3D Multiblock Structured Hexahedral Mesh
This is my first post so hello to all. :)
I am fairly new to the world to CFD and Ansys and as the first part of my project I have to carry out numerical validation of flow across a single cylinder for varying Reynolds number. I have successfully done the validation upto Re=150 using 2D multiblock structured quadrilateral mesh. Now I want to extend the mesh in 3D for validating higher Re. But I just can't figure out a way to do a 3D multiblock structured hexahedral mesh in Ansys Meshing. This is the mesh which I am looking for. Front view: http://i.imgur.com/EEyfh.png Cut section top view: http://i.imgur.com/4d038.png Any pointers how to achieve it? P.S.: These are the steps which I have done so far: 1. Created the 3D geometry in Ansys DesignModeler. http://i.imgur.com/3Xfew.png 2. Imprinted lines on the faces of the enclosing box in Ansys DM to facilitate edge sizing. http://i.imgur.com/fBPT8.png 3. Used Mapped Face Method for all the surfaces in Ansys Meshing and specified edge sizing on different lines and edges. http://i.imgur.com/Wt2jG.png 4. Cut section view of the mesh generated. http://i.imgur.com/A64P0.png |
The easiest way to achieve a "multiblock structured hexahedral" mesh in ansys meshing is to section the geometry in the design modeler.
Just model the "blocks" of your mesh as individual geometries. It works fine for the cylinder, I already did the same thing. But since this is quite a workaround, I recommend you learn ICEM if you ever want to mesh more complex geometries. |
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I've uploaded some pictures of a similar geometry of yours (I didn't know the exact dimensions). Then you just need to play around with the sizing functions in order to have the desired mesh. I can upload the WB.zip but I'm using V14 so if you are working with an older version you won't be able to open it, just let me know. |
Thank you all for your suggestions and help.
I did manage to create individual blocks in Ansys DM (as frozen) and mesh it individually in Ansys Meshing with the required edge sizing. http://i.imgur.com/jEvIZ.png But now the problem is when I open the mesh in Ansys Fluent it gives a series of warning saying: "Flow boundary zone xx is adjacent to a solid zone (y). This problem MUST be fixed before solution can proceed!" (where xx & y are boundary zone numbers and solid zone numbers) In the Boundary Conditions tab a single Named Selection is being split up into 2 parts in Fluent for e.g. inlet is being split up into inlet-part_1-solid and inlet-part_1-solid.1. (exception are backwall and frontwall) http://i.imgur.com/L6WCu.png My hunch is that the contacting faces of the blocks in the geometry is creating the problem. I tried changing the type for wall-part_1-solid to "interior" but it gave an error saying: "Cannot change wall-part_1-solid to interior because adjacent cell threads are of different types." How to overcome this problem in Fluent? |
@Far: Thanks for the video links. Due to time constraint I am forced to use Ansys Meshing for the current project. Maybe in future I would start learning Ansys ICEM. Do you have any good tutorials for beginners?
@Gweher: I am using Ansys v13. |
This is just a guess because I cant try it right now:
Are all cell threads defined as fluid zones in fluent? (check this under "cell zone conditions") Maybe there is a solid zone left which cannot form an interior boundary with the fluid zones. Did you group the parts in the design modeler? Maybe this could help. Just out of curiosity: Why do you use different grid spacings in z-direction? |
Your guess was correct flotus1. Some of the extruded blocks were of type solid while others were of type fluid in Ansys DM. After I changed all of them to type fluid, neither Fluent didn't give any warning and nor were the Named Selection faces split up.
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I thought of having finer mesh near the cylinder so I used a bias in the edge sizing on the radial egdes.
http://i.imgur.com/OoNJD.png But the mesh created in that region is no more a structured hexahedral mesh. http://i.imgur.com/hcRYV.png I have successfully done the same thing for the 2D case. Any idea why is it not happening for 3D case? |
Did you apply a "structured mesh" to the surfaces?
Did you set the "behavior" of the edge sizings to "strict"? For the 3D-geometry: Unless the walls in z-direction are no-slip-boundaries, there is no need to use a 3D-mesh. The results will be identical on a 2D-mesh with any RANS-based approach. |
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[Ref]C.H.K.Williamson - Oblique and parallel modes of vortex shedding in the wake of a circular cylinder at low Reynolds numbers Although I don't know how significantly does the change in the nature of vortex shedding affects the frequency of vortex shedding and heat transfer but if you say that the results will be identical then I would better go with 2D mesh because I am not keen on studying the end condition of the cylinder. |
You need 3d mesh for Re > 180 due to 3d nature of flow. It is good idea that you can check your mesh for 2d and 3d cases for Re< 180.
