yade-users team mailing list archive

[Jerome Duriez <jerome.duriez@xxxxxxxxxxxxxxx>]

Hello,

I'm focusing on the equations used in GlobalStiffnessTimeStepper. And I 
would like to link all this with what I've understood in the 1D case of 
mass spring systems (with the help for example of a former mail of 
Bruno, quoted in 
http://www.mail-archive.com/yade-users@xxxxxxxxxxxxxxxxxxx/msg00583.html, I 
did not find the original mail sorry)

The calculation is here apparently based on the diagonal terms, let us 
say Kd (=stiffness.x/y/z() in the code), of a stifness matrix defined 
for each body, matrix which takes into account all contacts involving 
this body. Because, I imagine, that this stifness matrix will appear in 
the matrixial write of a recursive scheme which would be equivalent to 
our integration "algorithm".

- So is the condition for the stability of the sheme in 3D coming from 
the same considerations about stability of recursive sheme as explained 
former by Bruno ? But by replacing the condition about this real alpha 
(alpha < 1) by one condition about the diagonal terms of a matrix ?

- If yes, why can we keep this sqrt(2) here in GSTS, while in 1D it was 
explained that this sqrt(2) has to disappear to take into account 
opposite motions between bodies ?

"FYI" all these questions because I want to write my kind GSTS for UDEC, 
and for now UDEC computes a timestep only by Dt = k * sqrt( Min(Mass of 
elements) / Max(Stifnesses of contact) ), with k = 0.2 generally. So, 
with few theory behind it seems, but with a smaller timestep at the end 
(is that also your feeling ?)...

And few more technical questions :
- Chiara : in my use of UDEC my rigidities are also proportionnel to 
sqrt(u). So I'd be glad to know how you use GSTS in your simulations : 
wich values of timestepSafetyCoefficient, and timeStepUpdateInterval do 
you use in your simulations ?

- Vaclav : would it be possible for you to send me also a copy of your 
thesis (or the paragraphs concerning time steps) ? Note that I finally 
found this reference 
http://www.emeraldinsight.com/Insight/viewContentItem.do?contentType=Article&hdAction=lnkpdf&contentId=878199 
(but I could not get the article)

- Bruno (?) : I'm very sorry but I could not find in the code what 
prevented GSTS to do its job (I saw all this active or isActivated() but 
I did not find where these things really act)

Thanks,

Jerome

--
Jérôme Duriez
ATER Iut 1 Grenoble, département GMP - Laboratoire 3S-R
04.56.52.86.49 (ne pas laisser de messages sur le répondeur)