic diameter]]. In fully developed pipe-flow the turbulent lenght-scale is 7% of the hydraulic diamater (in the case of a circular pipe the hydraulic diameter is the same as the diameter of the pipe) + It is common to set the turbulent length-scale to a certain percentage of a typical dimension of the problem. For example, at the inlet to a turbine stage a typical turbulent length-scale could be say 5% of the channel height. In grid-generated turbulence the turbulent length-scale is often set to something close to the size of the grid bars. In pipe-flows the turbulent length-scale can be estimated from the [[hydraulic diameter]]. In fully developed pipe-flow the turbulent length-scale is 7% of the hydraulic diamater (in the case of a circular pipe the hydraulic diameter is the same as the diameter of the pipe)

## Revision as of 10:53, 24 March 2006

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

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

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

$l = C_\mu \, \frac{k^\frac{3}{2}}{\epsilon}$

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

## Estimating the turbulent length-scale

It is common to set the turbulent length-scale to a certain percentage of a typical dimension of the problem. For example, at the inlet to a turbine stage a typical turbulent length-scale could be say 5% of the channel height. In grid-generated turbulence the turbulent length-scale is often set to something close to the size of the grid bars. In pipe-flows the turbulent length-scale can be estimated from the hydraulic diameter. In fully developed pipe-flow the turbulent length-scale is 7% of the hydraulic diamater (in the case of a circular pipe the hydraulic diameter is the same as the diameter of the pipe)