compressible low speed channel flow
 

Hi all

I am trying to simulate the low Reynolds (say Re=5) mass driven compressible flow in a rectangular microchannel. I used rhoCentralFoam with the following BC:
Pressure:
inlet
{
type zeroGradient;
}

outlet
{
type fixedValue;
value uniform 101315;
}

Velocity:
inlet
{
type fixedValue;
value uniform (0.79473 0 0);
}

outlet
{
type zeroGradient;
}
However, the results are not physical for example a reverse flow appears at the inlet!
I am wondering if I should use another solver or other BC?
Is there anyone who can help me?
Thanks.

Hi
I am really confused with the simple problem of straight rectangular channel.
I need to solve the flow but really cant find out what the problem is with the setting of the openFoam. I would be greatly thankful if any body has a promising feedback. :(

Dear Ali

@ inlet, fixed your pressure and @ outlet, assign a zeroGradient :)

The problem in your case is that the pressure gradient in your domain is too small. You should work with a solver using a "relative pressure".

Your case reminds me this topic: http://www.cfd-online.com/Forums/ope...on-fluent.html

You can also try another solver. rhoSimplecFoam / rhoPimpleFoam

Edit: put your full case here. Maybe your problem is not located in the boundary conidtion.

Dear Nima
Thanks a lot for your kind reply.
I changed the BC as you said but the solution is wrong again.
I also set the inlet pressure somewhat higher that the interior to avoid any back flow but surprisingly this leads to a stronger reverse flow.
can you please help me on this?

By the way, why do you need a compressible flow ?

And try to post your full case. Maybe you made a mistake somewhere else.

Dear Frédéric
Thanks for your comment. It really make scene and I think OF has some problem in solving the pressure field. Actually, I am new with OF and so I supposed rhoSimplecFoam / rhoPimpleFoam are for turbulent flow and cannot be used for low speed laminar cases.
My case is a compressible flow throw a microchannel at Re~1. Here are the details of my mesh and BC and solver settings.
It is a great help to me if you can show me the way.
Many many Thanks.

blockMeshDict:
[QUOTE][]
/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.0.1 |
| \\ / A nd | Web: www.OpenFOAM.com |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
object blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

convertToMeters 5e-5;

vertices
(
(0 0 0)
(10 0 0)
(10 1 0)
(0 1 0)

(0 0 1)
(10 0 1)
(10 1 1)
(0 1 1)

);

blocks
(
hex (0 1 2 3 4 5 6 7) (151 51 1) simpleGrading (1 1 1) // (141 51 51) simpleGrading (3292 0.047 0.047)
);

edges
(
);

boundary
(
inlet
{
type patch;
faces
(
(0 3 7 4)
);
}
outlet
{
type patch;
faces
(
(1 5 6 2)
);
}

bottom
{
type symmetryPlane;
faces
(
(0 4 5 1)
);
}
top
{
type patch;
faces
(
(3 7 6 2)
);
}
front_back
{
type empty;
faces
(
(0 1 2 3)
(4 7 6 5)
);
}

);

mergePatchPairs
(
);

// ************************************************** *********************** //]



thermoPhysicalProperties:

[QUOTE][
/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.0.1 |
| \\ / A nd | Web: www.OpenFOAM.com |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "constant";
object thermophysicalProperties;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

thermoType ePsiThermo<pureMixture<constTransport<specieThermo <hConstThermo<perfectGas>>>>>;

mixture
{
specie
{
nMoles 1;
molWeight 28.96;
}
thermodynamics
{
Cp 1007;
Hf 0;
}
transport
{
mu 1.846e-05;
Pr 0.711;
}
}


// ************************************************** *********************** //]


turbulenceProperties:

[QUOTE][
/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.0.1 |
| \\ / A nd | Web: www.OpenFOAM.com |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "constant";
object turbulenceProperties;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

simulationType laminar;


// ************************************************** *********************** //]

Pressure BC:
[QUOTE][/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.0.1 |
| \\ / A nd | Web: www.OpenFOAM.com |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class volScalarField;
object p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField uniform 101315;

boundaryField
{
inlet
{
type fixedValue;
value uniform 111315;
}

outlet
{
type zeroGradient;
}

bottom
{
type symmetryPlane;
}

top
{
type zeroGradient;
}
front_back
{
type empty;
}
}

// ************************************************** *********************** //]

Temperature BC:

