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Why should I make a subtracted body?

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Old   October 4, 2018, 03:52
Default Why should I make a subtracted body?
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Yuk Tong Cheng
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So I learn Ansys fluent myself and through many online tutorial, i know that we should create a enclosure, subtract the computational domain with the too bodies.
I believe we should do this because it simplifies the processes of setting up the domain (tell the computer that there is a object right here), i just wondering if there is another way to do it instead of using subtract option.

The reason why I come up with this problem is because I want to try relating Ansys fluent CFD result (pressure) with ABAQUS structural FEA. If I subtracted the body, I am not sure if I can transfer the data to FE model. How would you guys do it in Ansys Workbench?

Anyone has some experience?
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Old   October 4, 2018, 12:58
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You do this because in CFD you are modeling the flow, not the body.

Let's say you have a car (or a spaceship) and you have its CAD model. You can't do CFD of the car because the car is a solid object. What you do, is CFD of the flow around the car. So you can't just import the CAD of the car into a CFD program because that is not what the program needs, the car is not the computational domain. The computational domain is the region around the car, not the car itself. This region around the car is the actual CAD that you need to provide. A simple way to do this, since you already have a CAD model of the car is to do the subtract. Otherwise, you'd have to build a CAD model of the fluid volume around the car yourself, which is awkward for most people because they feel like it is the negative. It is the negative if you are a structural person, but it is the actual model you need if you are a fluid person.

You cannot simply tell CFD that the object is right there because fluids don't care what those objects are. Fluids interact with objects through surfaces. All a fluid cares is if it encounters a surface. It knows nothing about what is underneath the surface layer.
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