[morfeus80]

I finished to run the first simulation with simpleFoam and the realizableKe model for turbulence. I made 1700 iterations and I hadn't covergence problems, but the flow field I visualized is very far from reality.
I don't know if the problem can be caused by the fvSchemes, I'll try to change them.
Do you have any hints? Is there anybody has experience in simulation with this kind of turbulence model?

[bastil]

How does the field look like? What are your solver settings? Residuals?

[juho]

Hello

Here are some work in progress pictures of my not quite yet converged practise case:

http://www.students.tut.fi/~peltol20/it500.png

http://www.students.tut.fi/~peltol20/it500_2.png

simpleFoam
standard k-epsilon
3,1 million tetrahedral elements

[vinz]

Hi Juho,

Would it be possible to know the grid generator you used to create this simulation?
Do you have experimental or other software numerical results to compare some parameters like drag coefficient on your model?
Anyway, interesting start, and I'm waiting forward to see your converged solution.

Regards,

Vincent

[juho]

No data, it's an imaginary car I drew. Just learning to use OpenFOAM and some qualitative results on how geometry changes affect things. Velocity is 30m/s forgot to say.

Next I should try to figure out how to use liftDrag I guess.

The mesh is made in Gambit, my school has a license. Here's what checkMesh says:

=================================================

Domain bounding box: (-5.6 0.031 -1.10134e-15) (12.4 3.031 2.4)
Boundary openness (-3.90098e-17 3.3708e-16 -1.22955e-16) OK.
Max cell openness = 1.79272e-16 OK.
Max aspect ratio = 8.64372 OK.
Minumum face area = 2.77654e-06. Maximum face area = 0.114004. Face area magnitudes OK.
Min volume = 2.783e-09. Max volume = 0.0111914. Total volume = 127.35. Cell volumes OK.
Mesh non-orthogonality Max: 67.6634 average: 19.6029
Non-orthogonality check OK.
Face pyramids OK.
Max skewness = 0.878654 OK.
Min/max edge length = 0.0016882 0.607378 OK.

================================================

[vinz]

Thanks for your reply Juho.
I'm actually looking for a good and easy to use free (or not too expensive) mesher for this kind of complex meshes. But I still didn't find the perfect one.
I guess I'm putting the bar a little too high!:D
Problems come when beginning to use lifdrag, so don't be too hurry!
Anyway, nice draw!Good luck.

Vincent

[juho]

The ICEM CFD is good I hear, haven't used it. It probably isn't cheap though.

Does anyone actually know the price?

[vtk_fan]

ICEM/CFD can cost upwards of US $15,000 for a node-locked license. A floating license would probably cost a lot more.

[bastil]

ICEM has restrictions regarding hexcore meshes. Hexas all have the same size. His is not very well suited for Aerodynamic meshes.

[pbo]

What do you mean by restrictions on hexcore meshes?
I generated several multi-block structured meshes around wing and aircraft geometries using ICEM-CFD, all were featuring graded edges to properly resolve the boundary layer.

Gridgen from Pointwise is worth looking at (I prefer it to ICEM as far as I am concerned...), the academic licence is about 1500 GBP.

On the (almost) free side, there is CUBIT.

Another alternative: you can use the preprocessing utilities from EDGE (FOI CFD code with freely available binaries), and convert the generated mesh files to meet FOAM format.

[bastil]

Patrick:

You are takling about ICEM Hexa. This is good for block-structured meshes like wing flows. However, for real complex car shapes you will need lots of time to generate block structered grids. Therefore it is quite common to use more automatic methods like hexcore meshes. Hexcore meshes in ICEM Tetra can only generate hexas with a uniform edge length. His is a stupid restriction. It is better to uses hexahedrals of different sizes like T-Grid or ANSA can do or even more hex-dominant meshes (quality trobles!)

Regards

[morfeus80]

BastiL,
my residuals are very low (10^-8), but the wake has an absurd shape.

The solver settings I used are:

ddtSchemes
{
default steadyState;
}

gradSchemes
{
default fourth;
grad(p) fourth;
grad(U) fourth;
}

divSchemes
{
default none;
div(phi,U) Gauss SFCD;
div(phi,k) Gauss SFCD;
div(phi,epsilon) Gauss SFCD;
div(phi,R) Gauss SFCD;
div(R) Gauss linear;
div(phi,nuTilda) Gauss SFCD;
div((nuEff*dev(grad(U).T()))) Gauss linear;
}

laplacianSchemes
{
default none;
laplacian(nuEff,U) Gauss linear corrected;
laplacian((1|A(U)),p) Gauss linear corrected;
laplacian(DkEff,k) Gauss linear corrected;
laplacian(DepsilonEff,epsilon) Gauss linear corrected;
laplacian(DREff,R) Gauss linear corrected;
laplacian(DnuTildaEff,nuTilda) Gauss linear corrected;
}

interpolationSchemes
{
default linear;
interpolate(U) linear;
}

snGradSchemes
{
default corrected;
}

fluxRequired
{
default no;
p;
}


// ************************************************** *********************** //


solvers
{
p GAMG
{
tolerance 1e-08;
relTol 0;
smoother GaussSeidel;
nCellsInCoarsestLevel 40;
mergeLevels 1;
agglomerator faceAreaPair;
cacheAgglomeration off;
nPreSweeps 0;
nPostSweeps 2;
nFinestSweeps 2;
scaleCorrection true;
directSolveCoarsest false;
};
U BICCG 1e-07 0;
k BICCG 1e-06 0;
epsilon BICCG 1e-06 0;
R BICCG 1e-06 0;
nuTilda BICCG 1e-06 0;
}

