"LIGGGHTS WWW Site"_liws - "LAMMPS WWW Site"_lws - "LIGGGHTS Documentation"_ld - "LIGGGHTS Commands"_lc :c

:link(liws,http://www.cfdem.com)
:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

:line

fix check/timestep/gran command :h3

[Syntax:]

fix ID group-ID check/timestep/gran nevery fraction_r fraction_h :pre
ID, group-ID are documented in "fix"_fix.html command :ulb,l
check/timestep/gran = style name of this fix command :l
nevery = evaluate time-step size accuracy every this many time-steps :l
fraction_r = warn if time-step size exceeds this fraction of the Rayleigh time :l
fraction_h = warn if time-step size exceeds this fraction of the Hertz time :l
:ule

[Examples:]

fix ts_check all check/timestep/gran 1000 0.1 0.1 :pre

[Description:]

Periodically calculate estimations of the Rayleigh- and Hertz time
dt_r and dt_h for a granular system every 'nevery' time-steps. The user 
can specify two quantities {fraction_r} and {fraction_h}. A warning 
message is printed if the time-step size as specified via the 
"timestep"_timestep.html command exceeds either of dt_r * fraction_r 
or dt_h * fraction_h.

The former quantity is :

dt_r = PI*r*sqrt(rho/G)/(0.1631*nu+0.8766),

where rho is particle density, G is the shear modulus and nu is Poisson's 
ratio. The latter quantity is expressed by

dt_h = 2.87*(m_eff^2/(r_eff*Y_eff^2*v_max)^0.2.

The effective mass m_eff, the effective radius r_eff and the effective Young's 
modulus Y_eff are as defined in "pair gran"_pair_gran.html. v_max is the 
maximum relative velocity, taking mesh movement into account. Please note 
that the Hertz criterion will also be used if you use a different granular 
pair style (e.g. Hooke).

Additionally, this command checks the ratio of skin to the distance that 
particles can travel relative to each other in one time-step. This value 
should be >1, otherwise some interactions may be missed or overlap energy 
may be generated artificially. This command will warn you if this is the case.

These criteria are checked every 'nevery' time-steps. Rayleigh time dt_r is 
calculated for each particle in the simulation, and the minimum value is 
taken for further calculations. Hertz time dt_h is estimated by testing a 
collision of each particle with itself using v_max as the assumed collision 
velocity.

[Restart, fix_modify, output, run start/stop, minimize info:]

No information about this fix is written to "binary restart
files"_restart.html.  None of the "fix_modify"_fix_modify.html options
are relevant to this fix.  This fix computes a 3-vector, for access by 
various "output commands"_Section_howto.html#4_15.  The vector consists 
of the time-step size expressed as fraction of the Rayleigh and Hertz 
time-step sizes and the ratio of skin to the distance particles can 
travel relative to each other in one time-step, respectively. No 
parameter of this fix can be used with the {start/stop} keywords of 
the "run"_run.html command. This fix is not invoked during 
"energy minimization"_minimize.html.

[Restrictions:] none

[Related commands:] none

[Default:] none
