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Time step and CFL requirments for pimpleFoam case?

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Old   June 23, 2016, 04:07
Default Time step and CFL requirments for pimpleFoam case?
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Hello Foamers,

I have been trying to run an unsteady simulation using pimpleFoam on a C-D nozzle with an obstacle at the exit of the diffuser to validate with the literature. I have copied on of the cases provided in the solver, specifically pitzDaily. In that case, deltaT is 0.0001 and Courant number (CFL) is equal to 5. I created 2D mesh using gmsh to perform the simulations with air velocity of U=6m/s. The mesh spacing, which is shown below, has 0.02 between cells. I have been trying to adjust the value of CFL as well as deltaT, but I have no clue how to adjust these value since it is my first simulation in pimpleFoam. It seems that the unsteady simulation requires good experience unlike steady simulations. I am hopping if someone could suggest some documents or thoughts that bring up to speed to finish the validation. Also, if the grid below is not sufficient for the simulation, please suggest different strategy.

Thank you all,

mesh.jpg
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Old   June 23, 2016, 16:17
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Any thoughts.....
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Old   June 23, 2016, 17:23
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Any helps.....
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Old   June 23, 2016, 23:05
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Quote:
Originally Posted by CFD-Lover View Post
Hello Foamers,

I have been trying to run an unsteady simulation using pimpleFoam on a C-D nozzle with an obstacle at the exit of the diffuser to validate with the literature. I have copied on of the cases provided in the solver, specifically pitzDaily. In that case, deltaT is 0.0001 and Courant number (CFL) is equal to 5. I created 2D mesh using gmsh to perform the simulations with air velocity of U=6m/s. The mesh spacing, which is shown below, has 0.02 between cells. I have been trying to adjust the value of CFL as well as deltaT, but I have no clue how to adjust these value since it is my first simulation in pimpleFoam. It seems that the unsteady simulation requires good experience unlike steady simulations. I am hopping if someone could suggest some documents or thoughts that bring up to speed to finish the validation. Also, if the grid below is not sufficient for the simulation, please suggest different strategy.

Thank you all,

Attachment 48516

Guess expected maximum velocity and the mesh size you know. From this you could guess time step and from this info set the courant number.
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Old   June 23, 2016, 23:23
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Thank you for your kind reply. The maximum velocity might reach 12 m/s. What do you mean by mesh size? I mentioned earlier the mesh spacing, which is 0.02.

CFL = U dt/dx.

Could you please clarify? I could you provide you more details.
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Old   June 23, 2016, 23:26
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What do you think about the mesh attached?

mesh.jpg
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Old   June 23, 2016, 23:33
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Here is some information using checkMesh;

Create time

Create polyMesh for time = 0

Time = 0

Mesh stats
points: 687640
internal points: 0
faces: 1371459
internal faces: 683817
cells: 342546
faces per cell: 6
boundary patches: 7
point zones: 0
face zones: 0
cell zones: 1

Overall number of cells of each type:
hexahedra: 342546
prisms: 0
wedges: 0
pyramids: 0
tet wedges: 0
tetrahedra: 0
polyhedra: 0

Checking topology...
Boundary definition OK.
Cell to face addressing OK.
Point usage OK.
Upper triangular ordering OK.
Face vertices OK.
Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
Patch Faces Points Surface topology
front 342546 343820 ok (non-closed singly connected)
top 596 1194 ok (non-closed singly connected)
inlet 591 1184 ok (non-closed singly connected)
back 342546 343820 ok (non-closed singly connected)
Shroud 204 408 ok (non-closed singly connected)
bottom 596 1194 ok (non-closed singly connected)
outlet 563 1128 ok (non-closed singly connected)

Checking geometry...
Overall domain bounding box (0 -6 0) (12 6 1)
Mesh (non-empty, non-wedge) directions (1 1 0)
Mesh (non-empty) directions (1 1 0)
All edges aligned with or perpendicular to non-empty directions.
Boundary openness (2.47487e-17 4.14214e-16 1.58304e-12) OK.
Max cell openness = 2.21226e-16 OK.
Max aspect ratio = 2.63839 OK.
Minimum face area = 0.000337264. Maximum face area = 0.0309486. Face area magnitudes OK.
Min volume = 0.000337264. Max volume = 0.000747852. Total volume = 143.951. Cell volumes OK.
Mesh non-orthogonality Max: 52.1914 average: 3.28999
Non-orthogonality check OK.
Face pyramids OK.
Max skewness = 0.674821 OK.
Coupled point location match (average 0) OK.

Mesh OK.

End
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