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[Chiara Modenese <question159394@xxxxxxxxxxxxxxxxxxxxx>]

New question #159394 on Yade:
https://answers.launchpad.net/yade/+question/159394

Hi,

Two questions related to periodic case.

1. Servo-control mechanism using stiffness

In PerIsoCompressor.cpp, we find:
strain_rate=1/scene->dt*(goal[axis]-stress[axis])*cellArea[axis]/(stiff[axis]>0?stiff[axis]:1.);

My first observation is that a gain parameter is missing in this equation. Do you think that in case it was added, this servo-control could work? We apply here the same equation as for rigid walls. However, I remember Bruno saying that the calculation of stiffness in this case would be wrong. What is the main reason? I think it would be better to have one servo-control for the periodic case, other than two. Am trying to identify which algorithm would be more appropriate.

2. Simulation loop
	
In NewtonIntegrator.cpp, we find this note:
//NOTE : if the velocity is updated before moving the body, it means the current velGrad (i.e. before integration in cell->integrateAndUpdate) will be effective for the current time-step. Is it correct? If not, this velocity update can be moved just after "state->pos += state->vel*dt", meaning the current velocity impulse will be applied at next iteration, after the contact law. (All this assuming the ordering is resetForces->integrateAndUpdate->contactLaw->PeriCompressor->NewtonsLaw. Any other might fool us.)
	
Am not sure to understand the above issue. Can you help me out with that? I think it is important for me to see it. 

Thanks a lot,
Chiara

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