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Old   July 31, 2012, 23:56
Default Moving mesh in CFX
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Ali
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Hi Guys,

Does anyone know how I can move a domain and an object (like a flat plate) together like a rigid body? In CFX we only have two options: stationary domain and rotating domain. In my problem, which is a 2D simulation, I want to move the fluid within the domain as well as a flat plate, sinusoidally up and down?

Tnx
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Old   August 1, 2012, 08:00
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Can you explain what you are modelling and why you need this? It sounds very strange - and it is always difficult to model strange things as they are not physical. CFX is designed to model real systems so impossible systems are difficult to model.
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Old   August 2, 2012, 02:14
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My problem is a 2d simulation of a flat plate, which moves sinusoidally up and down. I'm trying to investigate the contribution of the added-mass effects to the lift and drag coefficients through this simulation. I have two ways of doing this simulation. First, I can just move the flat plate and get the results. The second way is to move the flat plate in conjunction with moving the fluid around the plate. In other terms, the plate and the fluid around it move together like a rigid body. The problem of doing the first way is, when the plate moves the mesh is deformed around the plate. For instance, when the plate moves up, the elements are stretched below the plate. As a result, the mesh at this region becomes coarse, where is of interest for the study. My question is can I do something like layering method in FLUENT to generate elements at this region. Do u have any idea on how I can keep the resolution fine in this region?

The good thing about the second option is, there is no mesh deformation around the plate and therefore the resolution around the plate remains fine. All the deformations occurs far from the plate. This case is not unrealistic, because in reality when you accelerate a body in a fluid some parts of the fluid moves with the plate like a rigid body. This is basically one of the added-mass effects.

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Old   August 2, 2012, 02:17
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Please have look at the tutorial posted on"https://www.box.com/shared/jicr9u3jgv". On page 3, you'll see a figure similar to the second way of doing my problem.
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Old   August 2, 2012, 20:13
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I seem to recall answering this question before. Please do not post the same question on multiple threads.

This is a simple moving mesh simulation, or you could do it using immersed solids.
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Old   August 2, 2012, 21:39
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I could move the flat plate sinusoidally, but as I mentioned in my last post, there is a problem in this simulation. When the plate moves the mesh is deformed around the plate. For instance, when the plate moves up, the elements are stretched below the plate. As a result, the mesh at this region, where is of interest for the study, becomes coarse, My question is how I can keep the resolution fine in this region (around the plate)?

Thanks,
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Old   August 3, 2012, 09:03
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The mesh motion will be simplified in this case if you move the fluid region adjacent with the body, then the mesh further away just needs to do a simple stretching to follow the motion. This is easy to implement using a subdomain where you specifiy the mesh motion.
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Old   August 3, 2012, 22:52
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Finally worked! thanks for your help
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Old   August 16, 2012, 16:10
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Hi Glenn,

Sorry to bother you again. I'm trying to run another simulation similar to the one I explained. In this simulation, the plate and the fluid around it move sinusoidally up and down and at the same time move sinusoidally to the left and right. Therefore, the motion of the plate is like a diagonal motion. In this case, I get a fatal error, which says "a negative volume detected". In this case, the mesh is deformed significantly. The angle between the edges of an element becomes negative and the error occurs. Do you have any idea how to solve this error?

Thanks,
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Old   August 16, 2012, 16:21
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Is mesh motion defined based on the velocity of the plate? Integration of the plate velocity with respect to time defines the mesh motion? Right?

Thanks,
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Old   August 16, 2012, 18:28
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Can you post an image of what you are doing and the motion you want to apply?

Mesh motion is usually applied as displacements on the boundary conditions.
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