[Andrew Gow]

Hi, I have modelled a small contraction-expansion micro-channel and get steady state laminar flow solutions (via SimpleFoam) for a range of fluids.

I want to check how the laminar flow develops into the steady state, so want to model it via a transient solver.

I have tried icoFoam, pisoFoam and pimpleFoam but consistently get 'nan' residuals.

Example below (laminar shear-thinning fluid flow using pisoFoam, with very very small timestep as I wanted to check that the Courant number was ok):

Create time

Create mesh for time = 0

Reading field p

Reading field U

Reading/calculating face flux field phi

Selecting incompressible transport model powerLaw
Selecting turbulence model type laminar
Selecting laminar stress model Stokes
No MRF models present

No finite volume options present

Starting time loop

Time = 1e-12

Courant Number mean: 1.78444e-12 max: 1.69643e-10
smoothSolver: Solving for Ux, Initial residual = 1, Final residual = 0.125661, No Iterations 1000
smoothSolver: Solving for Uy, Initial residual = 0.000204169, Final residual = 4.28595e-05, No Iterations 1
GAMG: Solving for p, Initial residual = 1, Final residual = 1.27784, No Iterations 1000
GAMG: Solving for p, Initial residual = 0.00590419, Final residual = 0.00283648, No Iterations 1000
time step continuity errors : sum local = 2.19094e-12, global = -7.73091e-14, cumulative = -7.73091e-14
GAMG: Solving for p, Initial residual = 0.0081096, Final residual = 0.0042503, No Iterations 1000
GAMG: Solving for p, Initial residual = 0.0802758, Final residual = 0.0285281, No Iterations 1000
time step continuity errors : sum local = 1.18382e-12, global = -4.21001e-14, cumulative = -1.19409e-13
GAMG: Solving for p, Initial residual = 0.181867, Final residual = 0.0826181, No Iterations 1000
GAMG: Solving for p, Initial residual = 0.13259, Final residual = 0.0732679, No Iterations 1000
time step continuity errors : sum local = 1.37441e-12, global = -4.97442e-14, cumulative = -1.69153e-13
ExecutionTime = 39.502 s ClockTime = 40 s

Time = 2e-12

Courant Number mean: 1.88248e-09 max: 3.55571e-07
smoothSolver: Solving for Ux, Initial residual = 0.999071, Final residual = 0.723957, No Iterations 1000
smoothSolver: Solving for Uy, Initial residual = 0.998321, Final residual = 0.0997865, No Iterations 412
GAMG: Solving for p, Initial residual = 0.992996, Final residual = 0.0988467, No Iterations 472
GAMG: Solving for p, Initial residual = 0.0426151, Final residual = 0.0042522, No Iterations 658
time step continuity errors : sum local = 2.3776e-09, global = 2.28602e-11, cumulative = 2.2691e-11
GAMG: Solving for p, Initial residual = 0.622031, Final residual = 0.0620596, No Iterations 404
GAMG: Solving for p, Initial residual = 0.141261, Final residual = 0.0140427, No Iterations 638
time step continuity errors : sum local = 2.31854e-09, global = -2.2299e-11, cumulative = 3.92025e-13
GAMG: Solving for p, Initial residual = 0.294358, Final residual = 0.0291968, No Iterations 340
GAMG: Solving for p, Initial residual = 0.0368593, Final residual = 0.00103414, No Iterations 1000
time step continuity errors : sum local = 1.02061e-10, global = -9.67482e-13, cumulative = -5.75458e-13
ExecutionTime = 63.389 s ClockTime = 64 s

Time = 3e-12

Courant Number mean: 2.78448e-06 max: 0.000431458
smoothSolver: Solving for Ux, Initial residual = 0.920662, Final residual = nan, No Iterations 1000
smoothSolver: Solving for Uy, Initial residual = 0.899158, Final residual = nan, No Iterations 1000
GAMG: Solving for p, Initial residual = nan, Final residual = nan, No Iterations 1000
GAMG: Solving for p, Initial residual = nan, Final residual = nan, No Iterations 1000
time step continuity errors : sum local = nan, global = nan, cumulative = nan
GAMG: Solving for p, Initial residual = nan, Final residual = nan, No Iterations 1000
GAMG: Solving for p, Initial residual = nan, Final residual = nan, No Iterations 1000
time step continuity errors : sum local = nan, global = nan, cumulative = nan
GAMG: Solving for p, Initial residual = nan, Final residual = nan, No Iterations 1000
GAMG: Solving for p, Initial residual = nan, Final residual = nan, No Iterations 1000
time step continuity errors : sum local = nan, global = nan, cumulative = nan
ExecutionTime = 104.086 s ClockTime = 105 s

Time = 4e-12

Courant Number mean: nan max: nan
smoothSolver: Solving for Ux, Initial residual = nan, Final residual = nan, No Iterations 1000
smoothSolver: Solving for Uy, Initial residual = nan, Final residual = nan, No Iterations 1000
GAMG: Solving for p, Initial residual = nan, Final residual = nan, No Iterations 1000


My fvSolution file is:

solvers
{
p
{
solver GAMG;
tolerance 1e-4;
relTol 0.1;
smoother GaussSeidel;
}

U
{
solver smoothSolver;
smoother symGaussSeidel;
tolerance 1e-4;
relTol 0.1;
}

pFinal
{
$p;
relTol 0;
}

UFinal
{
$p;
relTol 0;
}


}

PISO
{
nCorrectors 3;
nNonOrthogonalCorrectors 1;
pRefCell 0;
pRefValue 0;
}


}

relaxationFactors
{
equations
{
U 0.9;
".*" 0.8;
}
}

My controlDict file is:

application pisoFoam;

startFrom startTime;

startTime 0;

stopAt endTime;

endTime 0.5;

deltaT 0.000000000001;

writeControl runTime;

writeInterval 0.1;

purgeWrite 0;

writeFormat ascii;

writePrecision 6;

writeCompression off;

timeFormat general;

timePrecision 6;

runTimeModifiable true;


Does anybody have an tips? I get the same problem ('nan') with all transient solvers.

Thanks