[ms123]

Hi to all,

I'm trying to run a simulation in order to investigate the oil-behavior inside a gearbox. Unfortunately, after a few hours, I get the following error:

 --> FOAM FATAL ERROR: 
[4] Continuity error cannot be removed by adjusting the outflow.
Please check the velocity boundary conditions and/or run potentialFoam to initialise the outflow.
Total flux              : 0.023142523
Specified mass inflow   : 0.00064342005
Specified mass outflow  : 0.00064341938
Adjustable mass outflow : 0
[4] 
[4] 
[4]     From function bool Foam::adjustPhi(Foam::GeometricField<double, Foam::fvsPatchField, Foam::surfaceMesh> &, const Foam::GeometricField<Foam::Vector<double>, Foam::fvPatchField, Foam::volMesh> &, Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> &)
[4]     in file cfdTools/general/adjustPhi/adjustPhi.C at line 107.
[4] 
FOAM parallel run exiting

These are the initial conditions for my U, p_rgh, alpha.water and pointMotionU:

Velocity U

dimensions      [0 1 -1 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{ Gehaeuse       { 
                   type noSlip;  
	         }
  Symmetrieseite { 
                   type symmetry;                     
		 }
  Gearleft       { 
                   type movingWallVelocity;    
                   value $internalField; 
		 }
  Gearright      { 
                   type movingWallVelocity; 
		   value $internalField; 
		 }
}

pointMotionU

dimensions      [0 1 -1 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{ 
  Gehaeuse       
  {
                   type                  fixedValue;      
                   value                 $internalField;         
  }
  Symmetrieseite 
  { 
                   type                  symmetry;    
  }
  Gearleft       
  {               
                   type                  angularVelocity;  
                   axis                  (0 0 1);         
                   origin                (-0.075358 -0.07073 0.017); 
                   angle0                0.0;           
                   omega                 3.490658504; //2000rpm
                   value                 $internalField;
  }
  Gearright      
  { 
                   type                  angularVelocity;  
                   axis                  (0 0 -1);        
                   origin                (0.016142 -0.07073 0.017); 
                   angle0                0.0;       
                   omega                 3.490658504; //2000rpm  
                   value                 $internalField;
  }
  "procBoundary.*" { type processor; }		   
}

pressure p_rgh

dimensions      [1 -1 -2 0 0 0 0];

internalField   uniform 0;

boundaryField
{ Gehaeuse       { 
                  type fixedFluxPressure;
		  value uniform 0; 
		 }
  Symmetrieseite { 
                  type symmetry; 
		 }
  Gearleft       { 
                  type fixedFluxPressure;
		  value uniform 0; 
  
		 }
  Gearright      { 
                  type fixedFluxPressure;
		  value uniform 0; 
                 }
}

alpha.water

dimensions      [0 0 0 0 0 0 0];

internalField   uniform 0;

boundaryField
{
   Gehaeuse        { 
                     type     zeroGradient; 
		   }
  
   Symmetrieseite  { 
                     type     symmetry; 
		   }
  
   Gearleft        { 
                     type     zeroGradient; 
		   }

   Gearright       { 
                     type     zeroGradient; 
		   } 
}

Does anyone know how to help me?

[wouter]

hello ms123,
This problem occurs when there is a violation of the degrees of freedom. You probably set the inflow and the outflow, but you can only set one of those, the other has to be calculated.


hope this helps
wouter

[ms123]

Hello wouter,

First of all thank you for your quick response. Now I have an idea about the theoretical part of the error and I'm thankful for that. Although, I'm a little confused about what are my inlet and outlet, because it is a closed gearbox, it has no contact with atmosphere or what else. Do you mean that it could possibly work if I set f.e. p_rgh-boudary only on the one of my walls?
(I'll try a to run a simulation with the U-boundary fixedValue. I'll let you know if it works.)
Thank you again for your reply,

ms123

[piu58]

I recommend the following:
- set all boundaries to wall, with al wallfunction
- leave an amount of air in the box. Its compressibility gives room for changes in the oil volume due to temperature changes.

Or you make an inlet/outlet somewhere, some kind of pressure relief valve.

[ms123]

Hi piu58,

thank you too for your response. Perhaps I didn't described my simulation-model so good. The gearbox is not full of oil, there are oil and air inside of it.
All my boundaries (except the "symmetry") are set to wall. If you have any other ideas, please let me know,

ms123

[piu58]

I would try to set the pressure b.c. to internal field instead of 0.

[ms123]

Thanks a lot, I'll try it and I'll let you know if it works

[ms123]

Hi to all again,

unfortunately setting the U to fixedValue doesn't work. The continuity-error shows up again...
Dear piu58, I'm quite new and because of that, I would like to ask you what you really mean by setting the p_rgh to internal field instead of 0.
Do I have to let #internalField uniform 0;# as it is, and then:
(by Gearleft for example) {
type fixedFluxPressure;
value $internalField;
}
?

ms123

[piu58]

Gearleft  {
  type fixedFluxPressure;
  value $internalField;
}

Yes, something like that. You cannot have the pressure zero at all boundaries.

[ms123]

Hello again!
I've tried many different combinations but unfortunately it did not work. The continuity-error is still there, making my life difficult . I'm using the utility "mapFieldsPar" to map the U,p,pointMotionU and alpha.water fields from the one mesh to the other. I use in total 10 meshes. The error occurs just after the mapping and not during the calculations (of a single mesh). Does anzone have similar problem?

[queensuCFD]

Hello,

I've got the same thing happening. I have an enclosure with multiple fans. The continuity error is really small, but still causing a crash.

I'm betting there are some small sections where the MRF mesh is bridging some small gaps. Really frustrating.

Did you ever find a solution where your BCs fixed the issue.