Commit 0b24a3d8 authored by Dan S. Bolintineanu's avatar Dan S. Bolintineanu
Browse files

Added a 'mass_velocity' damping option to the new granular pair styles and... 

Added a 'mass_velocity' damping option to the new granular pair styles and granular wall interaction.
This is to more closely match the old style pair gran/hooke/* damping behavior.
Updated doc page accordingly, as well as adding some examples on how to match old pair gran/* styles.
parent d0826f28

doc/src/pair_granular.txt

+42 −13
Original line number Diff line number Diff line
@@ -24,7 +24,10 @@ cutoff = global cutoff (optional). See discussion below. :ul 
[Examples:]



pair_style granular

pair_coeff * * hooke 1000.0 50.0 tangential linear_nohistory 1.0 0.4 damping velocity:pre

pair_coeff * * hooke 1000.0 50.0 tangential linear_nohistory 1.0 0.4 damping mass_velocity :pre



pair_style granular

pair_coeff * * hooke 1000.0 50.0 tangential linear_history 500.0 1.0 0.4 damping mass_velocity :pre



pair_style granular

pair_coeff * * hertz 1000.0 50.0 tangential mindlin 1000.0 1.0 0.4 :pre
@@ -38,7 +41,7 @@ pair_coeff 2 2 hertz 200.0 100.0 tangential linear_history 300.0 1.0 0.1 rolling 


pair_style granular

pair_coeff 1 1 dmt 1000.0 50.0 0.3 0.0 tangential mindlin NULL 0.5 0.5 rolling sds 500.0 200.0 0.5 twisting marshall

pair_coeff 2 2 dmt 1000.0 50.0 0.3 10.0 tangential mindlin NULL 0.5 0.1 rolling sds 500.0 200.0 0.1 twisting marshall

pair_coeff 2 2 dmt 1000.0 50.0 0.3 10.0 tangential mindlin NULL 0.5 0.1 rolling sds 500.0 200.0 0.1 twisting marshall :pre



[Description:]
@@ -59,7 +62,7 @@ types, the mixing for the cross-coefficients can be carried out 
automatically. This is shown in the last example, where model

choices are the same for type 1 - type 1 as for type 2 - type2

interactions, but coefficients are different. In this case, the

coefficients for type 2 - type interactions can be determined from

mixed coefficients for type 1 - type 2 interactions can be determined from

mixing rules discussed below.  For additional flexibility,

coefficients as well as model forms can vary between particle types,

as shown in the fourth example: type 1 - type 1 interactions are based
@@ -188,6 +191,7 @@ other settings, potentially also the twisting damping). The options 
for the damping model currently supported are:



{velocity}

{mass_velocity}

{viscoelastic}

{tsuji} :ol
@@ -198,11 +202,23 @@ For {damping velocity}, the normal damping is simply equal to the 
user-specified damping coefficient in the {normal} model:



\begin\{equation\}

\eta_n = \eta_\{n0\}\

\eta_n = \eta_\{n0\}

\end\{equation\}



Here, \(\eta_\{n0\}\) is the damping coefficient specified for the normal

contact model, in units of {mass}/{time}.



For {damping mass_velocity}, the normal damping is given by:



\begin\{equation\}

\eta_n = \eta_\{n0\} m_\{eff\}

\end\{equation\}



Here, \(\gamma_n\) is the damping coefficient specified for the normal

contact model, in units of {mass}/{time},

Here, \(\eta_\{n0\}\) is the damping coefficient specified for the normal

contact model, in units of {mass}/{time} and 

\(m_\{eff\} = m_i m_j/(m_i + m_j)\) is the effective mass.

Use {damping mass_velocity} to reproduce the damping behavior of 

{pair gran/hooke/*}.



The {damping viscoelastic} model is based on the viscoelastic

treatment of "(Brilliantov et al)"_#Brill1996, where the normal
@@ -212,11 +228,10 @@ damping is given by: 
\eta_n = \eta_\{n0\}\ a m_\{eff\}

\end\{equation\}



Here, \(m_\{eff\} = m_i m_j/(m_i + m_j)\) is the effective mass, {a}

is the contact radius, given by \(a =\sqrt\{R\delta\}\) for all models

except {jkr}, for which it is given implicitly according to \(delta =

a^2/R - 2\sqrt\{\pi \gamma a/E\}\).  In this case, \eta_\{n0\}\ is in

units of 1/({time}*{distance}).

Here, {a} is the contact radius, given by \(a =\sqrt\{R\delta\}\) 

for all models except {jkr}, for which it is given implicitly according 

to \(\delta = a^2/R - 2\sqrt\{\pi \gamma a/E\}\).  For {damping viscoelastic}, 

\(\eta_\{n0\}\) is in units of 1/({time}*{distance}).



