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### fix check/timestep/gran command

Syntax:

    fix ID group-ID check/timestep/gran nevery fraction_r fraction_h

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

Examples:

    fix ts_check all check/timestep/gran 1000 0.1 0.1

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 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. 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. None of the fix_modify options are relevant to this fix. This fix computes a 3-vector, for access by various output commands. 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 command. This fix is not invoked during energy minimization.

Restrictions: none

Related commands: none

Default: none