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Re: [Yade-users] [Question #676841]: the definition of porosity


On 12/17/18 4:26 PM, Jan Stránský wrote:
Hi Bruno,

A) I am currently not so active contributor, so I do not insist on my point of view

Still the most active, it seems. :)

B) I like my point of view very much :-) and it makes to me much more sense and I wanted to present it here

No luck then. :(
We will have to not mix our replies.
Not a big issue, but an interesting philosophical question.

as I understand DEM and always explained it is that:

I agree, your way to put it is the most common. "Best" not implied.
And actually worst from both scientific and didactic points of view IMO.

> I would suggest that Cundall/Yade DEM makes no assumption of rigidity/overlaps.

but does not make any assumption on non-rigidity at the same time, right? :-)

It does. Starting from an elastic force-displacement equation in the first place really implies non-rigidity.

Relative motion between bodies says nothing about (non)rigidity of the bodies, or?

If you have a sticking (elastic) contact it means two material points from two different objects are co-moving. If at the same time the reference position-orientation of the inertial frames attached to each particle are not undergoing rigid body motion the only solution is that there is an internal deformation somewhere.

> Rejecting the notion of overlap is I think the only way to escape classical ill-posed questions on porosity. "Should overlapped volumes be removed?"

Such question always needs a close context. Missing context is IMO making it most ill-posed.

Yes, this is context-dependent in general. But my claim is that there is not one context in which the geometrical overlap would be justified.

- I have a dense assembly by triaxial compression I squeeze it ti 2/3 of its original volume.. probably in this case it makes sense to somehow treat overlap/deformation.

In that case you certainly need to account for volume change of the solid phase, yes.
Can you evaluate this volume change by the overlaps? NO! Absolutely not.
You'll need volume change as a component of the model.  Selecting an expression randomly (here cumulative overlapped volume) is not a valid replacement for a missing model. And actually overlaps amplify the real volume change of standard materials by far. It can even lead to porosity >1...

Well, you always have some limit on accuracy.
HM tells the volume change for one sphere-sphere contact, but many contacts with "big" overlaps would influence each other and you again ends with an approximation.

"Approximation" is always defined with respect to an ideal, more accurate, value.
2-sphere HM is only an approximation of multi-contact situation, I agree.
Volume change = 0 is then an approximation of the 2-sphere HM when normal disp. tends to zero. Overlapped volume is not an approximation of anything. It's first derivative is way off any physically justified quantity. It sounds better than zero because it is not constant, but it is actually not better. The error by using overlap is larger than by keeping zero (this is assuming that Poisson coefficient of the solid phase is between 0 and 0.5...).



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