Hey there,
It’s Robin from CFD Engine & much of this week has been spent chasing holes out of scan data, trying to make sure I don’t end up with a leaky model. But with more triangle-wrangling in store next week, I have to ask “am I wasting my time?”
I asked this question on LinkedIn a while ago & I’m not sure we reached a consensus. For some respondents a closed mesh was essential, for some it was preferable & some weren’t too bothered.
For my work in automotive external aerodynamics, I prefer a closed mesh, but it’s not a showstopper.
I’m not usually interested in the flow inside the vehicle, so it feels like a waste of cells to mesh it. But the cells in there are just as good quality as they are on the outside, so I could probably tolerate them. Plus snappyHexMesh doesn’t really care whether my input geometry is closed, watertight or even if it’s intersecting.
So why do I spend so much time trying to make sure that my models are closed & that mesh doesn’t get where I don’t really want it?
I think it harks back to pre-SHM days, when other meshing tools required a watertight input geometry – flashback to chasing red edges in ANSA until they turned up in my dreams.
But does it really matter any more?
If the leak paths are small (I’m not talking about leaving the window open, NASCAR-style) and the cells are good then perhaps it’s not really a problem?
Should I accept the higher cell count & longer run times as a worthwhile trade-off for less of my time spent tidying CAD & chasing leaks?
The answer is probably “YES” – but I can’t bring myself to do it. I’ve been obsessing over input geometry for too long to change my ways. But there’s also something else holding me back…
Fear of the unknown
Managing to keep the mesh where I want it helps me maintain an illusion of control over proceedings. Leaks immediately make me wonder…
- where are they?
- how big are they?
- how did I miss them?
- what else did I miss?
All questions that make it harder to have confidence in the results 😬
And that’s my main motivation for all the effort – the fear of the unknown.
- What if the holes are somewhere aerodynamically sensitive?
- What if they have a measurable effect on the forces?
- What if the quiescent flow in the cabin causes convergence issues?
The only way to really answer questions like these is to fix the leaks. So, just like the problem cells from last week’s email, I reckon it’s better to fix them at the outset, rather than have them bite you on the bum later on.
Other worlds
Whilst the open/closed thing is open for debate in my external-aero world, there’s a good chance that it isn’t in yours.
Leaky ship models aren’t great, neither are leaky heat transfer models.
Some meshing tools care very deeply about holey geometry, whilst other let you shrinkwrap the problem away.
And particle-based methods probably don’t care whether your geometry is closed or not.
As usual in CFD, there are many possible scenarios.
Where I’m at
I reckon I’m always going to obsess over the details of my input geometry & spend (considerable) time upfront to make sure I only get cells where I want them.
So, if you put a lot of effort into closing gaps & chasing leaks then I’m with you – I feel your pain 🙏
And if you couldn’t care less about gaps or leaks, then I’m jealous 🤫
Perhaps I need to move with the times?
The last thing I want is to be locked in my meshing “cave” with my headphones on, filling holes in CAD whilst the rest of you stopped that nonsense 10 years ago & you’re all in the bar having post-mesh cocktails 🍹
I’d love to hear your thoughts on this one, drop me a note.
While you’re at it, let me know if you’re interested in an email on leak mitigation with tips for finding, fixing & avoiding leaks.
Until next week, may the mesh be with you,
