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I: Re: Porosity


>Hi Bruno,
>thanks for your suggestions, I will try to put them in place.

>Some further considerations/corrections:
>For small range of final 
porosity I 
>was meaning having a fixed PSD and a constant friction angle. I 
was wrong in my 
>previous message, the porosity changes if I change the 
friction angle.
>Question: which value of friction angle do you use during 
the compaction phase? 
>Should be the final one or should be lower than that? 
Would it be possible to 
>run the analysis with a value equal to zero?
important point regarding 
>the unbalanced force. I agree with you that the 
value of unbalanced force 
>should come down just cycling (the time step is not 
zero) but that doesn't 
>happen in the way I should expect. For instance in 
your code, once you choose 
>the expansion method, after each expansion you are 
actually cycling without 
>expanding for a certain number of iterations (look 
at the bool 
>isARadiusControlIteration), so I would expect that the unbalanced 
force comes 
>down during this process (of course before I have to get some 
stresses at the 
>walls) but it remains always around 1 until the stresses on 
the walls become 
>higher. I would expect that also at a small level of stress 
on the walls, this 
>value of unbalanced force remains quite low. I suppose 
that the reason you (if 
>it were you) put the bool isARadiusControlIteration 
relays in the concept of 
>maintaining a quasi-static equilibrium as much as 
possible during the analysis. 
>To make a summary I see that the unbalanced 
force reaches low values (say 0.07 
>or whatever) only after having compressed 
the packing. Am I escaping something?
>Thanks, Chiara
Messaggio originale----
>>Da: bruno.chareyre@xxxxxxxx.
>>Data: 19/01/2010 
>>A: <yade-users@xxxxxxxxxxxxxxxxxxx>
>>Ogg: Re: [Yade-
>users] Porosity

>>> I would need your help concerning the Triaxial. Since 
>I need a 

>>> specific value of final porosity, I am trying to do something in 
code but 
>>> without success (BTW, I DON'T want to modify your code, if it 

>will work I will 
>>> manage to add a flag or something like that).

>problem. Just keep me updated so I know what happens in files.
>>> so 
>in the line 
>>> (1) it is like saying "cycle" without expanding if 
the value of 
>Unb force is 
>>> too high. Unfortunately "cycling" doesn't help 
(why? Should 
>it?), the computed 
>>> unbalanced force is always very high and 
around 1.
>>There is nothing "bad" in keeping a constant radius until 
unbF is small.
>> If 
>the unbalanced force does not approach 0 with cycles 
"_supposed to 
>>be_" doing 
>nothing else than dynamic integration, there is 
>>something wrong. The 
>timestep is not 0?
>>The only problem I 
see in your approach is you force the 
>porosity you 
>>want, which will result 
in very big overlaps and high stress 
>>likely. The porosity should be 
the result of a "realistic" packing 
>>> Could you give me 
>>> suggestions or directions how to do 
>that? I mean, how to fix the 
value of final 
>>> porosity..
>>Keep a 
>constant stress and decrease 
friction a little bit at each step. 
>>The packing 
>will self-reorganize, and 
porosity will decrease. Save the 
>>packing state when 
>porosity value cross 
the value you want.
>>You can use this function to modify 
>friction :

>>void TriaxialCompressionEngine::setContactProperties(Scene * ncb, 

>>frictionDegree)//I really mean "use", not copy ;)
>>All you have to do 

>is a loop that will keep assigning frictionDegree 
>>each n=~10 steps, with a 

>decreased frictionDegree each time.
>>Note that big porosity variations 
>occur with small values of  
>>friction, in the range 0° < f < 5°.

>>> I have 
>got that having a specific PSD and a 
>>> friction angle we can 
get only one 
>possible value of final porosity (or a very 
>>> small range of 
it) and that's 
>true with the DEM (cause we are dealing with 
>>> spheres and 
not irregular 
>shapes) but to me it is always possible to say that 
>>> for 
the same PSD I 
>would like to run the Triaxial with an higher porosity 
(higher than that 
>maximum dense packing I get with this method either with 

>>> compaction or 
>>What is small range? I tried that 
after your message and 
>I got different 
>>poro from different friction.

>>> Merci beaucoup.

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