yade-dev team mailing list archive

[Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx>]

The answer is (just for the record, I discussed that with Vincent) :

In a 2nd order finite difference scheme for Newtons law, positions and 
forces are defined at times t+Ndt, while velocities are defined at times 
t+(N+1/2)dt. If everything was defined at times Ndt, it would be a first 
order sheme.

v(t+0.5*dt) = v(t-0.5*dt) + a(t)*dt
p(t+dt) = p(t) + v(t+0.5*dt)*dt

Bruno



Vincent Richefeu a écrit :
> (I re-send this mail because it didn't appear in yade-dev list... 
> sorry if you receive it for a second time)
>
>
> Hi everybody,
>
> This morning I had a look at the file LeapFrogPositionIntegrator.cpp 
> and I a little scarred of what I saw (I exaggerate of course).
> Can someone enlighten me on that point:
> If I translate the c++ code into human readable operations, I've got:
>
> v(t+dt) = v(t) + a(t)*dt
> p(t+dt) = p(t) + v(t+dt)*dt + getMove
>
> 1. The first thing that seems odd to me is the use of v(t+dt) in the 
> evaluation of p(t+dt). There's no great danger if the displacements 
> are small and slow. However, it is know that even a quasistatic load 
> may be accompanied by small localized dynamic crises. If there is 
> indeed a problem here, no big deal because reducing the time step is 
> enough to correct it. But it's a pity!
>
> 2. The second that I don't understand is the method getMove that seems 
> to return a force (!) I know, I am probably wrong. Can anyone give me 
> details on what this function returns ?
>
> Thank you in advance for your helpful comments,
> VR
>
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