yade-users team mailing list archive

[Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx>]

Václav Šmilauer a écrit :
> kan napsal(a):
>   
>> Hey, guys,
>>  
>> I have a problem to get the critical timestep, could you please give
>> me hand?
>> if I need to calculate the critical time step for each time steps, how
>> can I calculate it?
>> From papers, I found that equation is:
>> critical_time_step=2*sqrt(mass/k),
>>     
> Bruno, how does this value compare to critical timestep calculated from
> elastic wave propagation speed along the particle calculated for
> cotinuum, i.e. , which is 2*particle diameter / sqrt(Young's modulus /
> density)?
Wait a minute, what is "Young modulus" for you?
If it is the real modulus of the spheres, it has no existence in Yade.
If it is the modulus of the packing, it is changing all the time, just 
because contacts are created/lost all the time (from 0 contacts at the 
begining to around 5 per sphere after some iterations).
I don't see how you can compute a constant Young modulus here.

> I would assume it underestimates the timestep, but may have
> the advantage that it has to be evaluated only once, since contact
> stiffnesses are derived (for purely linear elastic contacts) from the
> same values.
>
> I use this formula for preliminary estimation of computation time for a
> simulation.
>
>   
There is a simple way to evaluate if a timestep is good or not :
run the simulation and watch the evolution of unbalanced force (now 8th 
column in WallStresses files generated by TriaxialStateRecorder) while 
you increment dt manualy. When unbalanced force start to tend to a 
constant non-zero value, or even increases more and more, dt is too large.
You will see if your dt could be much larger without any trouble.

Honestly, given that the automatic computation of dt takes less than 5% 
of cpu time, I don't see any interest in using another approximate 
method. It would make sense only if you are sure that your approximate 
dt is only 5% smaller than the automatic one (so that you don't waste in 
extra iterations the time saved by not computing dt). Really unlikely...

Bruno





> Regards, Vaclav
>
>
>
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>   


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Chareyre Bruno
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