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Old   March 18, 2008, 13:00
Default Hi to all, sorry for my engl
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matteo_gautero
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Hi to all,
sorry for my english. I have some problems with my case. I have to study the aerodynamic of an airplane at cruise speed and altitude; the cruise speed is 60 m/s at an altitude of 1500 m, so the Mach number is equal to 0.2, then I assume that it is an incompressible case. I create a domain where is located the airplane; in particular, the mesh is composed by 16 millions of tetrahedra and it's finer near the airplane, coarser near the surface of the domain. I'm running this case with simpleFoam solver because I'm interested to the steady state solution. I impose the following boundary conditions:

- wall: on the airplane surface (velocity=zero, grad(p)=0)
- slip: on the lateral surface of the domain
- inlet: I impose the velocity that is equal to the cruise speed (grad(p)=0).
-outlet: classic outlet where the pressure is defined; in this case I put the pressure equal to zero.

I don't set the pref and the initial value for the pressure in the internal field is zero. For the velocity, the internal field is set to 50 m/s.

For now I use a laminar flow, so I'm not interesting to calculate the turbolence yet. I set the value of the viscosity to 1.646e-05 (viscosity at the altitude) and I set the schemes for the simulation. I used the default schemes for the different variables, imposing gauss upwind for the div schemes that are not set.

The problem is that the residuals are not acceptable, they are too high, 10^-2 after 2000 iterations for the pressure. I think that is a problem due to the choises about the boundary conditions.

Can anyone help me?

Thanks in advance,
Matteo.
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Old   March 18, 2008, 13:52
Default Matteo, I successfully ran
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Doug Baldwin
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Matteo,

I successfully ran a similar case. The one difference is that I used only three boundaries: wall, inlet, and outlet. I expect you could change slip into either inlet or outlet.

Doug
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Old   March 18, 2008, 14:30
Default Using wall or slip on the late
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Francesco Del Citto
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Using wall or slip on the lateral boundaries should not make any difference. If the lateral boundaries are parallels to inlet flux and far enough from the airplane, as they should, I'd set them as symmetryPlane.
The residuals issue can be related to mesh quality (bad orthogonality), solvers setting or numerical schemes used.
If with default schemes you mean the one shipped in the tutorial case, they are not so good, but it's something to start from.
If you don't do this yet, try to use GAMG solver for the pressure. And try to switch turbulence on (something simple, like Spalart-Allmaras), that usually helps in stabilizing the steady state solution when you have strong vortexes.
What non-orthogonal corrector values are you using?
Then you can play with fvScheme settings. Changing things there, accordingly to the case, can make a huge difference.

Francesco
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Old   March 19, 2008, 06:34
Default Hi to all and thanks for your
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matteo_gautero
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Hi to all and thanks for your response.
I tried to set the lateral surface like simmetryplane conditions at the first simulation. Now I can try to set the BC of the lateral surface like inlet with the same velocity of the inlet.The default schemes I mean the one set in the simpleFoam dictionary. Now I'll try to use GAMG solver for pressure. I set the non-orthogonal corrector value to zero, because I don't know how is the ortogonality of the mesh. Maybe checkMesh can help me? I try to set a value like 5 for the first simulation?

Thanks in advance,
Matteo.
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Old   March 28, 2008, 08:13
Default Hi to all, sorry if I'm posti
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matteo_gautero
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Hi to all,
sorry if I'm posting so late. The case with the new settings is not acceptable again, but there is an improvement because the residual is 2*10^-3. I consider the residual of the first cycle for the pressure because I think that this is the most important. Now I set the nNonOrthogonalCorrectors to 10, but I don't think that this is the problem, because the mesh seems good. I report the message of checkMesh:

/*---------------------------------------------------------------------------*\
| ========= | |
| \ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \ / O peration | Version: 1.4.1 |
| \ / A nd | Web: http://www.openfoam.org |
| \/ M anipulation | |
\*---------------------------------------------------------------------------*/

Exec : checkMesh . Socata_16M
Date : Mar 28 2008
Time : 12:57:48
Host : energrid
PID : 21238
Root : /home/asinari/OpenFOAM/Socata
Case : Socata_16M
Nprocs : 1
Create time

Create polyMesh for time = constant

Time = constant

Mesh stats
points: 2779762
edges: 18041380
faces: 29852789
internal faces: 28511887
cells: 14591169
boundary patches: 4
point zones: 0
face zones: 0
cell zones: 1

Number of cells of each type:
hexahedra: 0
prisms: 0
wedges: 0
pyramids: 0
tet wedges: 0
tetrahedra: 14591169
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
Superfici_laterali 5394 2767 ok (not multiply connected)
Inlet 626 349 ok (not multiply connected)
Outlet 632 352 ok (not multiply connected)
Socata 1334250 667127 ok (closed singly connected surface)

Checking geometry...
Domain bounding box: (-20.777 -12.175 -5.102) (13.038 12.175 6.168)
Boundary openness (-1.57555e-16 -2.43123e-18 3.81238e-17) OK.
Max cell openness = 7.07501e-16 OK.
Max aspect ratio = 25.879 OK.
Minumum face area = 9.8349e-07. Maximum face area = 1.08353. Face area magnitudes OK.
Min volume = 1.59774e-09. Max volume = 0.361471. Total volume = 9273.02. Cell volumes OK.
Mesh non-orthogonality Max: 85.4538 average: 15.2694
*Number of severely non-orthogonal faces: 44.
Non-orthogonality check OK.
<<Writing 44 non-orthogonal faces to set nonOrthoFaces
Face pyramids OK.
Max skewness = 1.7358 OK.
Min/max edge length = 0.00031438 1.89696 OK.
All angles in faces OK.
All face flatness OK.

Mesh OK.

End

Any suggestion? Thanks in advance,
Matteo.
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