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cfdlover August 6, 2009 17:23

quenstion in transient turbulence modeling
i have found that the RNG k-e model provides best data fit for branching flow under steady state conditions. Is it expected that this model will also give the best fit under transient conditions?
Please advice.

NewtonKF August 9, 2009 10:16

Hi cfdlover... I may be wrong but k-e models are not the best way to analyze transient flows... You have to set the correct constants and to do it, you have to use some PIV or LDA data.... I think the best way is to use LES or DNS. This last for academic pourposes of course....

Good luck....

vishyaroon August 9, 2009 19:06

I agree with Newton. LES (if not DNS) is the best for transient turbulent flows. Make sure you have the computational capability for LES. DES can be an alternative which can bring down the cell count, but if you know that you'd encounter massively separation. Also be careful in meshing for DES, as the solution is very sensitive to the mesh you generate.

As for RANS models, I'd not trust the results as much.

cfdlover August 12, 2009 17:04

thank you.
what is "DES"?
Also, i have around 10 million nodes in my geometry and the process is very rapid (5-20 seconds). Do you have any idea about the computation time limitation and time step in CFD codes?

vishyaroon August 12, 2009 17:26

DES is Detached Eddy Simulation. In simple words it is a hybrid RANS-LES method which considers the regions closer to the boundary as RANS and regions away from teh wall as LES.

Your time step will again depend on the smallest grid point you have in your domain. It's difficult to say from teh number of grid points as to what your minimum time step will be.

cfdlover August 12, 2009 18:16

thanks alot for the useful information.
i have the smallest grids in the y direction where there are 215 nodes in 4 cm length. Thus the smallest grid is around 0.19 mm.

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