Learning Forum

[MerlinWilliams]

Hi, I am new to ansys and am trying to create meshes of a model for the fluid and solid domains for an FSI simulation of blood flow in an aneurysm.

I started with an stl file of the geometry and imported it to spaceclaim converting it to a solid body. From here I made an outward shell to create the 'solid' domain for my problem (ie the aneurysm wall). I then trimmed the inlets and outlets of the domains to remove the new facets that had been added with in the shelling step. The output can be seen in the image below. I then converted the shelled part, seen in grey below, to a solid body.

[peteroznewman]

You will need to spend some time cleaning up your geometry before meshing. You can easily get rid of the sliver faces on the cut end face by simply using a plane to remove a thin slice of the solids.
Here is the Google Search Results with links to this topic on this site.
I have some videos on my YouTube channel on this topic.

[MerlinWilliams]

Ah that looks very helpful thank you very much. I will give them a look and let you know how I get on

[MerlinWilliams]

I have had a chance to watch your videos and I think it has helped a bit. Firstly when I tried to use the AutoSkin tool on the stl file it did not work and there were some errors that popped up which didn't allow me to do the auto skin. Shown in the images below:

So I then tried the process outlined in the third video you posted. I imported an stl file as a solid body into spaceclaim to have the same starting point as you of a solid body. I successfully completed the process and ended up with the thickened surface and a volume with the result shown below:
I then have now imported it into Ansys mesher to try create a mesh for both domains. However when I imported it I got the warning message 'A body was processed that faces but no facets. Mesh generation and mass property calculations may be impacted'. I could successfully produce a mesh for the fluid interior of the model, but when I tried to mesh the solid exterior of the model it failed (shown below) and gave me the error message 'A mesh could not be generated using the current meshing options and settings'. There were none of the sliver elements at the inlet and outlets as before and I am not sure what is wrong.


Could you please help me as to the next steps that I should take to successfully create the two separate meshes?
Also at the end of your third video you posted you mention that there are tools to smooth a body like the one you were using, do you have a video of that process? Because my current starting point is using a model that I have smoothed and refined in Meshmixer so I wonder if that is causing some problems and it might be simpler to do the refining in Ansys as well from the initial coarse stl file.
Thank you very much for your time.

[peteroznewman]

In SpaceClaim, did you use the Separate All button on the Facets ribbon? That will let you hide the main body and delete all the tiny bodies. Did you use the Check button?

[MerlinWilliams]

No I did not use that, at which stage of the process should I use that?

[peteroznewman]

Do that at the start when you first import the STL before you do anything else.

[MerlinWilliams]

Thank you I will try that again.
Is this in relation to trying the autoskin method or the method in your third video? Because for the other method in your third video I imported the model as a solid body not an stl file

[MerlinWilliams]

So I applied the separate all tool after importing the stl file and it kinda helped. The number of problem areas decreased so that there are no errors on the main body now but there are still some on the edges of the outlets as seen below.

[peteroznewman]

And the Auto Fix button does not repair that? What is the icon in the Structure tree, a closed facet body or open surface facets?
Create a plane slightly inside the body and use the Split button to slice a thin section off the end. Delete that end. That should give you a clean facet body.

[MerlinWilliams]

It is a closed facet body and the auto fix button does nothing. I cut off the thin sections off the end and tried auto skin again but the same problem appeared. as seen below
Could there be another way around the problem? The method in your third video seems exactly what I need as I need a fluid domain inside of a solid domain, or do you think the problems meshing with that method and the problem with auto skin are linked? What are the ways to smooth an stl file or solid model that you mention at the end of your third video?
I am very grateful for your time and help.

[peteroznewman]

Using Thicken is only helpful if you can assume a constant wall thickness. Arteries typically have varying wall thickness, so CT imaging delivers both the inner and the outer surface of the artery wall.
If you put the cleaned up version of your STL file into a .7z or .rar archive, you can attach it to your reply and I will take a look.

[MerlinWilliams]

for my research assuming a constant wall thickness is acceptable and the geometry I have been given is just the outer surface. Also for the purpose of the research it is acceptable to thicken the model outwards or inwards to create the artery wall.
That is very kind of you to have a look, I really appreciate it.
Merlin

[peteroznewman]

What version of ANSYS are you using?
Is the STL file in mm?

[MerlinWilliams]

Yes the stl file is in mm, im using Ansys 2021 R2 Student

[peteroznewman]

I have made some progress, but don't have the final geometry you want.
What is the wall thickness you want?

