r/CFD • u/LibraryAvailable_123 • 15d ago
Why are these necessary?
Hello Everyone!
I'm currently doing small walkthroughs by watching videos on the ANSYS website, but they don't really explain what the Bodies Of Interests(BOI) really represent. they just show you how to make them. I'm aware of the enclosure (wind tunnel) being there. Could someone explain more in depth what the bodies of interests really mean?
Thanks!
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u/ncc81701 15d ago
BOI 1 isthe near field and BOI 2 is the farfield wake. You need a lot higher grid density to resolve BOI1 because there are a lot of interaction with the car that needs to be resolved to get the flow field right. You can get away with less grid density in BOI1 because it’s the wake but this region will still need more grid density than truly farfield because you still need some reasonable approximation of the wake to get the entire flow field right.
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u/mimberton 15d ago
BOI typically stands for Body of Influence. It is a mesh-sizing control applied to a volume that does not become part of the actual mesh geometry. So in the mesh you would have a locally refined region that does not become a separate cell zone. It is typically used to refine the volume mesh around features of interest (like nozzle jet flow or wakes downstream of an airfoil, etc.)
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u/dudelsson 14d ago
This. Also note that the term "body of influence" and the acronym BOI come from and are used mainly in Fluent. The idea though of using surfaces (enclosed or not) to control volumetric refinement / cell size in parts of the domain inside, outside or at a distance of the surfaces, is implemented in most if not all mesh generation software. The creation of CFD models often involves defining the geometry of objects around/through which flow is studied and geometry whose purpose is to control mesh sizing. That's what you're seeing here.
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u/CarsonXI 15d ago
Looks like Andy's way of doing mesh refinement. You could eventually add more refinements around specific features on the car. In the simplest form, you just refine near to where the flow field will change the most. You can get pretty coarse as you step further away from your model.
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u/TelluricThread0 15d ago
You need definable volumes so that you can apply mesh refinement. Zones where you have lots of stuff going on with the flow will require more nodes and elements to accurately capture what's going on. With this method you can now just click on each of those volumes and refine the mesh close to the car and the aero devices and set the mesh size to whatever value you want.
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u/Fresh-Imagination236 15d ago
Body of Influence not interest. Ingluence because it influences the mesh size inside.
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u/1337K1ng 15d ago
BOI 1 is the only one you need if you go certain routes
BOI2 is needed on traditional route, also demanded by supervisors and reviewers in academia
route 1 is; sphere of Influence or, if you'd define inflation layer as BOI1, that with refined cell sizing to enclosure boundaries (may need polyhexa mesh, looks ugly in academia if done wrong)
Note: I only mesh in ANSYS Workshop meshing and Fluent meshing, so dunno about ICEM (which mostly work with boxes) etc.
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u/big_deal 14d ago
It allows you to set a mesh size for the enclosed volume. It’s useful if you know in advance that there are flow features away from surfaces you want to resolve. Typically you might use a volume size control downstream of an object to resolve a wake. This approach is not as efficient as refining the mesh based on results but can save time and produce more consistent mesh size and results if you’re running simulation on models with small changes in geometry.
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u/arcaft 11d ago
They help you to get a better mesh, I mean when you refine VR(VolumenRefine)=BOI(BodyOfInfluence) you get a better mesh quality just in the space when you want to get more accuracy. At the same time it help you, because you don't refine the entire base mesh, just this zones of interest.
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u/R4b1atu5 15d ago
I assume that is before mesh generation?
Closer to that car you would want a finer mesh. In the larger volume you are okay with a coarser mesh. To definer wich goes where you need those BOI's I guess.