Gravity Acting Oppo...
 

Gravity Acting Opposite for Some Parts and not Others  

 

pjay
 pjay
(@pjay)
Active Member
Joined: 5 months ago
Posts: 13
January 6, 2020 6:42 pm  

Hi All,

I am currently using the LOAD_BODY_Z card to apply gravity to my model (three parts), the first being SPH particles and the last two being Shells with MAT_RIGID properties. It appears that gravity seems to be acting in the correct direction for the SPH particles but not one of the shell parts (blue part seen at link below).

View post on imgur.com

Some model parameters set:

*DEFINE_CURVE

A1 O1

0.0 0.0

0.05 9.81

1000.0 9.81

*LOAD_BODY_Z --> uses the defined curve above.

Does anyone have any ideas why one of shells appears to float up while the SPH particles are acting as expected for a gravity load?

Cheers!


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Negative Volume
(@negativevolume)
Estimable Member Admin
Joined: 1 year ago
Posts: 171
January 7, 2020 1:00 am  

Hey @pjay,

Rigid parts can act weird for sure. The first thing I would do would be to check you *Mat_rigid cards and make sure that you don't have any global constraints toggled. 

The second thing would be to try changing you implementation of gravity. LS-Dyna recommends this odd method where the *Define_curve is actually just Time: 0-1000 (as long as your sim) 1.0-1.0 and the *Load_Body_Z has a scale factor of 9.81. This may have no effect at all, but this is Dyna and it can be weird sometimes.

Try those things and let me know if it is still an issue. 


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pjay
 pjay
(@pjay)
Active Member
Joined: 5 months ago
Posts: 13
January 7, 2020 2:56 am  

Hey @negativevolume,

It did not seem to have an effect at all, I think I might have to try using the *BOUNDARY_PRESCRIBED_MOTION_RIGID card or if you have any other ideas let me know! I am using a *MAT_RIGID card because it seems that when I use *MAT_ELASTIC the run time goes from 25hrs to ~1000hrs which does not seem reasonable (1850 shell elements, 990,000 SPH particles).

Cheers!


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Negative Volume
(@negativevolume)
Estimable Member Admin
Joined: 1 year ago
Posts: 171
January 9, 2020 3:30 pm  

@pjay

Is your Z axis pointing up or down relative to your model? A positive acceleration for *Load_Body_Z will apply gravity in the negative Z direction. I know, it seems backwards, but that's just how dyna calculates it. 

Also, you can also try to put all the parts of your model into a part set and reference that part set in *Load_Body_Parts. This is a card that tells LS-Dyna to only apply the Load body on these parts. 


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pjay
 pjay
(@pjay)
Active Member
Joined: 5 months ago
Posts: 13
February 3, 2020 12:14 am  

@negativevolume

Can't say I was able to figure it out but changing the material property to elastic instead of rigid seemed to fix the problem.


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