The {tsuji} model is based on the work of "(Tsuji et

al)"_#Tsuji1992. Here, the damping coefficient specified as part of
@@ -563,6 +578,20 @@ Finally, the twisting torque on each particle is given by: 


:line



The {granular} pair style can reproduce the behavior of the

{pair gran/*} styles with the appropriate settings (some very

minor differences can be expected due to corrections in

displacement history frame-of-reference, and the application

of the torque at the center of the contact rather than

at each particle). The first example above 

is equivalent to {pair gran/hooke 1000.0 NULL 50.0 50.0 0.4 1}.

The second example is equivalent to 

{pair gran/hooke/history 1000.0 500.0 50.0 50.0 0.4 1}.

The third example is equivalent to 

{pair gran/hertz/history 1000.0 500.0 50.0 50.0 0.4 1}.



:line



LAMMPS automatically sets pairwise cutoff values for {pair_style

granular} based on particle radii (and in the case of {jkr} pull-off

distances). In the vast majority of situations, this is adequate.
@@ -618,7 +647,7 @@ interactions is set to \(\mu_1\), and friction coefficient for type 
2-type 2 interactions is set to \(\mu_2\), the friction coefficient

for type1-type2 interactions is computed as \(\sqrt\{\mu_1\mu_2\}\)

(unless explicitly specified to a different value by a {pair_coeff 1 2

...} command. The exception to this is elastic modulus, only

...} command). The exception to this is elastic modulus, only

applicable to {hertz/material}, {dmt} and {jkr} normal contact

models. In that case, the effective elastic modulus is computed as:

src/GRANULAR/fix_wall_gran.cpp

+7 −1
Original line number Diff line number Diff line
@@ -53,7 +53,7 @@ enum{NONE,CONSTANT,EQUAL}; 
#define EPSILON 1e-10



enum {NORMAL_HOOKE, NORMAL_HERTZ, HERTZ_MATERIAL, DMT, JKR};

enum {VELOCITY, VISCOELASTIC, TSUJI};

enum {VELOCITY, MASS_VELOCITY, VISCOELASTIC, TSUJI};

enum {TANGENTIAL_NOHISTORY, TANGENTIAL_HISTORY,

      TANGENTIAL_MINDLIN, TANGENTIAL_MINDLIN_RESCALE};

enum {TWIST_NONE, TWIST_SDS, TWIST_MARSHALL};
@@ -184,6 +184,9 @@ FixWallGran::FixWallGran(LAMMPS *lmp, int narg, char **arg) : 
        if (strcmp(arg[iarg+1], "velocity") == 0) {

          damping_model = VELOCITY;

          iarg += 1;

        } else if (strcmp(arg[iarg+1], "mass_velocity") == 0) {

          damping_model = MASS_VELOCITY;

          iarg += 1;

        } else if (strcmp(arg[iarg+1], "viscoelastic") == 0) {

          damping_model = VISCOELASTIC;

          iarg += 1;
@@ -1158,6 +1161,9 @@ void FixWallGran::granular(double rsq, double dx, double dy, double dz, 
  if (damping_model == VELOCITY) {

    damp_normal = 1;

  }

  else if (damping_model == MASS_VELOCITY){

    damp_normal = meff;

  }

  else if (damping_model == VISCOELASTIC) {

    damp_normal = a*meff;

  }

src/GRANULAR/pair_granular.cpp

+6 −1
Original line number Diff line number Diff line
@@ -52,7 +52,7 @@ using namespace MathSpecial; 
#define EPSILON 1e-10



enum {HOOKE, HERTZ, HERTZ_MATERIAL, DMT, JKR};

enum {VELOCITY, VISCOELASTIC, TSUJI};

enum {VELOCITY, MASS_VELOCITY, VISCOELASTIC, TSUJI};

enum {TANGENTIAL_NOHISTORY, TANGENTIAL_HISTORY,

      TANGENTIAL_MINDLIN, TANGENTIAL_MINDLIN_RESCALE};

enum {TWIST_NONE, TWIST_SDS, TWIST_MARSHALL};
@@ -345,6 +345,8 @@ void PairGranular::compute(int eflag, int vflag) 


        if (damping_model[itype][jtype] == VELOCITY) {

          damp_normal = 1;

        } else if (damping_model[itype][jtype] == MASS_VELOCITY) {

          damp_normal = meff;

        } else if (damping_model[itype][jtype] == VISCOELASTIC) {

          damp_normal = a*meff;

        } else if (damping_model[itype][jtype] == TSUJI) {
@@ -779,6 +781,9 @@ void PairGranular::coeff(int narg, char **arg) 
      if (strcmp(arg[iarg+1], "velocity") == 0) {

        damping_model_one = VELOCITY;

        iarg += 1;

      } else if (strcmp(arg[iarg+1], "mass_velocity") == 0) {

        damping_model_one = MASS_VELOCITY;

        iarg += 1;

      } else if (strcmp(arg[iarg+1], "viscoelastic") == 0) {

        damping_model_one = VISCOELASTIC;

        iarg += 1;

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