Turbulence length scale
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The turbulence length scale, <math>l</math> , is a physical quantity describing the size of the large energy containing eddies in a turbulent flow. | The turbulence length scale, <math>l</math> , 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 | + | 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. |
| - | In the [[Standard k-epsilon model|k-epsilon model] the turbulent lenghts-scale can be | + | In the [[Standard k-epsilon model|k-epsilon model]] the turbulent lenghts-scale can be computed as: |
:<math>l = C_\mu \, \frac{k^\frac{3}{2}}{\epsilon}</math> | :<math>l = C_\mu \, \frac{k^\frac{3}{2}}{\epsilon}</math> | ||
| - | Where :<math> | + | Where :<math>C_\mu</math> is a model constant which in the standrd version of the k-epsilon model has a value of 0.09. |
==Estimating the turbulent lenght-scale== | ==Estimating the turbulent lenght-scale== | ||
It is common to set the turbulent lenght-scale to a certain percantage 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 lenght-scale to a certain percantage 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) | ||
Revision as of 21:03, 23 March 2006
The turbulence length scale, 
, 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.
In the k-epsilon model the turbulent lenghts-scale can be computed as:
Where :
is a model constant which in the standrd version of the k-epsilon model has a value of 0.09.
Estimating the turbulent lenght-scale
It is common to set the turbulent lenght-scale to a certain percantage 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)
