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massflow of inlet and outlet are not equal

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Old   April 1, 2014, 06:43
Default massflow of inlet and outlet are not equal
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Hello foamers!

In my mesh i have one inlet and one outlet, both have the same area.
I set the BC as followed:
--------------------------------------------------------------------------------
0/U

inlet
{
type flowRateInletVelocity;
flowRate 0.02;
value uniform (0 500 0);
}

outlet
{
type zeroGradient;

}

-----------------------------------------------------------------------------------------------------
0/p

inlet
{
type zeroGradient;
}

outlet
{
type fixedValue;
value uniform 86000;
}


the massflows are then:

MassFlows: inlet = 1.44
MassFlows: outlet = 0.880914

so they are not the same, but how can i tell the programm that the outlet massflow should be the same like the inlet massflow?

Maybe somebody has an idea?
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Old   April 1, 2014, 16:00
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Troy Snyder
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Is your solver incompressible? If the solver is compressible, the mass flowrates may not be the same.
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Old   April 2, 2014, 06:07
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Quote:
Originally Posted by tas38 View Post
Is your solver incompressible? If the solver is compressible, the mass flowrates may not be the same.
hi tas38!

Yes my solver is compressible, but the massflow should be always the same because it is in kg/sec, so everything that is going in should also go out?

so the massflow should be equal, but how do i can do that?
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Old   April 3, 2014, 21:48
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Orry Shamash
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mass flow rate is density*area*velocity

If this is incompressible flow, density shouldn't be a problem

area isn't changing

only thing changing velocity is friction

if you disable viscous dissipation there should be no viscous forces and therefore no loss of flow rate
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Old   April 4, 2014, 05:24
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Olivier
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hello,

Just to add some entropy ... is you simulation transient ?
by the way, you didn't tel give the solver name, neither other relevant info ... check How to give enough info to get help

regards,
olivier
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Old   April 30, 2014, 18:22
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My flow is subsonic,

0/U:

inlet type flowRateInletVelocity;
outlet type zeroGradient;
domain1 type temperatureDirectedInletVelocity;
domain2 type symmetryPlane;
domain3 type zeroGradient;

0/p:

inlet zeroGradient;
outlet type fixedValue;
domain1 type zeroGradient;
domain2 type symmetryPlane;
domain3 type outletInlet;

if i change the fixedValue of the pressure outlet i can change the outlet Massflow, but it never goes equal to the inlet massflow.
Also after the calculation converged, the massflows are not equal.
What should i change that is going to be equal?
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Old   April 30, 2014, 18:52
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Troy Snyder
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Regarding your initial post.

Is it possible that there may be an issue in specifying 500 in the second
element of the 'value' array? What is the basis for making this 500 and not leaving as 0?

Code:
Description
    Describes a volumetric/mass flow normal vector boundary condition by its
    magnitude as an integral over its area.

    The basis of the patch (volumetric or mass) is determined by the
    dimensions of the flux, phi.
    The current density is used to correct the velocity when applying the
    mass basis.

    Example of the boundary condition specification:
    \verbatim
    inlet
    {
        type        flowRateInletVelocity;
        flowRate    0.2;        // Volumetric/mass flow rate [m3/s or kg/s]
        value       uniform (0 0 0); // placeholder
    }
    \endverbatim

Note
    - The value is positive inwards
    - May not work correctly for transonic inlets
    - Strange behaviour with potentialFoam since the U equation is not solved
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Old   May 1, 2014, 03:40
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hello tas38,

yes of course i can also write 0 instead of 500, the velocity at the inlet
is just about 450m/s at the inlet. I think the velocity is calculated by the massflow at the inlet?
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Old   May 1, 2014, 12:32
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i want to calculate the pressure with groovyBC.

type groovyBC;
valueExpression "0.003/(sum(mag(Sf()))*(normal()&U))*287.058*(288.15-sqr(mag(U))/(2*1004.5))";
value uniform 90000;

so that the massflow here 0.003 is the same like for the inlet,
but they dont converge!
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