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[Václav Šmilauer <eudoxos@xxxxxxxx>]

Hi folks (Bruno),

where exactly does come the damping formulation we use in Yade? I
thought it was Cundall's damping (I don't have fulltexts of his papers,
sorry), but Hentz in his thesis reports formula from Cundall's book
"Analytical and Computational Methods in Engineering Rock Mechanics",
which is supposedly like this (3.21, pg. 71):

  Δa=-λ|a|(v/|v|)          (*)

(a=acceleration, v=velocity, λ=damping coeff. I write a=F/m instead of F
as he does)

In Yade, we damp per-component,

  Δaᵢ=-λaᵢsgn(Fᵢvᵢ'),   (**)

where vᵢ'=(vᵢ+½(aᵢΔt)), approximate of velocity between this and next
timestep.

I understand that in (*) you have to compute 1 square root, have to make
sure that |v|>0 etc, making (**) more efficient, but also less obvious
(it is not invariant to coordinate system rotation, for instance :-|)

In short, can I get some references for what we use?

Cheers, Vaclav