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Rotating Cylinder (Moving wall vs Moving reference frame vs Sliding mesh)

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Old   January 14, 2017, 10:36
Default Rotating Cylinder (Moving wall vs Moving reference frame vs Sliding mesh)
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Hamed Abdul Majeed
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Dear Users,

I am modeling a cylinder (with axis horizontal) rotating (rpm as input). It has an inlet at its axis and a radial outlet. This problem, as I understand, can be solved either:
1. Moving wall (rpm given to wall in the inertial frame)
2. Moving reference frame
3. Sliding mesh.

I have tried the three methods, they give same trends in velocity. I was wondering what is the actual way to model it!!

Any suggestion or comments are welcome.
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Old   January 14, 2017, 18:02
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Hamed Abdul Majeed
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I did figure out that in Fluent these cases are possible:

1. Axi-symmetric swirl flow
2. 3D swirl and rotating flow
3. Using single rotating reference frame
4. Using Multiple rotating reference frame
5. Using MRF with mixing planes at interfaces
6. Using sliding meshes.
7. Moving mesh approaches.
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Old   January 17, 2017, 13:18
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Lucky Tran
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Quote:
Originally Posted by hamed.majeed View Post
Dear Users,

I am modeling a cylinder (with axis horizontal) rotating (rpm as input). It has an inlet at its axis and a radial outlet. This problem, as I understand, can be solved either:
1. Moving wall (rpm given to wall in the inertial frame)
2. Moving reference frame
3. Sliding mesh.

I have tried the three methods, they give same trends in velocity. I was wondering what is the actual way to model it!!

Any suggestion or comments are welcome.
Newtonian fluids are Galilean invariant. The choice of how to model it is up to the user, and you may take the most convenient approach or most annoying approach depending on how much of a sadist you are. =)

You are really asking 3 different questions.

Do you want the reference frame to move? If so, then move it. If not, then don't move it.

Sliding mesh is only needed if you if the domain changes such that the computational grid must be deformed. For example if you have a rotating gear then the gear teeth in motion would occupy cell positions so that the cells must move out of the way.

The sliding mesh does not say anything about the motion of the boundaries, you still need to specify that the walls are moving. Just to demonstrate the point, you could use a sliding mesh in a problem where nothing is moving.

Some motions do not require a sliding mesh. Flow between two moving plates (or rotating cylinders). A sliding mesh is not needed because the mesh cells do not need to move out of the wall. The fact that the wall is moving is completely captured by the velocity of the no-slip condition.

A similar problem that does require a moving mesh is the squeezing of two parallel plates. As the plates move together, the mesh change shape.

The last question is the reference frame, which is completely up to you. How do you want to analyze the result? Some reference frames are convenient, some are not depending on what you plan to do.
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Last edited by LuckyTran; January 17, 2017 at 15:44.
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