Learning Forum

[71Charger]

Hello everyone, I am quite new to CFD simulation and i am working on simulation of a screw compressor as my undergraduate project. I intend to work with CFX using immersed body solver. Having created geometry in SOLIDWORKS I am attempting to mesh it in ICEM CFD. I am facing some problems like:

• model imports as hollow.


• unable to understand what mesh size and parameters even mean.


• while computing volume mesh it shows dialogue box:

          "The mesh size appears to be inadequate to represent the geometry.do you want to run with autosizing?"

• If I hit yes,sometimes it creates new surfaces inside geometry (I guess they are called tetras) or sometimes prompts to repair a hole lying somewhere.

I guess i have described my problem well. I'll be grateful if somebody offers me help at least in one of those problems.

Thanks in advance

Regards

[peteroznewman]

Hello Charger,

I can help make sure the geometry coming from SOLIDWORKS is clean. Create a zip file with your SOLIDWORKS parts and assemblies and attach it to your post.  The zip file must be < 120 MB.

I don't know much about ICEM meshing. I know a lot about ANSYS meshing. Why choose ICEM?

What version of ANSYS are you using 18.2 19.1 ?

Do you have access to both DesignModeler and SpaceClaim?

Regards

[pgl]

Just a heads up that we integrated many aspects of ICEM CFD meshing technology into the ANSYS Meshing App in ANSYS Workbench.  Our recommendation is to use the ANSYS Meshing application. 

[71Charger]

Thanks peteroznewman

First I wanna express how glad I am to see replies that much quicker.

I just came to know that mesh quality is much better in ICEM than workbench that is why I chose it. I don't know much about both of them either.

I am using ANSYS 15.0 and i do have access to design modeler. I'm going to attach parts with assembly ASAP.

Have a good day

[71Charger]

Thanks pgl!

your recommendation will surely benefit me.

Regards

[71Charger]

Hello Charger,

I can help make sure the geometry coming from SOLIDWORKS is clean. Create a zip file with your SOLIDWORKS parts and assemblies and attach it to your post.  The zip file must be < 120 MB.

I don't know much about ICEM meshing. I know a lot about ANSYS meshing. Why choose ICEM?

What version of ANSYS are you using 18.2 19.1 ?

Do you have access to both DesignModeler and SpaceClaim?

Regards

 

By the way i used ansys meshing earlier and encountered "update failed for the mesh component" error

[peteroznewman]

Hello Charger,

You are in luck that I kept an old computer with ANSYS 15.0 on it.  What version of SW are you using? I was able to open your files in SW2017 and export the Parasolids.  There is a lot of geometry cleanup to get to a good starting point for meshing. Here is the cross-section as it exists after I deleted the bearings and gears.  I will be filling the holes where the bearings went for example.

Is the square frame on top (which is not united with the casing) the inlet?  Is the tube at the bottom the outlet?

I will be uniting the frame to the casing. 

I have limited CFD experience. The last mesh was for a rotor and housing where I could define a fixed boundary in the fluid to separate the mesh for the rotor and the mesh for the stationary section.  That is not possible for this type of pump.

Question for the CFD experts: How should the fluid volume be treated?

Regards,

Peter

[peteroznewman]

Hello Charger,

I have cleaned up the geometry and used Fill in DesignModeler to fill the space around the two screws and the casing. I added two solids to cap off the inlet and outlet. The resulting fluid volume looks like this:

The ANSYS Mesher can mesh it. The image below is just to show feasibility. I haven't turned on inflation layers yet. 

I want to hear from experts about how the two rotating screws interact with the fluid volume. Attached is the ANSYS 15.0 archive.

Regards,
Peter

[pgl]

71Charger when time/resources allows I also recommend that you upgrade to a more recent release. ANSYS Meshing is enhanced each release, no need to stick with an older version unless your hardware/OS system is the restriction

Peter: Kudos on this sweet clean up and mesh. .  

[71Charger]

Hi Peter! I am using SW 2014 and:

Is the square frame on top (which is not united with the casing) the inlet?  Is the tube at the bottom the outlet?

Of course they are, and are supposed to be united.

 The last mesh was for a rotor and housing where I could define a fixed boundary in the fluid to separate the mesh for the rotor and the mesh for the stationary section.  That is not possible for this type of pump.

I am unable to understand that.

Is there any special reason to export in parasolid instead of iges or sldasm?

[71Charger]

Peter I went through a screw compressor simulation and came to know that it resembles with lobe pump simulation right here:

https://fetchcfd.com/view-project/730

I guess it might answer your questions.

Cheers!

[peteroznewman]

Hi Charger,

I helped mesh a centrifugal compressor that had a spinning impeller inside a an axial-inlet, volute-outlet housing. In the airspace between the impeller and the housing, a surface could be constructed that split the fluid and a "sliding mesh" method allowed the elements around the impeller to rotate, while the elements in the volute were stationary.

Since the screw pump has one screw rotating CW and an interleaved screw rotating CCW, there is no fixed boundary to use the "sliding mesh" method.

SOLIDWORKS uses Parasolid as its geometry kernel, and so does SIEMENS NX11, where I have the skills to quickly clean up geometry for meshing. Parasolid is also the geometry kernel for DesignModeler. What this means is that there is no translation from one geometry kernel to another when moving geometry between those three systems.  Things can go wrong when the geometry has to be translated into another kernel, such as ACIS, which is what SpaceClaim uses. See this example.

I downloaded the lobe pump example to get an understanding of what kind of mesh the solver works with. Now I know there is the "Immersed Body" method in CFX so I have updated the geometry to have the two screws and the boundary solid. This is analogous to the lobe pump.  I don't have a CFX license to use in Version 15, only the newer Student licenses that are available.  Attached is the updated Version 15 geometry suitable for "Immersed Solid" method.

Question for the CFX experts:  If the boundary is meshed with tetrahedral, do the immersed solid bodies need to also be meshed with tetrahedral elements or can they by hex meshes?

Here is how I used the Geometry file: I linked three Mesh components, then suppressed two of the three solids to mesh a single solid. I exported three Fluent format mesh files, which I was able to import into CFX 19.1

Cheers,
Peter

[71Charger]

Thanks Peter! for taking the trouble to help me. I wholeheartedly appreciate everything you’ve done for me.

A big THANK YOU!

[maaz007]

i have a confusion. do we really need to mesh the screw solids. as everything is being done on the fluid. shouldnt we leave the 2 screws suppressed?

[peteroznewman]

If you use the CFX Immersed Body method, you mesh the fluid and the solid bodies separately.

Read this post.