URANS in OpenFOAM
 

Dear all,

I am working with URANS in OpenFOAM with k-omega SST turbulence model. I was wondering about some of the issues in URANS.

I read a number of books about turbulence modeling. All they say that depending on the turbulence, ensemble averaging (unsteady turbulence- possess time derivatives) or time averaging (steady turbulence- don't possess time derivatives) can be done. However, the final form of the governing equations are almost same.


My point is

1. The basic procedure of CFD alogorithm is we discretize the G. Eqn and solve them. Finally we get the time averaged and fluctuating components. Then, where is the difference between the RANS and URANS? Is it only the time derivatives term or there are some other differences too in the algorithm?

2. When we say that time averaged RANS equation or ensemble averaged RANS equation, we actually don't do time or ensemble averaging right? Because the RANS or URANS equations are derived only by Reynolds decomposition, therefore when we assume that mean components are ensemble averaged then the time derivatives terms retains and when we assume that mean component is time averaged then the time derivative term vanishes. Therefore, to obtain RANS or URANS simulations results we actually don't do time or ensemble averaging, theses are just assumption how we define the mean component. Am I right?

I know these questions are very basic. Please help me to move on forward and build up my knowledge.

Thanks in advance.