yade-dev team mailing list archive

[Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx>]

Hi,

> where exactly does come the damping formulation we use in Yade?
It comes from Cundall (e.g. PFC manual). Either Cundall changed his mind 
at a point and it is not the same as in early papers, or perhaps Hentz 
made the same mistake as me when he saw the equation first. Note that 
(**) is (*) if you apply Einstein convention inside the bracket of (**).
In PFC manual, it actually reads :

"Δaᵢ=-λaᵢsgn(F_(ᵢ) v_(ᵢ) ') for i=1..3"

With the brackets on indices clearly excluding Einstein summation.

> 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 :-|)
>   
Yes, it really make this damping what it is : a numerical trick.
The funny part is that if you compute it like in (*), it is even worst : 
it generates locked-in forces and stable numerical solutions for 
non-equilibrium states, crazy. This is what I found when I used (*) for 
this model :
http://dx.doi.org/10.1061/(ASCE)0733-9399(2005)131:7(689)

> In short, can I get some references for what we use?
>
>   
In PFC manual, the reference for (**) is the one given by Hentz :

Cundall, P. A. “Distinct Element Models of Rock and Soil Structure,” in 
Analytical and Computational
Methods in Engineering Rock Mechanics, Ch. 4, pp. 129-163, E. T. Brown, 
Ed. London:
Allen & Unwin., 1987.

If this book really gives (*) (I don't know, to be honnest), you can 
cite my paper above for a reference to (**).

Bruno

p.s. For rotation, I vote "q"! :)