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Problem in PERIODIC BC in STAR CCM

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Old   January 19, 2015, 23:47
Default Problem in PERIODIC BC in STAR CCM
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mohammad
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Hello everyone,

I am simulating a blood pump in which the some surfaces are in the form of curved surfaces (strips) and have little width (~ 1 mm).

I generate Hexa mesh in ICEM and then bring the *.msh file in to STAR CCM+. The simulated part is 1/8 of the whole model.
The problem occurs when I assign the periodic BC in STAR CCM + on the periodic pars. here some part of the periodic surfaces become like small pieces of wall...ATTACHED IMAGE

(I do not know if the surface mesh is removes or...) and after the solution the create local strange velocity profile.

can anyone tell my how to solve this problem in ICEM to avoid this small walls
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Old   January 20, 2015, 09:37
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Sebastian Engel
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Hello mohammed,

unfortunately there is no picture attached. So the following text might not apply to your problem...
I could think of a possible problem. In the STAR CCM Manual there is a short paragraph about the mesh conversion between fluent mesh file format *.msh to starccm.
When creating inner interfaces in a domain, you should use a porous bc in icem instead of a interface bc. Then, in starccm, redefine it to an interface surface. Maybe the same applies to periodic mesh interfaces.

With regards,
Sebastian
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Old   January 20, 2015, 12:26
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Hi Sebastian,

Many thanks for your email.
The process which I do is:

1- Generating the hexamesh in ICEM using blocking technique.
2- Converting to unstructured mesh.
3- Saving as "STAR-CCM+" format ( this format and FLUENT format are both "*.msh", and both items exist on the ICEM saving menu, separately).
4-importing the into STAR-CCM+.

Up to this part everything is ok. The problem occurs after I assign the periodic BC to the walls inside STAR-CCM+. After this assignment and when I initialize the simulation, the bad elements appear. And their exist periodically on both sides,

Best regards,
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Old   January 27, 2015, 08:35
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Vivek Kumar
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Why do not you do meshing in starccm+ itself?

Then you avoid such kind of problems esp. if they are compatibity issues.
Starccm+ Mesher is much powerful and efficient than any other meshing tool and moreover seamlessly integrated with the solver and post.
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Old   January 28, 2015, 06:58
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Quote:
Originally Posted by vivekcfd View Post
Why do not you do meshing in starccm+ itself?

Then you avoid such kind of problems esp. if they are compatibity issues.
Starccm+ Mesher is much powerful and efficient than any other meshing tool and moreover seamlessly integrated with the solver and post.
Dear vivek...

I need to have full control on the hexahedra mesh and number of nodes along each edge. Starccm cannot do this task as it works based on the cell size.
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Old   January 28, 2015, 08:01
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Vivek Kumar
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In starccm+ there is directed Mesher for this purpose, I mean to control number of cells along edges. Have you tried it? May I ask why do you need to set or know the exact distribution of the cells? Is there some specific reason for this? If you need it for post processing you can do the following:

1. Solve on Poly- Or Trimmer Mesh (with or without the directed Mesher)
2. Import your ICEM Mesh in Starccm+ as a new region
3. Map the solution from starccm+ Mesh to ICEM Mesh using data Mapping toll of starccm

In this way you avoid using ICEM Mesh and still you will have solution on the ICEM Mesh.
I hope this would help.
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Old   January 28, 2015, 09:26
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Quote:
Originally Posted by vivekcfd View Post
In starccm+ there is directed Mesher for this purpose, I mean to control number of cells along edges. Have you tried it? May I ask why do you need to set or know the exact distribution of the cells? Is there some specific reason for this? If you need it for post processing you can do the following:

1. Solve on Poly- Or Trimmer Mesh (with or without the directed Mesher)
2. Import your ICEM Mesh in Starccm+ as a new region
3. Map the solution from starccm+ Mesh to ICEM Mesh using data Mapping toll of starccm

In this way you avoid using ICEM Mesh and still you will have solution on the ICEM Mesh.
I hope this would help.
Dear Vivek,

I highly appreciate your favor great reply.
About your question for "Why should I have control?"
1- The model is blood pump which in some parts the cavity height is 1 mm to 0.2 mm. And we need to solve the velocity profiles and friction coefficient. At the same time the impeller diameter is 80 mm. If I use the auto mesh (poly) with cell size I will have either a small number of cells or very high.

2- According to the definition of the solution and project plan we are supposed to use hexahedral mesh.

And two questions:

Q1: Since you look very experienced in Star CCM, How exact is the mapping process. Please notice that the mesh size will be different and hence different hexa cells fall inside an area of poly cells with different number of nodes. and This may results in some nodes with very close values ( as a result of interpolation process).

Q2: have you worked with "ANSYS meshing" or any other mesh generators for highly curved surfaces?

Regards,
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Old   January 28, 2015, 10:38
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Dear Mohamed
In starccm+ you have mesh refinement control under "Custom Controls". These custom controls can be used to locally refine your meshes. These control can be Volume, Surface or even on edges. This means the size if the cells can be very precisely controlled using custom controls in the required areas

It seems you have not explored Starccm+ Mesher. It is much powerful than any other meshing tool. For the Hexa, you can use Trimmer which mainly uses hexa-cells and just uses poly-cells between a boundary and core mesh.

Data mapping can be found under Tools=> Data Mappers. If your poly or trimmer mesh is locally well refined in the areas you need, this mapping would work without any problems.

ANSYS Meshing for me a no go as it needs at least 10 times more manual effort than starccm+ to generate similar quality of meshes.
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