My favourite trick was back at the university when the professor during fluid dynamics lectures (but you can play the same game with maths of structural mechanics) came up with a really-really long and complex equation, then he applied some conditions (saying for example that one or two parameters are neglectable compared to the others) and elegantly scored out 80% of the equation. From then on it could be solved easily. What should a CFD guy do if the geometry that he is about to use for a simulation contains a fully detailed, complete in-line six-cylinder natural gas powered engine?
The same. Let me show you how.
Some time ago I worked on a gas transmission pipeline evacuation compressor assembly in which the gas compressor was driven by an in-line six-cylinder natural gas powered engine. The device itself – as its name shows – is used to evacuate gas pipelines, sure it is not a cheap machine but as far as I know it pays itself back under 1-2 pipeline evacuations, so it is a good business for everyone.
Anyway, I got the extremely detailed otherwise very precise parasolid geometry (without feature history of course) from the engineer who was responsible for the mechanics. Starting from this I needed to make a CFD simulation of the surrounding air of the compressor unit. When I say extremely detailed, please think about this:
Got the V-belt, generator, fuel pipes and electrical wiring? And the infinite number of bolts? And the washers?
Meshing such a detailed model is almost impossible even for SC/Tetra. Not just because the small parts would require very detailed mesh with small element sizes but the parasolid model of the engine also contained some surface parts with gaps I was not able to stitch automatically and make a solid model out of them. No matter what I needed the engine as a closed volume to use it in the external flow simulation.
I could choose the path to remodel the whole engine thing in our CAD system and put that into the simulation knowing that what I do will be simple enough and 100% closed volume. But first: this would have taken too much time and second: this would have not been elegant. I know much better than that.
The simulation task itself did not require such a detailed geometry either. I needed the engine to explore air flow around it, so I needed the engine plus the parts attached to it as a closed volume but not too detailed however not too rough to represent the contour. How can I simplify the engine without sweating blood, but the elegant way like the professor did some years ago?
I pull a bed-sheet over it.
This is what we call Wrapping in SC/Tetra. As a first step the software creates a grid of hexa elements (cubes) around the geometry, of course the user can determine what cube size should be around each part. The ones I want to keep more detailed will have smaller the rest will get a bigger sized cube grid. The smaller the cube size is the tighter the bed-sheet shape will be. The resolution of the whole engine-compressor block was 15 mm and I used 10 mm cube size just for a few but important parts.
And the result is a tessellated model (like an .stl file) that contains all important details of the motor-compressor unit, makes a single closed volume that can be used further on for my simulation. Who could ever want more?
Wrapping is a jolly good tool for creating geometry of under-hood simulations. It can fill multiple cavities that you can find in electronics, headlamps, fog-lamps, etc. But this is the way I make a simplified contour of complex shaped castings like engine blocks and cylinder heads.
So if there is a task looking very complicated, have no fear. Just score out the half of it like the professor did or wrap it up like I do. It will work.
Dr. Robert Dul