Because Fluent is a commercial code, it is not always good to use the default turbulence model inside for research purposes. To define an omega equation based turbulence model, in Fluent, we can choose the k omega model, define our own user defined scalar (UDS) transport equations, and then solve the newly defined transport equations instead of the default k omega model. The k and omega values are also necessary to be copied back to the default k and omega field, because the momentum and energy equation needs the value of k and turbulent viscosity (computing the unknown Reynolds stress) as well.
So, my first question is, is there anyone has done similar things before?
I managed to write an UDF to define all necessary UDS, boundary conditions, and initialization, but I find the k and omega jump to negative values in some region of the flow field. Linearization of the source term, limiters for k and omega may help, but I don't exactly know how to do it. So, my second question is, how to deal with the solution divergence when using UDS k and omega equations.
My ultimate goal is to define all k omega based turbulence models, such as SST, and γ-Reθ SST.
Yes I will add that You can first calculate with the Fluent turbulence model to obtaina k-omega contours solution which will be usefull to initialize your model upgrades. You should also see if the "k-omega Fluent model" is solved with specific mathematical/numerical treatment. because I think UDS are solved with a known scheme which might be different.
In all cases, take a BenchMark case and calculate it with your and Fluent k-omega model and compare.
Hello, I have written an UDF code for k-w SST model firstly. (I enclosed my UDF here)
For compiling, I have done these steps:
1-go to define/user-defined/functions/compile and add the UDF and then bottom "Build" and "Load"
2-go to define/user-defined/scalars and put 2 UDS
3-go to model/viscous and hook Turbulent viscosity
4-go to "Fluid" and click on "Source terms" and hook two source terms for two UDS
4-go to Boundary conditions and select "plate" as wall and then select "fixed value" for both UDS, put zero for UDS1 (as turbulent kinitic energy) and select wall UDF for UDS2 (as specific dissipation rate)
5-go to pressure outlet boundary condition and click on "UDS" tap, then put the value of UDS1 and 2. I do the same procedure for velocity inlet boundary, too
6-go to solution control/equations, and select User Scalar 0" and "User Scalar 1". Moreover, I uncheck "Turbulence"
7-start calculation
But, this UDF does not work very well. It would be appreciated if you help me to solve the problem. I just want to get a simulation with this UDF and compare with Fluent one.
Hi Ali,
Have you got your UDF to work? I downloaded it and tried to run a simulation with it and received errors
Hello Ali,
Unfortunately it did not work very well. Just let you know, the procedure as explained in previous post works well; but the problem is in the code I think. I could not solve the wall boundary issue by UDF.
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