[QUOTE][//*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.0.1 |
| \\ / A nd | Web: www.OpenFOAM.com |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class volScalarField;
object T;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions [0 0 0 1 0 0 0];

internalField uniform 300;

boundaryField
{
inlet
{
type fixedValue;
value uniform 300;
}

outlet
{
type zeroGradient;
}

bottom
{
type symmetryPlane;
}

top
{
type fixedValue;
value uniform 350;
}
front_back
{
type empty;
}
}

// ************************************************** *********************** //]


Velocity BC:
[QUOTE][/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.0.1 |
| \\ / A nd | Web: www.OpenFOAM.com |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class volVectorField;
object U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField uniform (0.79473 0 0);

boundaryField
{
inlet
{
type fixedValue;
value uniform (0.79473 0 0);
}

outlet
{
type zeroGradient;
}

bottom
{
type symmetryPlane;
}

top
{
type fixedValue;
value uniform (0 0 0);
}
front_back
{
type empty;
}
}

// ************************************************** *********************** //]


controlDict:

[QUOTE][/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.1.1 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "system";
object controlDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

application rhoCentralFoam;

startFrom latestTime;

startTime 0;

stopAt endTime;

endTime 1e-02;

deltaT 1e-6;

writeControl adjustableRunTime;

writeInterval 1e-5;

cycleWrite 0;

writeFormat ascii;

writePrecision 10;

writeCompression off;

timeFormat general;

timePrecision 6;

adjustTimeStep yes;

maxCo 0.2;

maxDeltaT 1;


// ************************************************** *********************** //]


fvSchemes:

[QUOTE][/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.1.0 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "system";
object fvSchemes;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

fluxScheme Kurganov;
//fluxScheme Tadmor;

ddtSchemes
{
default Euler;
}

gradSchemes
{
default Gauss linear;
}

divSchemes
{
default none;
div(tauMC) Gauss linear;
}

laplacianSchemes
{
default Gauss linear corrected;
}

interpolationSchemes
{
default linear;
reconstruct(rho) vanLeer;
reconstruct(U) vanLeerV;
reconstruct(T) vanLeer;
//reconstruct(rho) upwind;
//reconstruct(U) upwind;
//reconstruct(T) upwind;
}

snGradSchemes
{
default corrected;
}


// ************************************************** *********************** //]

fv solution:

[QUOTE][/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.1.0 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "system";
object fvSolution;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

solvers
{
"(rho|rhoU|rhoE)"
{
solver diagonal;
}

U //"(U|e)"
{
solver smoothSolver;
smoother GaussSeidel;
nSweeps 2;
tolerance 1e-09;
relTol 0.01;
}

e
{
solver smoothSolver;
smoother GaussSeidel;
nSweeps 2;
tolerance 1e-10 relTol 0;
}

h
{
$U;
tolerance 1e-10;
relTol 0;
}
}


// ************************************************** *********************** //]

Instead of copying all the file here, you can actually upload them. There is a function "Attach Files" just below the submit button.

rhoSimplecFoam can be either turbulent or laminar. You can use k-epsilon, k-omega, Sparlat Allmaras, or laminar. And if you use rhoPimpleFoam, you can use all those model and even LES.

The official description is:
So yes, this solver can do the job...

I'm not familiar with rhoCentralFoam but I don't think it is suitable for you. rhoCentralFoam is a density based solver (which was primarily designed for supersonic flow) while rhoSimplecFoam is a pressure based solver (which was primarily designed for subsonic flow).

Dear Frédéric
I have solved the flow with an incompressible CFD code but whereas the pressure drop in microchannels are considerable and also there is a considerable temperature gradient in my problem, I am curious to see what happen if I use a compressible solver. Actually I have to compare my results with those of compressible case.

Thanks for you comments dear Frédéric
(I am a row new member in this site :) )OK; I will try the rhoSimplecFoam hope it work.
Then I will tell you about the results.
Again thanks for your kind informative comments.

Well, you didn't mention that before... I'm not familiar with micro channels. Where does the heat come from ?

Also, are you sure your flow is really 100% laminar ? How is the flow at the inlet of your simulation ? Does it come from a "tube", how long ? If yes, you might need to consider the turbulence of the upstream flow.

The problem in micro channel, if I remember well, is that the boundary layers tend to meet at the middle of the channel. Maybe a fine mesh with a k-omega SST model can handle this problem. You might need to compare two simulations (laminar vs k-omega sst).