How many diameters you are taking in 3rd dimension? Try to search some references that what should be 3rd dimension in terms of dia of cylinder. You can make the mesh independence study for x-y coordinates for the Re< 180 and therefore for Re>180 you just check the mesh requirements in the Z-direction. |
I haven't finalized the dimensions in the spanwise direction (Z-axis) yet but the domain which I have chosen right now has a depth of 40D (D=Diameter of cylinder).
In this report on St-Re relationship they have conducted experiments with cylinders having L/D>50: A new Strouhal–Reynolds-number relationship for the circular cylinder in the range 47<Re<2×105 By the way do you have any idea of how big or small the time step should be? |
Reference 1 (When to perform domain size, mesh density and time step study)
http://www.tfd.chalmers.se/~lada/pro.../proright.html Quote:
Reference 2 (For span wise depth) http://www.iawe.org/Proceedings/5EACWE/103.pdf Quote:
http://www.cfd-online.com/Forums/ans...cylinders.html Quote:
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Thanks Far for the elaborate description. As a coincidence I had stumbled upon this post of yours while browsing the net before I signed up on cfd-online. :)
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As always, it depends...
If you want to simulate the flow past the cylinder using a LES or even a DNS approach, you will need a 3-dimensional mesh. In this case, there are a lot of other things to worry about when setting up the case, the structure of the mesh is just one of these issues. But if you are using a turbulence model based on a RANS-approach (like k-epsilon for example) and there is no gradient of the mean velocity along the z-axis, the results will be the same on a 2D and a 3D mesh. And yes, a strouhal number can be obtained with an unsteady RANS simulation. The heat transfer will be in rather poor agreement with experimental data in this case, since the intrinsic assumptions of the RANS-approach are not necessarily fullfilled in a detached flow. |
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new to ansys 13
hi to all.. am new to ansys 13.0 ... i finished the vortex shedding problem in fluent 6.3.. now i want to do it in ansys 13.0... am unable to create the geometry in DM... i need step by step tutorial.... thanks in advance...
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Hi All,
I've split the main block into smaller blocks by using the extrude>add frozen method in DM. However when I go into Fluent, it shows that there are multiple walls at the boundary conditions. May I know is this the correct method to split the main block into smaller blocks? If so will the multiple walls shown under the boundary conditions in Fluent affect the results? Thanks! Regards, Hee |
I think you should get rid of the "surface body" and the non-frozen solid body in DM before proceeding.
Now the next step depends on the type of interface you want between the blocks. Method 1: non-nonformal interface Proceed with the multiple independent frozen bodies. This allows you to mesh every block independently. However, you will get an interface between each block which affects your results. Method 2: conformal interface Select all the bodies, right-click and "form new part". There will be no additional interfaces created in fluent since the mesh transition between the blocks is conformal. |
I've got it, thanks so much for your quick response!
Another question is that which model should i use in FLUENT to simulate an airflow of Re= 10,000? Is there any reference materials which i can refer to to obtain information such as boundary conditions settings (intensity & viscosity ratio), solution methods, time-step sizing and etc? Thanks! Regards, Hee |
Sorry for jumping in :
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hi , would u mind sending me wb.zip too?
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cylinder with finite length
Hi everyone,
I would like to make a 3d mesh with a circular cylinder in it, but the cylinder has a finite length, so it doesn't "touch" the boundaries of the domain. I don't know how to create a mesh above and below the cylinder. Anyone has some idea about his? Thanks |
cylinder with finite length
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Hi darazsbence,
I know its late but just wanted to show a couple of meshing methods (for future). Even if it has finite length, you can mesh it the exact same way as mentioned above. But assuming you have another body other than the cylinder (cylinder+body) in the domain, then the basic idea for a mesh is shown in the attached pictures. Best, Rahul |
hi, can i know the step to do those meshing? when i use multizone, i got an error which is found free block in swept body. I am doing with a sphere instead of cylinder
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Hi Karl,
Multizone is not a magic option that will automatically generate a hexa mesh for all geometries. Here the error is quite obvious "found free block in swept body". You need to partition your geometry using slice planes in order to help the mesher and successfully achieve the mesh you want. I would recommend you to look at tutorials on blocking strategies to gain more insights on how to partition your geometries. Have fun ;) |
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