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-   -   Convergence difficulties (http://www.cfd-online.com/Forums/fluent/38757-convergence-difficulties.html)

Freeman November 28, 2005 14:54

Convergence difficulties
 
Hi all!

I am trying to simulate a flow around a car in 2D. First of all I use a k-e standard with std. wall-functions and first order scheme in the momentum and turbulence quantities (standard scheme for pressure)

Then, when solution is converged with these parameters I change settings into more complex ones: RNG+non-equilibrium+2nd order schemes. The fact is that when I make these kind of changes, convergence problems appear (residuals diverge). I know that there are no grid problems. Can anybody help me with that?

Freeman November 28, 2005 16:42

Re: Convergence difficulties
 
I've made simulations again plotting the Mass-Weigthed Average of the outlet velocity and it not flattens: it keeps fluctuating and it doesn't stabilize itself so I am starting to thing I have an unsteady problem (quite reasonable since I have a bluff body). But many other questions arises:

1. I don't understand the meaning of those Mass-Weigthed Average quantities, as you are allways advising to monitor, since they throws valuable information about the convergence. I don't also understand the other report parameters such as the integral average, Flow rate, mass flow rate, mass-imbalance, etc. and in Fluent user's guide I have not found the theory behind those parameters.

2. When in unsteady problems, I think Mass-Weigthed Average for example will no longer stabilize to any value, so it isn't a parameter to see convergence... is this correct? If so, which is the convergence criterion you use?

Thanks in advance!


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