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flowRateInletVelocity and timeVaryingFlowRateInletVelocity Problems

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Old   July 22, 2011, 15:29
Default flowRateInletVelocity and timeVaryingFlowRateInletVelocity Problems
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Hello,

I am trying to use flowRateInletVelocity to set the mass flow on an inlet in a compressible solver. I am running into issues, however. I expect that when I increase the mass flow rate, the velocity should increase to match, but what I am actually seeing is that the pressure increases and the velocity stays relatively low. This problem does not always occur; sometimes I can fix it by making slight variations in my BCs that shouldn't have an effect. I also am having this problem to a very bad degree using timeVaryingFlowRateInletVelocity, where my velocity starts fairly high and then plummets, while the pressure sky rockets to 50 atmospheres.

Does anyone have any suggestions?
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Old   July 25, 2011, 06:28
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Samuel ARNAUD
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Hi Cyberholmes,

If you want a better help, you should explain your case, tell us in which OpenFoam version you're working, which OS (and so on) and join your files (boundaries, constant, schemes, ...). It will help us to give you more effective advices.

Nevertheless, I would guess, as usual, there is a problem with your BCs. I don't think (with those limited informations) the problem comes from the velocity BC but better check your pressure BC.

As I said, help us to help you

Best regards
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Old   July 25, 2011, 11:44
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Ok, here goes:

I am using OpenFOAM 2.0.0 running on Ubuntu Natty Narwhal.

I am modeling a 2D square chamber, 100 mm across, with a 2 mm inlet in the center of the bottom face and and a 2 mm outlet at the center of the right face. I am using the solver reactingFoam, currently with reactions turned off, because I will later want to use reactions.

My BCs are as follows:

p:
inlet: zeroGradient;
outlet: fixedValue; value uniform 1e5;
otherWalls: zeroGradient

T:
chamber starts at 600 K
inlet: fixedValue; value uniform 800;
outlet: zeroGradient; <--- because if I fix a value here, I get a janafThermo error, I don't know why
otherWalls: fixedValue; value uniform 600;

U:
inlet:

type timeVaryingFlowRateInletVelocity;
flowRate 0.000; // Volumetric/mass flow rate [m3/s or kg/s] <--- What does this value do?
value uniform (0 0 0); // placeholder
fileName "$FOAM_RUN/project/buildup/varyingFlowRateTest/constant/time-series";
outOfBounds repeat; // (error|warn|clamp|repeat)

outlet: zeroGradient;
otherWalls: fixedValue; value uniform (0 0 0)

My time-series file looks like this:

(
// Time field
(0 0)
(0.01 0.001)
(0.02 0.002)
(0.03 0.003)
(0.04 0.004)
(0.05 0.005)
(0.06 0.006)
(0.07 0.007)
(0.08 0.008)
);

My other model is of a perfect sphere with a small flat circular inlet and a much larger flat circular outlet. There is a heat source at the center of the sphere, created by altering the reactingFoam source files to include a heat source term.

In this second model, I only get issues when I use flowRateInletVelocity for both the inlet and outlet (setting the outlet value to the negative of the inlet value). Otherwise, the BCs are the same as my first case, except I hold the outlet at a fixed temperature.

In both cases, the pressure at the inlet rises to an extremely high value, and the velocity does not reach the value I expect it to.

Thank you.
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Last edited by Cyberholmes; July 25, 2011 at 11:51. Reason: Typo
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Old   July 26, 2011, 04:36
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Samuel ARNAUD
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first guesses (haven't read everything for the moment):
- Don't put "uniform 1e5" for p at the outlet, but either "interalField" or even better " {
type outletInlet;
outletValue $internalField;
value $internalField;
}"
-For U, try: " {
type inletOutlet;
inletValue uniform (0 0 0);
value $internalField;
}" for the Outlet (it avoids the flow to get back into the system)
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