Body Intersection Issue in Autodesk CFD During CAD Import

[dylan.edmiston03]

Hi, 

I am trying to run a simulation of 1 stage of an axial compressor I have designed and I am running into a weird issue when I try to import the CAD.  I have 3 bodies in my CAD that I am importing as a .step file. They are the fluid, the impeller and housing (these are modeled as one component for simulation purposes), and the rotating region.  In CAD, the rotating region is overlapping and intersecting both the fluid and the housing.  When I import into CFD the software is chopping every intersection into different bodies. And because of this my rotating region has essentially disappeared and has just left split bodies on the fluid and impeller and housing where the rotating region once was.  Another way to say this is I have gone from what should be 3 bodies to 5 inside CFD (splitting along the intersecting bodies of the modeled rotating region). But there isn't two bodies overlapping at the same location anymore. 

This is making it where I cannot have a continuous flow path and a rotating region at the same time.  And I cannot set the body where the fluid is rotating as both a Fluid and a Rotating Region.  So I do not see how I could solve this simulation like this. 

How do I fix this? Is there a way to import two different files or do I need a different file type or is there some other solution? 

[marwan_azzam]

Hi @dylan.edmiston03 ,

Yes, CFD will create a new body for the intersection volume.

If your CAD model has two bodies that overlap you will have 3 bodies in CFD.

- How do I fix this?

The CAD model has to be modified somehow.

- Is there a way to import two different files?

No. You can only import one CAD file at a time.

- Do I need a different file type?

All the different file types will behave the same.

- Is there some other solution? 

No idea. Will have to see the model in order to recommend a solution so share if you can.

Marwan

[dylan.edmiston03]

Hi Marwan

"The CAD model has to be modified somehow."

Okay I don't understand how it could be modified and still work?  Not because I am not proficient at CAD, but because all of the rotating region autodesk CFD tutorials I have watched seem to have the rotating region intersecting the fluid as well as parts of the machinery.  

Am I correct in understanding that there needs to be a continuous fluid path as well as another overlapping body to define the rotating region for this fluid? Or does the Rotating Region take on the fluid properties of the fluid that it is touching (in this case Air)? As you can see from the cad I have attached, when the rotating region splits the fluid body there is not air touching air in the simulations.  It goes inlet -> air -> rotating region -> air -> outlet. 

[marwan_azzam]

Hi @dylan.edmiston03 ,

Thank you for sending the model.

Please see the attached model. It's three parts in CAD as well as CFD.

The rotating region should enclose the solid rotating part and also float in the fluid.

Marwan

[dylan.edmiston03]

Marwan

The issue with what you did is that I cannot simply enlarge the fluid passage as this is an axial jet engine and the fluid passage size is a fixed diameter.  This is not a fan in free stream. 

Additionally, you moved the rotating region to encompass the stator and inlet guide vanes.  This will not work. 

Lastly, I cannot have the rotating region 'floating' in the fluid as the blades of the rotor extend all the way to the edge of the housing of the engine (technically they wont be touching and there will be a very small gap so the rotor doesn't rub, but this is of course not modeled in for the CFD). 

So right now I have the rotating region dissecting the fluid.  I have included a photo. I am running a simulation now and am on time step 338 and this seems to be working.  So it appears that the rotating region does indeed take on the fluid properties of which it is touching.  I would love to hear this confirmed though since I am running a compressible flow simulation and the properties of the air (and eventually exhaust gas) need to be well modeled in the rotating region of the simulation. 

Another problem I have noticed that is immediately apparent to me is that, as you can see from the photo, there is a pressure gradient occurring at the boundaries of the rotating region and the fluid.  I am sure this have something to do with my ignorance of how to mesh correctly.  I plan to make another post soon to discuss the best methods for running compressible turbomachinery simulations once I have gathered a little more experience. 

Thank you.

[dylan.edmiston03]

I apolagize, that last photo you could not see what I was describing about the pressure change at the boundary of the rotating region.  This should demonstrate things better. 

[marwan_azzam]

@dylan.edmiston03 ,

I don't believe I changed the diameters of your model. I placed the Rotating Region between:

R1= 38.1 mm
R2 = 40.132 mm

Feel free to shorten the RR volume in the axial direction as you see fit.

Marwan