Calculation of yPlus in MRF simulations

MatejAndrej · February 17, 2015, 05:27

Dear All,

I'm simulating airflow passing a propeller using MRF. I'm using incompressible solver simpleFoam and kOmegaSST turbulence model with nutkWallFunction as a wall function at nut. I don't have problems with simulations, it converges beautifully to nice results, but I would like to ask how do I calculate yPlus in my situation?

Since it is High-Re type of simulation I ran yPlusRAS utility, but resulting yPlus is not ok even though I recalculated U by adding \omega x R (rotating term).

After a little bit of research in doxygen I found that since I'm using nutkWallFunction the utility yPlusRAS consequently uses equations: u_{\tau}=C_{\mu}^{1/4}\sqrt{k} together with y+ definition y+=y u_{tau}/\nu. Since turbulent kinetic energy k is being calculated in rotating frame of reference it apparently doesn't result in correct y+ calculation?!?

Then I changed kOmegaSST in constant/RASProperties to SpalartAllmaras and I changed all nutkWallFunction in nut file to nutUWallFunction in the last simulation time folder. Then I ran yPlusRAS again and this time it calculated "logical" y+ distribution.

nutUWallFunction uses an iterative calculation in order to calculate y+ described in: http://www.cfd-online.com/Wiki/Two_equation_models (standard wall function).

Since I ran simulation with nutkWallFunction and calculated yPlus with nutUWallFunction I don't know what to think about these results?!?

How do you calculate yPlus in MRF?

Thank you very much for your help.

Best
Matej

be_inspired · April 28, 2015, 13:01

Dear MatejAndrej,

Excuse for using this same topic but I think you can help me the little I need.
I am trying to use the same approach that yours to simulate a wind turbine using MRF approach. All goes ok until I activate turbulence ( I am trying to detect where the problem is).
For the walls inside the rotating region I have put nutkWallFunction, KqRWallFunction and epsilonWallFunction.
I have also tried nutUSpaldingWallFunction with no luck

Is it needed to change in some way the wall function for MRF? The k and epsilon values at wall increase a lot and then the simulation crash.

I am using OF2.3.x and kEpsilon model ( the mesh is ready for y+ around 100)

February 17, 2015, 05:27	Calculation of yPlus in MRF simulations	#1
MatejAndrej New Member Matej Andrejasic Join Date: Sep 2010 Posts: 2 Rep Power: 0	Dear All, I'm simulating airflow passing a propeller using MRF. I'm using incompressible solver simpleFoam and kOmegaSST turbulence model with nutkWallFunction as a wall function at nut. I don't have problems with simulations, it converges beautifully to nice results, but I would like to ask how do I calculate yPlus in my situation? Since it is High-Re type of simulation I ran yPlusRAS utility, but resulting yPlus is not ok even though I recalculated U by adding \omega x R (rotating term). After a little bit of research in doxygen I found that since I'm using nutkWallFunction the utility yPlusRAS consequently uses equations: u_{\tau}=C_{\mu}^{1/4}\sqrt{k} together with y+ definition y+=y u_{tau}/\nu. Since turbulent kinetic energy k is being calculated in rotating frame of reference it apparently doesn't result in correct y+ calculation?!? Then I changed kOmegaSST in constant/RASProperties to SpalartAllmaras and I changed all nutkWallFunction in nut file to nutUWallFunction in the last simulation time folder. Then I ran yPlusRAS again and this time it calculated "logical" y+ distribution. nutUWallFunction uses an iterative calculation in order to calculate y+ described in: http://www.cfd-online.com/Wiki/Two_equation_models (standard wall function). Since I ran simulation with nutkWallFunction and calculated yPlus with nutUWallFunction I don't know what to think about these results?!? How do you calculate yPlus in MRF? Thank you very much for your help. Best Matej

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April 28, 2015, 13:01		#2
be_inspired Senior Member M. Montero Join Date: Mar 2009 Location: Madrid Posts: 155 Rep Power: 17	Dear MatejAndrej, Excuse for using this same topic but I think you can help me the little I need. I am trying to use the same approach that yours to simulate a wind turbine using MRF approach. All goes ok until I activate turbulence ( I am trying to detect where the problem is). For the walls inside the rotating region I have put nutkWallFunction, KqRWallFunction and epsilonWallFunction. I have also tried nutUSpaldingWallFunction with no luck Is it needed to change in some way the wall function for MRF? The k and epsilon values at wall increase a lot and then the simulation crash. I am using OF2.3.x and kEpsilon model ( the mesh is ready for y+ around 100)