gran command

Syntax

fix ID group-ID check/timestep/gran nevery fraction_r fraction_h keywords vales:pre 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
zero or more keyword/value pairs may be appended
keyword = warn or error or vmax

warn value = yes or no
error value = yes or no
vmax value = v_max
  v_max = maximum particle velocity to be used as minimum for evaluation Hertz criterion

Examples

fix ts_check all check/timestep/gran 1000 0.1 0.1

Description

Periodically calculate estimations of the Rayleigh- and Hertz time $t_r$ and $t_h$ for a granular system every ‘nevery’ time-steps. The user can specify two quantities fraction_r ( $f_r$ ) and fraction_h ( $f_h$ ). A warning message is printed if the time-step size as specified via the timestep command exceeds either of $t_r \cdot f_r$ or $t_h \cdot f_h$ .

The former quantity is :

$t_r = \frac{\pi r}{0.1631 \nu + 0.8766}\sqrt{\frac{\rho}{G}}$

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

$t_h = 2.87 \left( \frac{m_{eff}^2}{r_{eff} Y_{eff}^2 v_{max}} \right)^{0.2}$

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). If keyword vmax is used, a user-defined maximum velocity is used as a minimum in the formula above, i.e. the maximum of v_max of the particles in the simulation and $v_{max}$ specified by the user is used.

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. The Rayleigh time $t_r$ is calculated for each particle in the simulation, and the minimum value is taken for further calculations. The Hertz time $t_h$ is estimated by testing a collision of each particle with itself using $v_{max}$ as the assumed collision velocity.

Keyword warn can be used to turn off the warning message. Keyword error can be used to have Aspherix® issue an error message and abort the simulation if any of the criteria is violated.

Restart, fix_modify, output, run start/stop

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.

Restrictions

none

Related commands: none

Default: warn = yes, error = no, $v_{max} = 0$