CFD Online Discussion Forums - smoothSolver diverges

Hi all!
I have 2 points to discuss:

1st one:

I wrote a heat transfer solver based on pimpleFoam, adding heat transfer equation.
I use high Courant numbers (up to 5) to speed up calculations. Flow simulation runs without problems (GAMG solver for pressure, smoothSolvers for U and turbulent parameters). By analogy I used smoothSolver also for TEqn in my solver:

Code:

    T

    {

        solver          smoothSolver;

        smoother      GaussSeidel;

        tolerance       1e-05;

        relTol          0;

    nSweeps        1;

    }

But it diverges just at the beggining of simulation with error in GaussSeidel smoother method.

Then I just switched TEqn solver to PBiCG with DILU preconditioner

Code:

    T

    {

        solver          PBiCG;

        preconditioner    DILU;

        tolerance       1e-05;

        relTol          0;

    }

and every thing runs Ok. I compared transient solution at different times with pisoFoam solution + heat transfer, and all fields correspond realy good to each other.
So what is the reason for such behavior?

2nd point:

Why am I using pimpleFoam? I solve more complicated task with solidification and turbulent flow. For some cases I search for steady state. One approache is to use steady state solvers, another - to apply transient or cvazi-transient solvers to reach steady state when solution will become stable.

I tried to use simpleFoam just to test such posibility for my task. It reached solution which transient solvers achieve and do not change later, but instanteniously solution deverged! Residuals got a big jump and solution crashed... Am wondering why it happend? Stable solution for that simmetric geometry which I used was assimetric, because there are 3 jets and in 2D they cannot be stable because two zones of low pressure do not have a connection. Is assymethry of solution the case for steady state solver to deverge?

So now I an forced to use PISO or PIMPLE technics to solve my problem... But it is time-consuming!

So, waiting for your opinion! Have a good day!