[cristopf]

Good morning everybody,

I have a question regarding boundary conditions. I want to suck a flow from the outlet. Normally you set p=0 at the outlet and a velocity at the inlet. But that means that you push the flow into let´s say a pipe.
If you want the flow to be sucked from the outlet you actually have to switch the BC. I know that it´s not possible but is there any way that you can suck the flow?
And in addition does anyone know why it´s not possible to set p=0 at the inlet and the velocity at the outlet?

Regards
Christof

[TobiF]

Hello Christoph,

it is possible to set the BCs for the velocity at the inlet as zeroGradient and outlet as fixedValue and for the pressure inverse. (I use this BCs in some cases without problems)

It is also possible to set a negative pressuregradient from inlet to outlet to simulate suction.

You have to look that you do not overdetermine your system by setting fixedValues for U and for p

[jherb]

Perhaps you can use the fixedMean boundary conditions for the velocity at the inlet with a vector pointing out of your flow domain.
https://github.com/OpenFOAM/OpenFOAM...FvPatchField.H

This avoids an non-physical jump in the velocity field at your "inlet" because it allows a flow profile on the boundary similar to the flow profile inside your flow domain.

[chpjz0391]

Originally Posted by cristopf

Good morning everybody,

I have a question regarding boundary conditions. I want to suck a flow from the outlet. Normally you set p=0 at the outlet and a velocity at the inlet. But that means that you push the flow into let´s say a pipe.
If you want the flow to be sucked from the outlet you actually have to switch the BC. I know that it´s not possible but is there any way that you can suck the flow?
And in addition does anyone know why it´s not possible to set p=0 at the inlet and the velocity at the outlet?

Regards
Christof

Hi Christof,

Have you found the answer to your question?
I want to simulate an exhaust ventilation system. The air will be sucked from the outlet. Which BC should I use?

Kind Regards.
Ping

[vbchris]

Why does a negative velocity BC not work for this problem?

If I have a column, with an inlet at the bottom and outlet at the top. I can set suction by the following BC.

outlet: U = -1 m/s P=0
inlet: U = zeroGradient P=zeroGradient

or I can set a dP.

outlet: U = zeroGradient m/s P=0
inlet: U = zeroGradient P=2 kPa

Can you clarify what the problem is.

Chris

[chpjz0391]

Originally Posted by vbchris

Why does a negative velocity BC not work for this problem?

If I have a column, with an inlet at the bottom and outlet at the top. I can set suction by the following BC.

outlet: U = -1 m/s P=0
inlet: U = zeroGradient P=zeroGradient

or I can set a dP.

outlet: U = zeroGradient m/s P=0
inlet: U = zeroGradient P=2 kPa

Can you clarify what the problem is.

Chris

Hi Chris,

This should be work, you can set the pressure difference at inlet or outlet. And set velocity BC as pressureinletoutletveloctiy. It should be work.

Kind regards,

Ping

[smodh]

I am using codedFixedValue bc for implementing ΔV=0 ;where V=x2+y2+z2
my boundarycondition at 0/phi is:
inlet
{
type codedFixedValue;
value uniform 0;
redirectType velocitySquareInlet;

code
#{
const fvPatch& boundaryPatch = patch();
const vectorField& Cf = boundaryPatch.Cf();

scalarField& field = *this;

forAll(Cf, faceI)
{
(
(Cf[faceI].x()=0) &&
(Cf[faceI].y()=0) &&
(Cf[faceI].z()=0)
)
{
field[faceI] = ((Cf[faceI].x()*Cf[faceI].x())
+(Cf[faceI].y()*Cf[faceI].y())
+(Cf[faceI].z()*Cf[faceI].z()));
}
}
#};


and I will have following error at terminal
Using dynamicCode for patch inlet on field phi at line 25 in "/home/sachin/OpenFOAM/Conduction/charge-20/0/phi.boundaryField.inlet"
Creating new library in "dynamicCode/velocitySquareInlet/platforms/linux64GccDPInt32Opt/lib/libvelocitySquareInlet_f5eecc7ba736b15b9d60f3aa108 fc0cf20d513de.so"
Invoking "wmake -s libso /home/sachin/OpenFOAM/Conduction/charge-20/dynamicCode/velocitySquareInlet"
wmakeLnInclude: linking include files to ./lnInclude
Making dependency list for source file fixedValueFvPatchFieldTemplate.C
/home/sachin/OpenFOAM/Conduction/charge-20/0/phi.boundaryField.inlet: In member function ‘virtual void Foam::velocitySquareInletFixedValueFvPatchScalarFi eld::updateCoeffs()’:
/home/sachin/OpenFOAM/Conduction/charge-20/0/phi.boundaryField.inlet:39:35: error: assignment of read-only location ‘(&(& Cf)->Foam::Field<Foam::Vector<double> >::<anonymous>.Foam::List<Foam::Vector<double> >::<anonymous>.Foam::UList<T>:perator[]<Foam::Vector<double> >(faceI))->Foam::Vector<Cmpt>::x<double>()’
/home/sachin/OpenFOAM/Conduction/charge-20/0/phi.boundaryField.inlet:40:35: error: assignment of read-only location ‘(&(& Cf)->Foam::Field<Foam::Vector<double> >::<anonymous>.Foam::List<Foam::Vector<double> >::<anonymous>.Foam::UList<T>:perator[]<Foam::Vector<double> >(faceI))->Foam::Vector<Cmpt>::y<double>()’
/home/sachin/OpenFOAM/Conduction/charge-20/0/phi.boundaryField.inlet:41:35: error: assignment of read-only location ‘(&(& Cf)->Foam::Field<Foam::Vector<double> >::<anonymous>.Foam::List<Foam::Vector<double> >::<anonymous>.Foam::UList<T>:perator[]<Foam::Vector<double> >(faceI))->Foam::Vector<Cmpt>::z<double>()’
/home/sachin/OpenFOAM/Conduction/charge-20/0/phi.boundaryField.inlet:43:17: error: expected ‘;’ before ‘{’ token
/home/sachin/OpenFOAM/Conduction/charge-20/0/phi.boundaryField.inlet:34:26: warning: unused variable ‘field’ [-Wunused-variable]
/opt/openfoam4/wmake/rules/General/transform:8: recipe for target 'Make/linux64GccDPInt32Opt/fixedValueFvPatchFieldTemplate.o' failed
make: *** [Make/linux64GccDPInt32Opt/fixedValueFvPatchFieldTemplate.o] Error 1


--> FOAM FATAL IO ERROR:
Failed wmake "dynamicCode/velocitySquareInlet/platforms/linux64GccDPInt32Opt/lib/libvelocitySquareInlet_f5eecc7ba736b15b9d60f3aa108 fc0cf20d513de.so"


file: /home/sachin/OpenFOAM/Conduction/charge-20/0/phi.boundaryField.inlet from line 25 to line 30.

From function void Foam::codedBase::createLibrary(Foam::dynamicCode&, const Foam::dynamicCodeContext&) const
in file db/dynamicLibrary/codedBase/codedBase.C at line 206.

FOAM exiting




Does anyone know the solution?
Thanks in advance!!!!!