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TWiki> Main Web>RichardHowardLESwithQold (revision 1)

TWiki> Main Web>RichardHowardLESwithQold (revision 1)

DNS data can be used to plot and compare it with different powers of

From this we can get an idea of how the dependence of should change with .

For example

,

,

,

,

This sort of way at looking at suggests that we require at least 5 different models for or in another sense the sensitivity of the turbulent viscosity is different at different distances from the wall.

In order to obtain the y dependence we can make use of and the approximate assumption that . In addition we can convert the wall associated dependence to a local flow dependence

,

,

,

,

Since the dimensions of are the same as : and and also has no dependence, we can use with to provide the necessary dimensions for

,

,

,

,

For larger values of a turbulent viscosity associated with high Reynolds numbers needs to be developed.

The most common way of dealing with the wall shear stress in high Reynolds number wall flows is to use a power law or log law.

We must remember that we are trying to model part of the momentum equation for the near wall grid

and the model is represented as

In effect at high Reynolds numbers because the the viscosity contribution becomes negligeable.

The friction velocity can either be obtained from a power law *e.g.*

which can be written as

(Werner Wengle, , )

or a log law

which can be written as

(, )

In both cases the value of the friction velocity can be obtained as a function of the local grid velocity, wall distance and **the molecular viscosity**:

and this is the value that is included in the momentum equation.

For the power law shown above the relation is

so

This suggests that at high Reynolds numbers the turbulent viscosity should be modelled as

and

The wall laws and power laws discussed above demonstrate that the turbulent viscosity needs to have some molecular viscosity dependence at high Reynolds numbers.

,

,

Topic revision: r1 - 2008-05-27 - 13:41:28 - RichardHoward

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