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Old   March 22, 2011, 14:21
Default Accuracy problem
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Ivan
Join Date: Sep 2010
Location: Russia , Moscow.
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Hello everyone ! I have a quite strange problem with solving of two equations:

Code:
fvVectorMatrix uEqn
    (
        fvm::ddt(U) ==  (fvc::div(sigma) - fvc::grad(p))/rho
    );

    uEqn.solve() ;
    
    pU = p*U ; 
    
    fvScalarMatrix EEqn
    (
        fvm::ddt(E) == -fvc::div(pU)/rho
    ); 
    EEqn.solve() ;
initial condition for U is uniform (0 0 0) for E uniform 215250, for p i made a step-like distribution with funkySetFields :

Code:
gap
    {
     field p; 
     expression "2e05";
     condition "(pos().x)<=(pi)";
         keepPatches 1; 
    }
Solution domain is one-dimensional with 10000 cells. The problem is that second equation gives that E changes throughout all solution domain with a small fluctuations about e-12, and after each time step this errors accumulates.
Here 1st 100 values of E :

Code:
215250
215250
215250
215249.999999999971
215250
215250
215250
215249.999999999971
215250
215249.999999999971
215250
215250
215250
215249.999999999971
215250.000000000029
215250
215250
215250
215249.999999999971
215249.999999999971
215249.999999999971
215250
215249.999999999971
215250
215250
215250
215250
215250
215249.999999999971
215250.000000000029
215249.999999999971
215250
215250
215249.999999999971
215249.999999999971
215249.999999999971
215249.999999999971
215249.999999999971
215249.999999999971
215249.999999999971
215250
215250
215250
215249.999999999971
215250
215250
215249.999999999971
215250
215249.999999999971
215250
215250
215249.999999999971
215249.999999999971
215250
215250
215249.999999999971
215249.999999999971
215249.999999999971
215249.999999999971
215249.999999999971
215249.999999999971
215249.999999999971
215250
215249.999999999971
215250
215249.999999999971
215249.999999999971
215249.999999999971
215249.999999999971
215249.999999999971
215250
215250
215250
215250
215249.999999999971
215249.999999999971
215250
215250
215250
215250
215249.999999999971
215250
215250
215249.999999999971
215249.999999999971
215250
215250
215250
215250
215249.999999999971
215249.999999999971
215250
215250
215250
215250
215249.999999999971
215250
215250
215249.999999999971
215250
215249.999999999971
I use Gauss linear scheme for div(pU) term. I've tried various linear solvers with difference tolerance values, but it doesn't help.
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