CfdOF for hollow airbox

[Uncle Peter]

OS: Windows 10 Version 2009
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.24267 +99 (Git)
Build type: Release
Branch: Branch_0.19.3
Hash: 6530e364184ce05ccff39501e175cf2237e6ee4b
Python version: 3.8.6+
Qt version: 5.15.2
Coin version: 4.0.1
OCC version: 7.5.3
Locale: English/Canada (en_CA)


Hello,

I would like to use the CfdOF workbench to simulate airflow through an airbox that I designed. The box has a circular opening (intake) in the front, which I would like to send air through and four openings on the side (exhausts), through which I would like to see the airspeed. See attached photo for reference.

I've tried following this tutorial below to generate a flow, but it does not work for me since the intake/exhaust holes in my airbox do not have faces (therefore I cannot specify them as inlets and outlets). The pipe in the tutorial is also non-hollow, whereas my object is hollow (technically mine is a hollow object made of solids).

https://www.youtube.com/watch?v=EpzLt1WIm8A

Is there a better way to do this using this workbench? Or a way to modify my object that will allow me to use this workbench for flow simulation?

I cannot attach the FreeCAD file as it is too large, unfortunately.


-Peter

airbox.png (72.71 KiB) Viewed 1911 times

[thschrader]

Hi,
for cfd you must mesh the inside of your body.
That means: you must design an "inverted" model (the air inside is the new body)
Can you strip down your model?
Have a look at this:
https://forum.freecadweb.org/viewtopic. ... i&start=30

engine inlet:
https://forum.freecadweb.org/viewtopic. ... 6&start=90

[KAKM]

In general, a good model for CAD is not a very good model for CFD. You have to do a bit of tweaking to get it to mesh properly.

I would suggest starting by selecting all the faces that will be in contact with the airflow and cloning/copying them into a new model. Depending on how you modeled this, you might need to play with the Draft Upgrade/Downgrade tools to be able to just select the faces you are interested in. From there, you can close the inlet and outlet by selecting the edges around the gap, cloning them, and upgrading them until you get a face (you might need to downgrade first to be able to access the edges). You can also consider removing any features that aren't really important to your simulation. Alternately, you could make a whole new model of just the flow path or try using a boolean cut with a solid around your existing model.

Make sure that your resulting model is "watertight"–that it doesn't have any gaps at the surface.

[Syres]

To create the solid for Cfd in the attached very simple example, the following procedure was used:

1) Install/Update using AddonManager or manually install Curves Wb, this is for the Parametric Solid function
2) Using View menu > Clipping Plane and in my case using the X axis plane 'open up' access to the faces inside the model (as long as you can select them it doesn't matter if the clipping plane has chopped them in half)
3) Switch to the Curves workbench and using Ctrl key select all the internal faces and once verified that very single one has been highlighted then click the Parametric Solid button on the Curves Wb toolbar (this means even if you change the size of the actual air intake model then the Cfd model will change accordingly).

Tested using:

OS: Windows 7 Version 6.1 (Build 7601: SP 1)
Word size of FreeCAD: 64-bit
Version: 0.20.27428 (Git)
Build type: Release
Branch: master
Hash: 27460358508a2057e0ec57a418641435f12628dd
Python version: 3.8.6+
Qt version: 5.15.2
Coin version: 4.0.1
OCC version: 7.5.3
Locale: English/United Kingdom (en_GB)

[Uncle Peter]

Thanks for the responses everyone.

Turns out modelling the "airflow" was quite easy given the way I modeled the airbox. Was able to run the analysis with great success and get some nice looking flows.

One more question I had. As you can slightly see in my original image, inside the airbox is a solid cylindrical ring with several cylindrical holes in it, that runs the length of the airbox. This was my attempt to simulate airflow through an air filter (which I know is incorrect).

There is a a toolbar function in the CfdOF workbench that lets you create a porous zone. Ideally, I would like to replace this ring of holes with a solid ring and define it as a porous region. The issue I see is that once I fuse all the solids into a single solid, how can I define the porous region?

I've posted a diagram below, hopefully this makes things more clear. Basically I want to be able to define the inner cylindrical ring as a "porous" region, while keeping the outer cylindrical ring and inner cylinder as normal "air" regions.

porous.png (41.86 KiB) Viewed 1610 times

[thschrader]

...how can I define the porous region?

The inner cylindrical ring can be handled as a separate object to define the porous zone.
It is not necessary to add the inner ring to the simulation domain.
For instance you can define the inner ring as a mesh-refinement volume, so the ring can be a separate object.
Same works with the porous-zone definition.
Example:
https://www.opensim.co.za/training.html

adding_porous_zone.JPG (79.68 KiB) Viewed 1530 times