yade-users team mailing list archive

[chiara modenese <c.modenese@xxxxxxxxx>]

On 7 May 2010 16:40, Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx> wrote:

>
>
>> I tried the sample example with the two balls and the time step estimated
>> by PWave. I attach the figure.
>>
> Mmmm... It is not "that" better. I expected better matching with corrected
> kinetic (as in the simple mass-spring test).
>
>
>  Now I suppose that if the kinetic energy is computed correctly (is it?)
>> the problem relies on the elastic energy. You said yourself that the total
>> energy is "approximately" constant.
>>
>
> ret+=.5*(b->state->mass*(b->state->vel+b->state->accel*dt/2).SquaredLength()+(b->state->angVel+b->state->angAccel*dt/2).Dot(diagMult(b->state->inertia,(b->state->angVel+b->state->angAccel*dt/2))));
>
> Ah! I see. At time "t" we have contact forces F(t-dt) and velocities
> V(t-dt/2).
>
Now I see as well. Thanks!

So, you need in fact to __substract__ accel*dt/2, and you will get at time
> "t" the energy of time "t-dt". Sorry if I wrote something different in
> previous mails.
> It explains why it match better with non-shifted curves, you shifted the
> wrong way. ;)
>
> Now replying to another question :
>
> > plasticDissipation +=
> (shearDisp+(1/currentContactPhysics->ks)*(shearForce-prevForce)).dot(shearForce)
> > Is shearDisp the total shear displacement, right? I would compute the
> dissipation incrementally, something like
> > plasticDissipation += [delta_us + (Fs_max_current - Fs_max_prev)/ks] *
> Fs_max_current
>
> The equations are almost the same. shearDisp is the increment.
> The problem in your equation is it doesn't handle the transition
> plastic->elastic or elastic->plastic (when the increment of displacement is
> partly elastic and partly plastic), it is the reason to use
> shearForce/prevForce instead of max_current/max_previous. If the increment
> is fully plastic, it makes no difference : in such case
> shearForce=max_current and prevForce=max_previous.


Perfect! Thanks for explain me ;)

Chiara

>
>
> Bruno
>
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