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Doubts about cundall-strack law between sphere-facet interaction

Asked by jacopo

Hi, i would like To have some informations about the interaction between facet And sphere using cundall-strack law. I read The "DEM beckgraund" where IT explains the Logic of The contact law between sphere-sphere. As Is well explained, there Are springs between The two balls, And knowing The young modulus And The elastic lenght of the "Springs" during the contact , IT can obtain The forces.

Then, considering a facet- ball contact, How can IT recover The force? I mean, facets ( considering them dynamic, with mass And inertia) don t have thickness. How can Yade consider The spring?

I m facing this problem Becouse i clumped more facets togheter( from a mesh file). Clumping them toghether, i obtained a rigid body. Rigid body Is rigid for definition, But Modifyding The facets material, i obtain different forces during the rigid body impact( IT impacts against spheres body) . How can yade calculate IT if the body Is rigid And doeSn t have thickness Becouse Is Made by facets?

I hope you can understand what i 'm trying To explain. Thanks for your time.

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Bruno Chareyre
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Jan Stránský (honzik) said : 		#1

Hello,
for the contact computation, the facet is considered to have radius=2*sphereRadius [1]
cheers
Jan

[1] https://github.com/yade/trunk/blob/master/pkg/dem/Ig2_Facet_Sphere_ScGeom.cpp#L115

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jacopo (varrialeee) said : 		#2

thanks, i think i understand. To be more clear : when the facet hit the ball, it is considered as a thickness facet with a thickness = 2*sphereradius? is it right?.

some more questions:

->Does exist a way to give a predefined facet thickness? i know that facet does not have thickness, but maybe could exist a way to give it, like a wall. Becouse i would like to make a comparison if is possible.

-> then a clumped body made of facets can not be considered perfectly rigid becouse during the contact it acts like a "spring". Is it right?

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Best Bruno Chareyre (bruno-chareyre) said : 		#3

Hi,
> i know that facet does not have thickness, but maybe could exist a way to give it, like a wall. Becouse i would like to make a comparison if is possible.

Walls have no thickness either. Boxes have thickness, but it is still completely unrelated to contact behavior.
You could write facet.thickness=10 in python script for a comparison, it will not change the result.

> then a clumped body made of facets can not be considered perfectly rigid

Along this line nothing is perfectly rigid in yade since all contact models include elasticity. I agree with this view.
The only place where we use a perfectly rigid approximation is when we calculate motion, defined by just translation and rotation.

Bruno

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jacopo (varrialeee) said : 		#4

Thanks Bruno Chareyre, that solved my question.