Zitat von Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx>:
I tried to understand how to calculate torsional moments for clumps.
Please have a look at the pictures in the attachment (see link above).
There two clumps are shown and I do not understand how their torsional
moments can be calculated. In the first picture there is box clumped
with a "half sphere". In the second picture you can see a box inside a
sphere, like I want to model my sand grains in future. How is
torsional moment calculated in this cases?
I don't understand the question, sorry. Torsional/bending moments are
defined for interactions, not for individual bodies (even if the bodies
are clumps, it makes no difference). Your pictures have only one body,
how could you define torsional moments?
Back to theory:
In DEM one gets a contact force F_c from the contact law. From F_c
one gets angular moment M of the body (cross product with r_c and
sum over all contacts c). The angular moment is given by
multiplication of torsional moment J and angular acceleration a_a.
M = J*a_a (1)
For spheres J = 2/5 * m * R^2, where m is mass and R is radius. Right?
Then one gets angular velocity by rearranging and time integration of (1).
a_a = int(5M/(2mR^2))dt (2)
In the case of a box J is a tensor, not a scalar. Calculation of
angular velocities would be also no problem, just a little bit more
complicated. (I hoped this is done by RotationEngine, but now I
think its not.)
My question is, how can one obtain J (and I think one needs it to
calculate angular velocities and displacements) in case of clumped
box-sphere systems?
Or another question also comes in my mind: How is it done with
clumped spheres?
Can this be simulated with RotationEngine already?
RotationEngine is not related to your problem, it is only prescribing
motion.
For box-box interactions, there is a serious background in 3-DEC from
itasca, where arbitray polyhedral shapes are modelized. It should apply
to boxes.
Hm, I think in the case of cubids (boxes) it should be much easier,
than a general polyhedral shape.
Sure, but you will find interesting ideas in 3DEC regarding contact
formulation.
Ok, I will have a look at it.
My only question is if it is really necessary to modelize angular
vertices instead of rounded shapes.
If I want to model rounded shapes, I could clump spheres,
Not really, a rounded box has flat surfaces that you will never have by
clumping spheres. The contact between two flat surfaces will transmit
moments.
Right, but isnt it too expensive in terms of CPU time? I mean a box
has eight vertices and every vertex will be replaced by numerous
triangles with a lot of vertices in the rounded case ...
Bruno