[peteroznewman]

10 mm thick wall

[MerlinWilliams]

Oh wow that looks amazing thank you very much! The wall thickness that I preferably want is 0.3679mm if it is thickening it outwards.
How should I now progress to get to something like the patch solid model that you linked, but with the correct wall thickness? I can't thicken it as its not faceted, so would I use shell and shell outwards and then use volume extract to create the fluid domain? However, I am not sure how I might use the thickened facets file you linked.

[MerlinWilliams]

I am very grateful for you helping me but, the model 'patch solid' has increased in size, did you specify the units as mm when opening it? The diameter of the largest opening should be about 3mm, how would I go about making it back to its original size?

[peteroznewman]

STL files have no units associated with them. When you open an STL file in SpaceClaim, it defaults to Automatic. I have no idea what that means.
It is vital to know the units the STL file was created with. In your case, it is mm. Unfortunately, I didn't get that information before I started playing with the file.
If you know the size of a feature in the model, you can use the Scale tool in SpaceClaim to resize the geometry to the correct size.
Now that you have provided a specific wall thickness and direction, I can use that in a fresh attempt.

[peteroznewman]

[MerlinWilliams]

Thank you so much for your time and help, I am incredibly grateful.
One final question. Upon creating the fluid domain by using the volume extract tool and then importing it to the meshing software in Ansys I will need to create a mesh where the faces and nodes of the outer surface of the fluid domain mesh and the inner surface of the solid domain mesh match up. Do you have any idea how I would do this? Would the match control work, or would I have to create shared topology between the fluid and solid domain?
Thank you very much again for your continued help
Merlin

[peteroznewman]

After you do the volume extract in SpaceClaim, use the Share button on the Workbench tab. That will ensure the nodes at the interface are shared between the fluid and the solid.

[MerlinWilliams]

,ok thank you.
And will this ensure that the nodes at the interface are shared even if I add in a feature like an inflation layer for the fluid domain?

[peteroznewman]

Yes.

[MerlinWilliams]

Amazing thank you very much.

[MerlinWilliams]

I am now trying to create the mesh for the solid domain and I want to make the solid domain two elements thick. How can I do this? I have seen the thread linked below but I am still not sure.
Is it also possible to specify the two element thickness for only the solid domain and not the fluid domain and make sure that the elements and nodes still match for the inner face of the solid domain and outer face of the fluid domain?

[peteroznewman]

Here is an artery mesh with 4 elements through the thickness.
Here is the fluid mesh with inflation layers.
There is a different mesh on the artery than the fluid, but an identical mesh is not required for one-way FSI where the fluid flow is solved first and the pressure used as a load on the artery
Tell me more about how you want to do your FSI. The Student license only permits one-way FSI.

[MerlinWilliams]

So I am actually doing the FSI simulation on OpenFOAM not Ansys and am just going to export the mesh from Ansys to OpenFOAM.
The fsi I need to implement is a 2 way fully coupled partitioned approach. Will this mean that I have to use the same type of mesh for each domain?

[peteroznewman]

I don't know OpenFOAM, you will have to ask on their support forum. Maybe they have a contact definition, I don't know, but you can't go wrong with a congruent mesh that shares nodes at the interface.

[MerlinWilliams]

Ok thank you anyway

[MerlinWilliams]

please can you tell me how you made two different meshes on the fluid and solid domain?

[peteroznewman]

I meshed the solid body that represents the fluid domain, then I extruded the faces of that mesh by a constant thickness for 4 elements.

[MerlinWilliams]

Ah ok thank you

[Smiles]

Hello, i am an MSc student working on my dissertation. please i created this model in revit then imported it to the geometry but when i meshed it i encounter failed mesh on the windows and the walls around the window. please how can i correct this error.

[aneesfahim]

I am working on the blood flow simulations of the carotid arteries. I have imported the .stl file into SpaceClaim
Listing a few issues, I have come across.
Once I tried to use the separate all tool, Spaceclaim commented that No bodies are separated
Even after trying to convert into solid using the Autoskin tool, I am getting a geometry with 2 faces
Could you please look into the issue?
I am attaching the model herewith

[aneesfahim]

Sorry to mention the thickness of the artery.
It's 1 mm throughout.

[peteroznewman]

What version of ANSYS are you are using?

[aneesfahim]

2021 R1

[clown 1935]

Hello, I am a college student. Could you please share your artery STL file? Thank you very much. If you'd like, here's my email: 1584087554@qq.com.