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Steady microchannel simulation with simpleFoam

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Old   April 29, 2015, 05:32
Default Steady microchannel simulation with simpleFoam
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Join Date: Jul 2013
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Hi,

I am trying to simulate an existing microchannel setup with simpleFoam. I use laminar flow, as there shouldn't be turbulences at that scale. This is my layout:

0==============
||||||||||||||||||||||||||||||||||||||
||||||||||||||||||||||||||||||||||||||
||||||||||||||||||||||||||||||||||||||
===============0

One inlet, one outlet (the 0s), two wider manifolds (===) and 60 small channels connecting the manifolds (|||).

I have pressure versus flow results from the real experiment, but the simulation is far off these values.
Here are my p/U fvScheme/fvSolution:

U:
Code:
FoamFile
{
  version     2.0;
  format      ascii;
  class       volVectorField;
  object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //                                                                                                                                                                

dimensions      [0 1 -1 0 0 0 0]; // [m/s]                                                                                                                                                                                                     

internalField   uniform (0 0 0);

boundaryField
{

  inlet
    {
      type        fixedValue; // incompressible -> Q=A*v                                                                                                                                                                                       
      value       uniform (0 0 0.21);
    }

  outlet
    {
      type            zeroGradient;
    }

  wall
    {
      type            fixedValue;
      value           uniform (0 0 0);
    }
}

// ************************************************************************* //
p:
Code:
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //                                                                                                                                                                

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

internalField   uniform 0;

boundaryField
{

    inlet
    {
        type            zeroGradient;
    }

    outlet
    {
        type            fixedValue;
        value           uniform 0;
    }

    wall
    {
        type            zeroGradient;
    }
}
fvSolution:
Code:
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    location    "system";
    object      fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //                                                                                                                                                                

solvers
{
    p
      {
        solver           GAMG;
        tolerance        1e-7;
        relTol           0.001;
        minIter          5;
        maxIter          100;
        smoother         GaussSeidel; // DIC; //DICGaussSeidel; //FDIC;                                                                                                                                                                        
        nPreSweeps       1;
        nPostSweeps      3;
        nFinestSweeps    3;
        scaleCorrection true;
        directSolveCoarsest false;
        cacheAgglomeration on;
        nCellsInCoarsestLevel 50;    // 500                                                                                                                                                                                                    
        agglomerator     faceAreaPair;
        mergeLevels      1;    // 3                                                                                                                                                                                                            
      }

    "(U|k|epsilon)"
      {
        solver           smoothSolver;
    smoother         GaussSeidel;
    tolerance        1e-6;
    relTol           0.01;
    nSweeps          1;
    maxIter100;
      };
}

SIMPLE
{
  nNonOrthogonalCorrectors 0;
    residualControl
      {
           p               1e-2;
          U               1e-3;
    "(k|epsilon)"   1e-3;
      }
}

relaxationFactors
{
  p               0.3;
  U               0.7;
  k               0.5;
  epsilon         0.5;
  R               0.5;
  nuTilda         0.5;
}

cache
{
  grad(U);
}
fvSchemes
Code:
FoamFile
{
  version     2.0;
  format      ascii;
  class       dictionary;
  location    "system";
  object      fvSchemes;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //                                                                                                                                                                

ddtSchemes
{
 default         steadyState;
}

gradSchemes
{
 default         Gauss linear;
}

divSchemes
{
 default         none;
 div(phi,U)      bounded Gauss limitedLinearV 1;
 div(phi,k)      bounded Gauss limitedLinear 1;
 div(phi,epsilon) bounded Gauss limitedLinear 1;
 div(phi,R)      bounded Gauss limitedLinear 1;
 div(R)          Gauss linear;
 div(phi,nuTilda) bounded Gauss limitedLinear 1;
 div((nuEff*dev(T(grad(U))))) Gauss linear;
}

laplacianSchemes
{
 default         Gauss linear limited 0.33;                       
}

interpolationSchemes
{
 default         linear;
}

snGradSchemes
{
 default         limited 0.33; //corrected;                                                                                                                                                                                                    
}

fluxRequired
{
  p;
}
I use the sample function to get the pressure at inlet and outlet with following sampleDict:
Code:
FoamFile
{
  version     2.0;
  format      ascii;
  class       dictionary;
  object      sampleDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //                                                                                                                                                                  
// see /OpenFOAM-2.3.0/applications/utilities/postProcessing/sampling/ for manual                                                                                                                                                              


setFormat raw;

surfaceFormat vtk;

formatOptions
{
  ensight
    {
      format  ascii;
    }
}

interpolationScheme cellPoint;

fields
(
 p
 U
 );

surfaces
(
  inlet_interpolated
    {
      type            patch;
      patches         ( ".*inlet.*" );
      interpolate     true;
      // Optional: whether to leave as faces (=default) or triangulate                                                                                                                                                                         
      // triangulate     false;                                                                                                                                                                                                                
    }
  outlet_interpolated
    {
      type            patch;
      patches         ( ".*outlet.*" );
      interpolate     true;
      // Optional: whether to leave as faces (=default) or triangulate                                                                                                                                                                         
      // triangulate     false;                                                                                                                                                                                                                
    }
 );
The pressure at the outlet is of course 0, but the pressure at the inlet gives me for example at v=0.21 m/s=10 ml/min a pressure of -32.76 which I have to multiplicate with rho to get Pa (is that right?) that is 0.5 bar but it should be 1.2 bar.

Am I missing something like fluid-wall interactions or special micro-scale settings?
I'm quite new to OpenFoam, as you can guess...
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