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obtaining significantly different Flow profiles after only changing Mesh size

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Old   June 30, 2020, 13:06
Default obtaining significantly different Flow profiles after only changing Mesh size
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berat
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Hello,
so i am new to OpenFOAM and simulation in general and i am currently trying to simulate a two dimentional pipe flow. I managed to obtain a satisfiying result/flow profile in my simulation.
Now I am looking to making the simualtion bigger by simply changing the value form the "converttoMeter" command in the BlockMesh file. When i do so i am getting a completly unrealistic flow profile, even though every other part of the Simualtion is exactly the same. I allready tried increasing/decreacing the velocity at the input patch, this didnt change anything though.

BlockMesh of the working Simulation:

convertToMeters 0.000001;

vertices
(
(0 0 0)
(35000 0 0)
(35000 3500 0)
(0 3500 0)
(0 0 65)
(35000 0 65)
(35000 3500 65)
(0 3500 65)
);


blockMesh of the faulty simulation

convertToMeters 1;

vertices
(
(0 0 0)
(35000 0 0)
(35000 3500 0)
(0 3500 0)
(0 0 65)
(35000 0 65)
(35000 3500 65)
(0 3500 65)
);

as i already mentioned the rest of the simulation is exactly the same, despite me tryed to change up the velocity which didnt change anything though.

now i am curious what i am doing wrong. i would appreate the sightest hints which would help me solve my problem.
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File Type: jpg faulty.jpg (61.8 KB, 9 views)
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Old   June 30, 2020, 16:07
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Troy Snyder
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In your faulty simulation, the length scales are increased dramatically. Correspondingly, your Reynolds number has increased significantly given the same U and fluid properties rho and mu.

Is the solution converging for the high-Re case?

Is the flow modeled as laminar or turbulent?
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Old   July 1, 2020, 12:28
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berat
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thank you, I never concidered the reynolds number. But i calculated the Reynolds number by hand and get 2.17. so i should still have laminar flow.

Quote:
Is the solution converging for the high-Re case?
to be hornest i didnt know what convergence in fluid dynamics are up till now, after looking it up. i can tell you that i didn't implement anything to help with convergence. so my simulation probably doesn't converge for high-Re cases.
Do i have any other options of assuring convergence besides implementing relaxationFactors?

Quote:
Is the flow modeled as laminar or turbulent?
i am using the piso solver and i have the simulationType set to laminar in the turbulenceProperties file.
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Old   July 1, 2020, 13:14
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Troy Snyder
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Using incompressible piso in openfoam, the only fluid property to specify is kinematic viscosity. What value did you prescibe for this and the inlet velocity for the faulty mesh case? I
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Old   July 1, 2020, 13:22
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berat
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Code:
transportModel  Newtonian;

nu              nu [ 0 2 -1 0 0 0 0 ] 1;

CrossPowerLawCoeffs
{
    nu0             nu0 [ 0 2 -1 0 0 0 0 ] 1;
    nuInf           nuInf [ 0 2 -1 0 0 0 0 ] 1;
    m               m [ 0 0 1 0 0 0 0 ] 1;
    n               n [ 0 0 0 0 0 0 0 ] 1;
}

BirdCarreauCoeffs
{
    nu0             nu0 [ 0 2 -1 0 0 0 0 ] 1;
    nuInf           nuInf [ 0 2 -1 0 0 0 0 ] 1;
    k               k [ 0 0 1 0 0 0 0 ] 0;
    n               n [ 0 0 0 0 0 0 0 ] 1;
}
Code:
dimensions      [0 1 -1 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{
    inlet
    {
        type            fixedValue;
	value		uniform (0.62e-3 0 0);

    }
    outlet
    {
        type            zeroGradient;
    }
    walls
    {
        type		noSlip;
    }
}
but i should mention here that i am am also using this line of code here. so the units are in micro. e.g μg, μm etc.

Code:
DimensionedConstants
{
    unitSet             micro;

microCoeffs
{
universal
{
c c [ 0 1 -1 0 0 0 0 ] 2.99792e+8; // speed of light in vacuum (m/s)
G G [ -1 3 -2 0 0 0 0 ] 6.67429e-20; // gravitational constant (m^3/(pgs^2))
h h [ 1 2 -1 0 0 0 0 ] 6.62607e-13; // Planck's constant (pgm^2/s)
}
electromagnetic
{
e e [ 0 0 1 0 0 1 0 ] 1.602176634e-4; // elementary charge (pC)
}
atomic
{
me me [ 1 0 0 0 0 0 0 ] 9.10938e-16; // electron mass (pg)
mp mp [ 1 0 0 0 0 0 0 ] 1.67262e-12; // proton mass (pg)
}
physicoChemical
{
mu mu [ 1 0 0 0 0 0 0 ] 1.66054e-12; // atomic mass unit (pg)
k k [ 1 2 -2 -1 0 0 0 ] 1.38065e-8; // Boltzman constant (pgm^2/s^2K)
}
standard
{
//- Standard pressure [pg/ms^2 ]
Pstd Pstd [ 1 -1 -2 0 0 0 0 ] 1e2; // 1 pg/ms^2
//- Standard temperature [degK]
Tstd Tstd [ 0 0 0 1 0 0 0 ] 298.15; // should be same as in SI unit system
}
}

}
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Old   July 1, 2020, 13:28
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berat
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kinematic viscosity is 1
velocity is (0.62e-3 0 0)

i should also metion, that i have been using the "unitSet micro" command which is supposed to change the units into micro. so its not kg,m etc anymore, but rather μg, μm etc.
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