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[Question #683708]: Elastic potential energy of polyhedra


New question #683708 on Yade:


Following the discussion in [1], the normal stiffness "kn" in the Law2_PolyhedraGeom_PolyhedraPhys_Volumetric can have different units depending on the value of "volumePower"; e.g. for the default value volumePower=1, kn is in [N/m^3].

When calculating the elastic potential energy, we currently use [2,3,4]: 
where "kn" should be in [N/m] for the units to give energy values [Joules=N*m]. 

I think the energy calculation in the normal direction should change either as 0.5*(normalForce.squaredNorm()/(phys->kn*phys->area), 
where area the projection of the overlapping volume perpendicularly to the normal direction (this would work only if volumePower=1), or more safely, use the equivalentPenetrationDepth to begin with (which works regardless of the units of kn). Btw, looking at the calculation of the equivalentPenetrationDepth [5,6], is the exact "area" calculated or is an estimation made?


[1] https://answers.launchpad.net/yade/+question/681383
[2] https://gitlab.com/yade-dev/trunk/blob/master/pkg/dem/Polyhedra.cpp#L517
[3] https://gitlab.com/yade-dev/trunk/blob/master/pkg/dem/Polyhedra.cpp#L584
[4] https://gitlab.com/yade-dev/trunk/blob/master/pkg/dem/Polyhedra.cpp#L589
[5] https://gitlab.com/yade-dev/trunk/blob/master/pkg/dem/Polyhedra_Ig2.cpp#L121
[6] https://gitlab.com/yade-dev/trunk/blob/master/pkg/dem/Polyhedra_Ig2.cpp#L219

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