LIGGGHTS WWW Site - LAMMPS WWW Site - LIGGGHTS Documentation - LIGGGHTS Commands

### fix insert/pack command

Syntax:

    fix ID group-ID insert/pack seed seed_value distributiontemplate dist-ID general_keywords general_values pack_keywords pack_values ...

- ID, group-ID are documented in fix command
- insert/pack = style names of this fix command
- seed = obligatory keyword
- seed_value = random # seed (positive integer)
- distributiontemplate = obligatory keyword
- dist-ID = ID of a fix_particledistribution_discrete to be used for particle insertion
- one or more general keyword/value pairs can be appended
- general_keywords = verbose or maxattampt or insert_every or overlapcheck or all_in or random_distribute or vel constant or vel uniform or vel gaussian or orientation or omega

        verbose = yes or no
        maxattempt value = ma
          ma = max # of insertion attempts per atom (positive integer)
        insert_every value = once or ie
          once = value to signalise that isertion takes place only once (the step after the fix is issued)
          ie = every how many time-steps particles are inserted - insertion happens periodically (positive integer)
        start value = ts
          ts = time-step at which insertion should start (positive integer larger than current time-step)
        overlapcheck value = yes or no
        all_in value = yes or no
        random_distribute value = exact or uncorrelated
        vel constant values = vx vy vz
          vx = x-velocity at insertion (velocity units)
          vy = y-velocity at insertion (velocity units)
          vz = z-velocity at insertion (velocity units)
        vel uniform values = vx vy vz vFluctx vFlucty vFluctz
          vx = mean x-velocity at insertion (velocity units)
          vy = mean y-velocity at insertion (velocity units)
          vz = mean z-velocity at insertion (velocity units)
          vFluctx = amplitude of uniform x-velocity fluctuation at insertion (velocity units)
          vFlucty = amplitude of uniform y-velocity fluctuation at insertion (velocity units)
          vFluctz = amplitude of uniform z-velocity fluctuation at insertion (velocity units)
        vel gaussian values = vx vy vz vFluctx vFlucty vFluctz
          vx = mean x-velocity at insertion (velocity units)
          vy = mean y-velocity at insertion (velocity units)
          vz = mean z-velocity at insertion (velocity units)
          vFluctx = standard deviation of Gaussian x-velocity fluctuation at insertion (velocity units)
          vFlucty = standard deviation of Gaussian y-velocity fluctuation at insertion (velocity units)
          vFluctz = standard deviation of Gaussian z-velocity fluctuation at insertion (velocity units)
        orientation values = random or template
          random = randomize rotational orientation
          template = use orientation from particle template
        omega values = constant omegax omegay omegaz
          constant = obligatory word
          omegax = x-comonent of angular velocity (1/time units)
          omegay = y-comonent of angular velocity (1/time units)
          omegaz = z-comonent of angular velocity (1/time units)

- following the general keyword/value section, one or more pack keyword/value pairs can be appended for the fix insert/pack command
- pack_keywords = region or volumefraction_region or particles_in_region or mass_in_region or ntry_mc

      pack_keywords = where exactly one out of volumefraction_region or particles_in_region or mass_in_region has to be defined:l
        region value = region-ID
          region-ID = ID of the region where the particles will be generated (positive integer)
        volumefraction_region values = vol
          vol =  desired volume fraction for the region (positive float, 0 < vol < 1)
        particles_in_region values = np
          np =  desired number of particles in the region (positive integer)
        mass_in_region values = m
          m =  desired mass in the region (positive float, m > 0)
        ntry_mc values = n
          n = number of Monte-Carlo steps for calculating the region's volume  (positive integer)

Examples:

    fix ins all insert/pack seed 1001 distributiontemplate pdd1 insert_every once overlapcheck yes volumefraction_region 0.3 region mysphere ntry_mc 10000

Description:

Insert particles into a granular run either once or every few timesteps within the specified region, as defined via the region keyword.

The verbose keyword controlls whether statistics about particle insertion is output to the screen each time particles are inserted.

This command must use the distributiontemplate keyword to refer to a fix_particledistribution_discrete (defined by dist-fix-ID) that defines the properties of the inserted particles.

