setting viscous damping coefficients in Hookean systems

dbreton's picture
Submitted by dbreton on Mon, 11/21/2011 - 19:56

In the LIGGGHTS documentation, it explains how to use

fix ID all property/global gamman peratomtypepair ... and
fix ID all property/global gammat peratomtypepair ...

to set the normal and tangential viscoelastic damping constants. It also talks about a gamman_abs and gammat_abs alternative to that fix.

However, I cannot seem to set them myself in an input script. The function "deriveContactModelParams" in pair_gran_hooke_history.cpp seems to override any settings I want to make.

A snippet of my contact parameters section looks like this:

fix m1 all property/global youngsModulus peratomtype 1.0e5 1.0e5 1.0e2
fix m2 all property/global poissonsRatio peratomtype 0.3 0.3 0.3
#values are for pair interactions: 1-1, 1-2, 1-3, 2-1, 2-2, 2-3, 3-1, 3-2, 3-3
fix m3 all property/global coefficientRestitution peratomtypepair &
3 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001
fix m4 all property/global coefficientFriction peratomtypepair &
3 0.000001 0.000001 0.000001 0.000001 0.000001 0.000001 0.000001 0.000001 0.000001
fix m5 all property/global characteristicVelocity scalar 0.1
fix m6 all property/global cohesionEnergyDensity peratomtypepair &
3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.0
fix m7 all property/global gammat peratomtypepair &
3 1.0 1.0 1.0 1.0 1.0 1. 1. 1. 1.0
fix m8 all property/global gamman peratomtypepair &
3 10.0 10.0 10.0 10.0 10 10 10 10 10.0

Any thoughts on this? I have tried both the gamman and gamman_abs versions of this, no luck.

Thanks
DAN

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raguelmoon's picture

raguelmoon | Tue, 11/22/2011 - 04:27

Dear,
LIGGGHTS uses new contact model in which the stiffness and damping coefficients k_n, k_t, gamma_n, gamma_t are derived from the material properties. If you want to use LAMMPS style model then dont use YoungsModulus etc, but just use

fix ID all property/global gamman peratomtypepair ...
fix ID all property/global gammat peratomtypepair ...
etc

See doc-pages for detail.

Thanks,
Ram

Ram

dbreton's picture

dbreton | Tue, 11/22/2011 - 19:10

hi Ram,

Thanks for the input -- LIGGGHTS seems pretty insistent about having youngsModulus, coefficientRestitution, etc. specified when using gran/hooke or hooke/history, but at Christoph suggests above I think I want to use gran/hooke/simple instead.

I will forge ahead

Thanks
DAN

ckloss's picture

ckloss | Tue, 11/22/2011 - 09:16

Hi Dan,

what pair style are you using? gran/hooke/history does not use gamman, gammat, but pair/gran/hooke/history/simple does!

Cheers, Christoph

dbreton's picture

dbreton | Tue, 11/22/2011 - 19:06

Hi Christoph-
I am using a pair style of my own for sintering interactions, but it is based off of gran/hooke/history. I'll try re-doing things using the simple version

Thanks for the help,
DAN

ckloss's picture

ckloss | Tue, 11/22/2011 - 21:55

Hi Dan,

the coefficients are read in the init_substyle() function, so in your own pair style you will have to implement reading these coefficients to be able to use them for your model. Have a look at how it is solved in pair/gran/hooke/history and pair/gran/hooke/history/simple and you will see how to do it.

Hope I could help

Cheers, Christoph