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Message #14447
Re: [Yadeusers] [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 nonrigidity at the same time,
right? :)
It does. Starting from an elastic forcedisplacement equation in the
first place really implies nonrigidity.
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 comoving. If at the same time the
reference positionorientation 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 illposed questions on porosity. "Should overlapped volumes
be removed?"
Such question always needs a close context. Missing context is IMO
making it most illposed.
Yes, this is contextdependent 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 spheresphere 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.
2sphere HM is only an approximation of multicontact situation, I agree.
Volume change = 0 is then an approximation of the 2sphere 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...).
Cheers
B
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