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Meshing Domains of Very Different Sizes

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Old   September 8, 2009, 18:02
Default Meshing Domains of Very Different Sizes
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Claudio
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Hi,

I am new to Ansys and CFX and I haven't been able to resolve this issue. I'm modelling flow through a cylindrical duct in which there is a cylindrical obstacle, leaving a small gap between the obstacle and walls of the duct for the fluid to flow thorugh. The duct radius is 25 mm, while the obstacle radius is 24.45 mm. Thus, the gap is only 0.55 mm wide. I'm having trouble meshing this geometry in Ansys.

Is there any way I can have greater control of the mesh inside the gap?

I was thinking, one option was to create a separate domain for the fluid in the gap, and limit the mesh element size in order to have a finer mesh there, and use fluid-fluid interfaces to connect that domain to the rest of the duct.

Does this sound right to you or would there be a better way to do it?
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Old   September 9, 2009, 00:24
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Glenn Horrocks
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Any geometry with a large range in dimensions is tricky to mesh. Do the tutorial examples and maybe some training in the meshing and that will help.

Generally it is preferable to have the mesh contiguous but you can split it up and link with GGIs if necessary.
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Old   September 18, 2009, 20:45
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Hello,
Iíve been reading some tutorials, and I still canít figure out how to mesh my geometry. What Iím trying to model is flow inside a cylindrical duct with a smaller cylindrical obstacle inside, i.e., flow only occurs through the annular gap that surrounds the obstacle. Since the problem is axisymmetric, Iím only modeling a ďsliceĒ of the geometry. The fluid region can be seen in image number 1.

I know that due to symmetry, this model could be solved in 2d, but I want to learn how to solve it in 3d.

Iím looking to obtain a mesh similar to image 3, but of course, in 3d. Iíve been fooling around with the different mesh sizing options, and canít get it right. (I want a mesh that is good enough to capture the flow through the gap correctly and that then transitions to a coarser mesh as it moves away from the gap)

I think it would be much easier if the annular gap was a separate ďentityĒ that could have its own mesh sizing options, different from the rest of the fluid region. How could that be done? One idea I have is to use the Design Modeler to create a multi-body part and try to get a connected mesh across each body. How do I indicate to the mesher that I want a connected mesh at that interface? Also, then how can I make a fluid-fluid interface, in order for CFX to consider all the fluid regions as a continuous unity?
By the way, Iím using Meshing 12.0.01 that comes with the Ansys Workbench.

Thanks a lot!
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Old   September 18, 2009, 23:28
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Why can't you set a Face Spacing on your obstacle cylinder, with an influence radius of 1mm?
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