Learning Forum

[Lee]

Hi everyone,

  I want to do a 3D simulation that a rigid body moves in an elliptical path, now I have:

1. Center axis of the ellipse(in vector form);

2.Length of semi-major axis a and semi-minor axis b, as is shown in Figure below;

3. Rotation speed of the object (rev/min)

  How can I realize this motion, do I need to use UDF?

and furthermore, any user-defined path.

Regards

 

[Rob]

Is the object spinning too? Is there anything around the object? What drives the motion in the real case?

[Lee]

 

 

Hi, Rob:

(1) Yes, in real, the moving object rotate around its own axis;

(2) Yes, the moving object is connected with another driving object(Let’s say a rod);

(3) The driving object(the rod) pushes the moving object to move along the ellipse path, but I do not know the force between them.

  Figure 2 shows the simplified situation.

Regards

Figure 2

 

 

[Rob]

Split the two motions. So, the green object spins using MRF or sliding mesh. Moving mesh can then be used to bodily move the green object. Note, that's moving mesh with no remeshing. Or, you can mess with the gravity vector and just spin the green object. Either way I suspect it's UDF based but defining the path mathematically may not be easy. 

[Lee]

Thanks Rob! For the revolution, is there an UDF in Fluent can define the path(I can obtain the function of ellipse)?

I have an idea about the elliptical motion, but I am not sure it will work, the thing is: I obtain the velocity function in X, Y, Z directions, and put these three functions in the “CG_MOTION” to realize this motion. But this method is a little bit complex. The ideal method in my mind is to define the expected path by input the ellipse function, and drive the object by the revolution speed, but I do not know if it is possible to realize.

[Rob]

CG_MOTION is likely a good way of doing it, DEFINE_MESH_MOTION is another. The rotational functions in Fluent are designed for mixers, turbines etc inside another fluid (or casing) rather than eccentric motions. We often use compound systems to rotate components relative to another moving part, but again, those motions are circular. Hence questioning how you plan on causing the motion profile in reality as that can give you ideas on how to model it.