SIMPLE
{
nNonOrthogonalCorrectors 1;
pRefCell 0;
pRefValue 0;
}

relaxationFactors
{
p 0.3;
U 0.4;
k 0.5;
epsilon 0.5;
R 0.7;
nuTilda 0.5;
}




----------------------------------

Hrv suggested these settings in another topic of this forum.
What's your opinion?

[hjasak]

? Did I really - this looks pretty bizarre to me.

gradSchemes
{
default fourth;
grad(p) fourth;
grad(U) fourth;
}

Nope. Use Gauss linear or leastSquares if your mesh is bad.

div(phi,k) Gauss SFCD;
div(phi,epsilon) Gauss SFCD;
div(phi,R) Gauss SFCD;

Here, Gauss upwind is probably good enough.

For momentum convection, try

div(phi,U) Gauss GammaV 0.5; and see if it converges. If not, we can talk more...

Hrv

[morfeus80]

I'm sorry, Vincent RIVOLA used these settings.

On manday I'll test with the schemes you suggest. According your opinion, are the solvers OK? Is it possible make iterations faster?

Another thing: in each time step, that should be an iteration counter for steady solvers, there are several iterations inside. Why?

[bastil]

Mattia,

how bad is your mesh? Btw, how did you create it? Could you post the output from checkMesh, please. It might be worth trying to reduce nNonOrthogonalCorrectors to 0 if your mesh is not too bad. This will give you an aditional speedup.
Youre right, a timesteps corresponds to what other solvers call iteration. The output of iterations give you the inner iterations for each variable (called sweeps in STAR). Hrv please correct me if I am wrong.

Regards

[morfeus80]

BastiL,
I use a tetrahedral mesh of 1.3mil cells and I think it's quite good. Max cell skewness is less than 0.40 . However on monday I'll post the checkMesh result.

[bastil]

Mattia,

quality sounds good. I am looking forward to see the checkMesh results. What tool did you use? Is it komplex geometry? Cell count is low so I guess it is a more simple shape?

In general perver hexahedral over tetras so aim for more hex-dominant meshes.

Regards

[juho]

I'm baffled by the pressure results in my case.

My free stream velocity is 30m/s

After 3000 iterations, and max. pressure hasn't changed much during the last 1500 of them, simpleFoam gives a stagnation pressure of 672 at the front of the car.

Shouldn't it be 30²/2 = 450?

What am I missing?

[morfeus80]

Hallo,
this is the output of checkMesh:


Mesh stats
points: 262914
edges: 1633690
faces: 2644429
internal faces: 2450187
cells: 1273654
boundary patches: 7
point zones: 0
face zones: 0
cell zones: 0

Number of cells of each type:
hexahedra: 0
prisms: 0
wedges: 0
pyramids: 0
tet wedges: 0
tetrahedra: 1273654
polyhedra: 0

Checking topology...
Boundary definition OK.
Point usage OK.
Upper triangular ordering OK.
Topological cell zip-up check OK.
Face vertices OK.
Face-face connectivity OK.
Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces ...
Patch Faces Points Surface
Ahmed_body 55893 28228 ok (not multiply connected)
simmetry 59808 30528 ok (not multiply connected)
side 5778 3066 ok (not multiply connected)
outlet 546 305 ok (not multiply connected)
inlet 539 301 ok (not multiply connected)
road 66226 33517 ok (not multiply connected)
top 5452 2902 ok (not multiply connected)

Checking geometry...
Domain bounding box: (-11 -9.1e-06 -0.05) (5 1.5 1.5)
Boundary openness (-1.88276e-17 1.94498e-15 2.05761e-15) OK.
Max cell openness = 1.70742e-16 OK.
Max aspect ratio = 7.26466 OK.
Minumum face area = 1.3057e-06. Maximum face area = 0.012724. Face area magnitudes OK.
Min volume = 6.59628e-10. Max volume = 0.000420286. Total volume = 37.1445. Cell volumes OK.
Mesh non-orthogonality Max: 67.8068 average: 19.6179
Non-orthogonality check OK.
Face pyramids OK.
Max skewness = 0.97498 OK.
Min/max edge length = 0.00165297 0.196461 OK.
All angles in faces OK.
All face flatness OK.

Mesh OK.

[bastil]

Looks like a pretty good mesh. Try zero non-orthogonal correctors. This will speed up your solution. What software did you use for meshing?

Regards