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[Jan Stránský <question403385@xxxxxxxxxxxxxxxxxxxxx>]

Question #403385 on Yade changed:
https://answers.launchpad.net/yade/+question/403385

Jan Stránský proposed the following answer:
Hi Mohsen,

by MWE (Minimal Working Example [4]) I meant a script simulating the
cylinder and computing resulting parameters (e.g. computing strain parallel
to cylinder axis might be a bit tricky). I f you can provide such script,
still it would be good, thanks :-)

> So by changing this, you change the explicit micro-macro
> relationship,preserving the model consistency.
> If micro-macro relationship changes, i think the model consistency would
> change too.


ok, it depends on the definition of consistency :-) you can define the same
material model only changing the definition of "interaction cross section
area" r*r or pi*r*r. The micro-macro parameters relation would not be the
same, but only multiplied by a constant factor..

NeverDamage is active! also I decreased it to 1e-3


even if neverDamage=True, there might be some plasticity (which is not
deactivated by neverDamage)..


1- there is no rule that "micro and macro parameters should be the
same".


exactly


> 2- macro parameters must be independent of geometry (However I am
> simulating a cylinder trying to keep CoordinationNumber, porosity and
> fabric constant and the macro parameters vary about 20%).


**ELASTIC** macro parameters must be independent on geometry... actually it
does depend on geometry, because every time the interaction network is
random. Also the number of particles should be large enough to decrease
these random effects and also boundary effects (boundary behaves
differently from bulk, by increasing number of particles, the boundary
become less and less significant). But the variation should not be "large".

3- For calibration by selecting the appropriate contact law and setting the
> geometry constant:
> 3-1- The porosity should be the same as reality (first parameter suggested
> by Bruno @6)


If you simulate physical grains, then yes. In some application (typically
concrete simulated with CPM) particles are just artificial discretization
of in reality not porous material, and then this criterion does not make
sense, just you should keep the simulation porosity constant for different
simulations (e.g. randomDensePack with constant particle radius results in
porosity very close to 38%).

3-2- The micro parameters should change in such a way that desired macro
> ones obtained (I do not have any idea about how to set coordination number
> and fabric isotropy). In the first try it is preferable to select micro
> parameters the same as macro.


coordination number is controlled by aabbEnlargeFactor and
interactionDetectionFactor. By keeping it constant, coordination number
should be constant, too
fabric isotropy should be "autimatic" by default.
The "first try" of micro parameters is a tricky part. For normal models, a
good idea is to use default values or proposed in some example script. By
changing microparameters, you observe change of macroparameters and you can
use some optimization technique to obtain the desired values.
You can also use dimensional analysis to make the process with less
variables. E.g. for elastic parameters, the dimensional analysis shows that
macro Poisson's ratio only depends on micro poisson and that macro Young's
modulus is linearly proportional to micro young.


3-3- It is better to validate the obtained results (micro parameters and
> the three suggested by Bruno) by changing the geometry; However the
> parameters should be kept constant.


It is a nice approach. For example (any other tests are possible :-)
Calibrate parameters on cylinder and then use it on 3-point bending if you
have experimental or theoretical data.

cheers
Jan

[4] https://yade-dem.org/wiki/Howtoask


2016-10-26 14:58 GMT+02:00 mohsen <question403385@xxxxxxxxxxxxxxxxxxxxx>:

> Question #403385 on Yade changed:
> https://answers.launchpad.net/yade/+question/403385
>
> mohsen posted a new comment:
> @ Bruno
>
> Thanks! I should investigate your suggestion. It looks great.
>
> @ Jan
> >please really provide a MWE :-) it would be much easier to determine the
> >source of problem..
>
> So according to #1 I mentioned the way i computed E and nu. But i think
> i did not understand the meaning of MWE. Can you tell me what it is?
>
> > So by changing this, you change the explicit micro-macro
> relationship,preserving the model consistency.
> If micro-macro relationship changes, i think the model consistency would
> change too.
>
> >1e-2 is relatively high, there might be already some inelastic processes
> (with the MWE I could tell if yes or no)
> NeverDamage is active! also I decreased it to 1e-3
>
> >Next time plese be more specific what "great difference" is (10%? 1000%?)
> Jan I did not consider Coordination number in the simulations. Hence the
> previous results can not be compared.
>
> I want to conclude:
> 1- there is no rule that "micro and macro parameters should be the same".
> 2- macro parameters must be independent of geometry (However I am
> simulating a cylinder trying to keep CoordinationNumber, porosity and
> fabric constant and the macro parameters vary about 20%).
> 3- For calibration by selecting the appropriate contact law and setting
> the geometry constant:
> 3-1- The porosity should be the same as reality (first parameter suggested
> by Bruno @6)
> 3-2- The micro parameters should change in such a way that desired macro
> ones obtained (I do not have any idea about how to set coordination number
> and fabric isotropy). In the first try it is preferable to select micro
> parameters the same as macro.
> 3-3- It is better to validate the obtained results (micro parameters and
> the three suggested by Bruno) by changing the geometry; However the
> parameters should be kept constant.
>
> Regards
>
> Mohsen
>
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