At each insertion step, fix insert/pack tries inserts as many particles as needed to reach a defined target, which can be either a region volume fraction (keyword volumefraction_region), the total number of particles in the region (keyword particles_in_region), or the total particle mass in the region (keyword mass_in_region). Exactly one out of the keywords volumefraction_region, particles_in_region, mass_in_region must be defined.

The frequency of the particle insertion can be controlled by the keyword insert_every, which defines the number of time-steps between two insertions. Alternatively, by specifying insert_every once, particles are inserted only once.

The start keyword can be used to set the time-step at which the insertion should start.

Inserted particles are assigned the atom type specified by the particledistribution defined via the fix_particledistribution_discrete and are assigned to 4 groups: the default group "all" and the group specified in the fix insert command, as well as the groups specified in the fix_particledistribution_discrete and fix_particletemplate_sphere command (all of which can also be "all").

The keyword overlapcheck controls if overlap is checked for at insertion, both within the inserted particle package and with other existig particles. If this option is turned off, insertion will scale very well in parallel, otherwise not. Be aware that in case of no overlap check, highly overlapping configurations will be produced, so you will have to relax these configurations.

If overlapcheck if performed, the number of insertion attempts per particle can be specified via the maxattempt keyword. Each timestep particles are inserted, the command will make up to a total of M tries to insert the new particles without overlaps, where M = # of inserted particles * ma. If unsuccessful at completing all insertions, a warning will be printed.

The all_in flag determines if the particle is completely contained in the insertion region (all_in yes) or only the particle center (all_in no). Currently all_in yes is not yet supported for all types of insertion.

Keyword random_distribute controls how the number of particles to be inserted is distributed among parallel processors and among the particle templates in the particle distribution. For style exact, the number of particles to be inserted each step is matched exactly. For style uncorrelated, the number of particles to be inserted for each particle template will be rounded in an uncorrelated way, so the total number of inserted particles may vary for each insertion step. However, statistically both ways should produce the same result. Style uncorrelated may be faster in parallel since it does not need global MPI operations. Please note that if the # of particles to be inserted is calculated e.g. from a particle mass to be inserted, the number of particles to be inserted each insertion step will vary by 1, irrespective of the random_distribute settings. This is because in this case the # of particles to insert in each step will be a floating point number, and applying a simple floor/ceil rounding operation would lead to a statistical bias.

The initial velocity and rotational velocity can be controlled via the vel and omega keywords. vel constant simply patches a constant velocity to the inserted particles, vel uniform sets uniformly distributed velocities with mean and amplitude. vel gaussian sets Gaussian distributed particle velocities with mean and std. deviation.

Description for fix insert/pack:

This command must use the region keyword to define an insertion volume. The specified region must have been previously defined with a region command. Dynamic regions are not supported as insertion region. Each timestep particles are inserted, they are placed randomly inside the insertion volume.

The volumefraction option specifies what volume fraction of the insertion volume will be filled with particles. The higher the value, the more particles are inserted each timestep. Since inserted particles should not overlap, the maximum volume fraction should be no higher than about 0.6.

To determine the volume of the insertion region, a Monte Carlo approach might be used for some cases where the volume is difficult to calculate or where the volume calculation is simply not implemented by the region. The ntry_mc keyword is used to control the number of MC tries that are used for the volume calculation.

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

Information about this fix is written to binary restart files. This means you can restart a simulation while inserting particles, when the restart file was written during the insertion operation.

None of the fix_modify options are relevant to this fix. A global vector is stored by this fix for access by various output commands. The first component of the vector is the number of particles already inserted, the second component is the mass of particles already inserted. 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:

The overlapcheck = 'yes' option performs an inherently serial operation and will thus not scale well in parallel. For this reason, if you want to generate large systems, you are advised to turn overlapcheck off and let the packing relax afterwards to generate a valid packing.

Keywords duration and extrude_length can not be used together.

Currently all_in yes is not yet supported for all types of insertion.

Dynamic regions are not supported as insertion region.

Related commands:

fix_insert_stream, fix_insert_rate_region, fix_deposit, region, fix_pour

Default:

The defaults are maxattempt = 50, all_in = no, overlapcheck = yes vel = 0.0 0.0 0.0, omega = 0.0 0.0 0.0, start = next time-step, duration = insert_every, ntry_mc = 100000, random_distribute = exact