Learning Forum

[yingtao]

Hello, I am writing to seek your assistance regarding to definition of rigid part rotating around certain location and axis in LS-DYNA.

My task is to load an angular acceleration curve in Y axis into model and compare the results in 2 methods, the rotation center point is CG (red dot) for method 1, the rotation center point is OC (black dot) for method 2 in the below picture, the pulses are same for 2 methods.

I created a rigid part, used *Boundary_Prescirbed_Motion_Rigid to load the curve, then Constraint_extra_nodes was added to the rigid part, the nodes are in brain.

My problem is the method2, I created a local coordinate and VID 99, changed DOF to 8 and VID 99 in the below picture, but the output animation shows the brain rotate center point is still CG, not change to OC. No difference between method 1 and 2.

Later, I tried to use *Part_Interia to define the mass center of the rigid part, but the result from method 2 is still the same as method 1.

Can you please give some advice about the issue? Thanks for your help!

 

 

[Reno Genest]

Hello Ying,

For a rigid body, the axis of rotation is always through the center of mass. Here is an excerpt from the LS-DYNA User Manual Vol I for *BOUNDARY_PRESCRIBED_MOTION:

 

So, I guess the center of mass of your rigid body is aligned with the CG point correct? Have  you tried to moved the rigid body so that its center of mass is aligned with point OC for method 2?

 

Let  me know if this helps or not.

 

Reno.

[yingtao]

Hi Reno,

I created a rigid part - 1900012 used for loading, the rigid body is moved to align with the OC point, but the rotation center is still the CG. Then I tried to assign the new mass of center for part 1900012 through xc, yc and zc in *Part_Interia. But the result has no change. Not sure what's the issue.

[Reno Genest]

Hello Ying,

Don't use *PART_INERTIA. Just move the rigid body to the OC point. Make sure you constrain the rigid body in all direction exept the axis of rotation using the constraints in *MAT_RIGID.

 

Let me know how it goes.

 

Reno.

[yingtao]

Hi Reno,

Tried the constraints in *MAT_RIGID, the rotation is different from without constraints in *MAT_RIGID. The result with constraints is the correct one, right?

But I re-ran method 1 (green, rigid part is in the black circle) and method 2 (yellow, rigid part is in the black circle), both have the constraints in *MAT_RIGID, the rotation of the model are still the same for method 1 and 2.

 

 

 

I changed DOF to 6 this time.

 

Can you give some suggestions?

Thanks,

Ying

[Reno Genest]

Hello Ying,

Does the rigid body rotate about its center of mass? If so, then the behavior is correct. If not, then something is wrong. The rigid body should rotate about its center of mass.

Also, you mentioned the following:

Tried the constraints in *MAT_RIGID, the rotation is different from without constraints in *MAT_RIGID. The result with constraints is the correct one, right?"

 

I don't know which one is correct; it is for you to decide. Is the behavior what you want? 

 

I suggest you build a small model to test the keywords. Once you are satisfied with the behavior, implement the settings on the real model.

 

Note that Ansys employees are not allowed to upload or download files on the Ansys Forum. You can still post your model and another user may be able to help you.

 

Let me know how it goes.

Reno.

 

 

 

 

[yingtao]

Hi Reno,

I set up a simple model, the rotation pulse was applied to samll rigid part located in the circle ( 1 for model 1 in blue, 2 for model 2 in orange). The tube is deformable, connected to rigid part through *constraint extra node.

The animation shows no difference for blue and orange (below are the 21 and 50ms). The constraint in *MAT RIGID is applied. I still have the issue. Do you have any suggestions?

   

 

[Reno Genest]

Hello Ying,

Is the center of mass of both rigid bodies on the same axis of rotation? If so, then you won't see a difference between 1 and 2.

 

Reno.