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-   -   Calculating the Friction Velocity (http://www.cfd-online.com/Forums/openfoam/67554-calculating-friction-velocity.html)

 cbarry August 19, 2009 03:18

Calculating the Friction Velocity

Hi everyone,

I'm trying to implement new wall functions for the kOmegaSST model in OpenFOAM 1.5.

There is just one major hurlde I keep coming across. Does anyone know how I can calculate the friciton velcoity in OpenFOAM?

Best regards,

Chris

 ngj August 19, 2009 04:03

Hi Chris

It depends on whether or not you want to use low- or high Reynolds version of SST. If you are using low RE, then (nu() + nut()) U.snGrad() computed at the wall yields satisfactory results.
If on the other hand you are having high RE, then an assumption of a log-profile would give reasonable results.

Best regards,

Niels

 cbarry August 19, 2009 05:34

Thank you very much Niels.
I am trying to implement a universal, non-linear wall function. For y+ > 30, i.e. the log layer, I want the wall function to take the following form:

omega = u*/ (sqrt(beta2) * kappa * y)

where u* = friction velocity

It is thus in effect a high Reynolds approach. How would you recommend I calculate the friction velocity in that case?

Best regards,

Chris

 ngj August 19, 2009 05:38

Hi Chris

Then the current implementation of SST is what you want. As I understand it, it is doing exactly that.

Best regards,

Niels

 cbarry August 19, 2009 06:00

Hi Niels,

as far as I can see the current implementation of SST calculates omega in the wall region as follows:

omega= sqrt (k) / (Cmu25 * kappa * y)

When I looked earlier on I couldn't see how the two expressions are equivalent. Are they actually equivalent then? If so, how?

In that case I'll try leaving that expression as it is and adding different functions for the buffer layer and the viscous sublayer.

Thank you for your quick help. :)

Best regards,

Chris

 ngj August 19, 2009 07:58

The following is from [1]:

k = u_f^2 / sqrt(C_mu)

and if you combine this with your equation for omega, you obtain the implemented result.

Best regards,

Niels

[1]: Wall Function Boundary Condition Including Heat Transfer and Compressibility, RH Nichols and CC Nelson, AIAA, vol. 42, no. 6., june 2004.

 cbarry August 19, 2009 08:34

Thank you :) . It all makes sense now