# yade-dev team mailing list archive

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

```>> I like my point of view very much :-) ...
> No luck then. :(

it was just explanation why I did the answer. In general I agree with both
options, just wanted to "defend" my approach.
I also think we come to philosophy area in many places here :-) especially
because finally one arrives to the same results and formulas from both
sides.

>>> 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.

see my previous (slightly modified):
- assuming the particles rigid
- computing the force under assumption that in reality there would be no
overlap and some deformation.
Then the elastic force-displacement equation just computes force assuming
some deformation in reality, but the model of particle may remain rigid.

>> 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.

Relative motion is just relative motion. You can define it both for rigid
and deformable particles. In both cases it is just a consequence of the
previous paragraph and decision if particles are or are not rigid :-)

> But my claim is that there is not one context in which the geometrical
overlap would be justified.
> Can you evaluate this volume change by the overlaps? NO! Absolutely not.
> Overlapped volume is not an approximation of anything.

Yes, I agree that from physical point of view, the "purely geometrical"
overlaps does not make much sense. In the launchpad question I just
described how I would do it, but with no experience in the topic..
But it would be nice to actually compare this geometrical and physical
approaches :-) maybe "by chance" the geometrical (non-pysical) overlap
justification is not that far from physical evaluation, even for the first
sight there is no reason for it (I have a fresh experience with something
similar).

cheers
Jan
```