fix insert/stream command — LIGGGHTS v3.X documentation

# fix insert/stream command

## Syntax

    fix ID group-ID insert/stream seed seed_value distributiontemplate dist-ID general_keywords general_values stream_keywords stream_values ...

- ID, group-ID are documented in fix command
- insert/stream = style name of this fix command
- seed = obligatory keyword
- seed_value = random # seed (prime number greater 1000)
- 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 nparticles or mass or particlerate or massrate or insert_every or overlapcheck or all_in or random_distribute or*vel constant* or vel uniform or vel gaussian or orientation or omega or set_property

    verbose = yes or no
    maxattempt value = ma
      ma = max # of insertion attempts per atom (positive integer)
    nparticles values = np or INF
      np =  number of particles to insert (positive integer)
      INF =  insert as many particles as possible
    mass values = mp
      mp = mass of particles to be inserted (positive float)
      INF =  insert as many particles as possible
    particlerate values = pr
      pr = particle insertion rate (particles/time units)
    massrate values = mr
      mr = mass insertion rate (mass/time units)
    insert_every value = ie
      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
    compress_tags value = yes or no
    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 or constant q1 q2 q3 q4
      random = randomize rotational orientation
      template = use orientation from particle template
      constant = use constant quaternion for orientation
      q1 q2 q3 q4 = quaternion values for constant orientation
    omega values = constant omegax omegay omegaz
      constant = obligatory word
      omegax = x-component of angular velocity (1/time units)
      omegay = y-component of angular velocity (1/time units)
      omegaz = z-component of angular velocity (1/time units)
    set_property values = property-varname val
      property-varname = variable name of a fix property/atom holding a scalar value for each particle
      val = value to initialize the property with upon insertion

- following the general keyword/value section, one or more stream keyword/value pairs can be appended for the fix insert/stream command
- stream_keywords = duration or parallel or insertion_face or extrude_length

    insertion_face value = mesh-ID
      mesh-ID = ID of the fix mesh/surface or fix mesh/surface/planar to use as starting face for particle generation
    extrude_length values = L
      L =  length for extruding the insertion face in normal direction so to generate in insertion volume
    parallel values = yes or no
      yes, no = pre-calculate location of overlap of processor subdomains and extrusion volume to some extent
    duration values = du
      du =  duration of insertion in time-steps

## Examples

fix ins all insert/stream seed 123457 distributiontemplate pdd1 nparticles 5000 vel constant 0. -0.5 -2. particlerate 1000 overlapcheck yes insertion_face ins_mesh extrude_length 0.6

## Description

Insert particles into a granular run either once or every few timesteps within a specified region until either np particles have been inserted or the desired particle mass (mp) has been reached.

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

Each timestep particles are inserted, they are placed randomly inside the insertion volume so as to mimic a stream of poured particles. The insertion volume is generated by extruding the insertion face as specified via insertion_face in the direction of the face normal. The sign of this face normal is automatically flipped so that it is opposite to the normal component of the insertion velocity.

To specify the number of particles to be inserted, you must use either the nparticles or the mass keyword (but not both). In the latter case, the number of particles to be inserted is calculated from the mass expectancy given by the particle distribution. The start keyword can be used to set the time-step at which the insertion should start.

Likewise, you can use the particlerate or the massrate keyword (but not both) to control the insertion rate. Particles are not inserted continuously, but in packets (for efficiency reasons). Particles are inserted again after enough time has elapsed that the previously inserted particles have left the insertion volume.

One of the two keywords insert_every and extrude_length must be provided by the user (but not both).

In case insert_every is defined, this sets the frequency of the particle insertion directly, i.e. the number of time-steps between two insertions. The number of particles to be inserted at each insertion event is calculated from the insertion rate and insert_every.

If extrude_length is specified, the amount of extrusion is fixed and the insertion frequency is calculated from extrude_length and the insertion velocity normal to the insertion face.

When defining insert_every, you have the possibility to define the duration of each insertion via the duration keyword. duration < insert_every will generate a “pulsed” stream as opposed to a continuous stream. Example: Setting insert_every = 1000 and duration = 600 will produce a stream that pours material for 600 time-steps, will pause for 400 time-steps, pour for another 600 time-steps etc.

As mentioned above, particles are inserted again after enough time has elapsed that the previously inserted particles have left the insertion volume. Until the time these particles reach the insertion face, no other forces affect the particles (pair forces, gravity etc.). Fix insert/stream internally issues a special integrator to take care of this. This procedure guarantees that the specified velocity, omega etc. values are perfectly met at the specified insertion face.

The larger the volume, the more particles that can be inserted at one insertion step. Insertions will continue until the desired # of particles has been inserted.

Note

The insertion face must be a planar face, and the insertion velocity projected on the face normal must be non-zero.

Note

Keywords insert_every and extrude_length may not be used together

Note

Keywords duration and extrude_length cannot be used together.

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.

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 existing 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).Using all_in = yes requires you to use an insertion face of style fix mesh/surface/planar

Note

You also have to use fix mesh/surface/planar if there is a run command between the definition of the insertion face and the fix insert/stream command. Otherwise, a fix mesh/surface/planar will do.

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.

If keyword parallel is set to ‘yes’, LIGGGHTS(R)-PUBLIC tries to pre-calculate more accurately the overlap of process subdomains and extrusion volume. For cases where the insertion volume is highly divided between different processes, this can lead to a speed-up of insertion as random number generation is more efficient. For cases where the extrusion volume is divided among few processes this will impose a small computation overhead.

If keyword compress_tags is set to ‘yes’, then atom IDs are re-assigned after each insertion procedure. The default is ‘no’. This is typically only recommended if some models internally store arrays which have the length defined via the max ID of any atom.

Warning

Setting compress_tags to ‘yes’ will result in all contact history to be lost at that point in time. External post-processing tools will give wrong results because they typically track atoms via their IDs.

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. The insertion velocity must be non-zero.

The set_property option can be used to initialize scalar per-particle properties such as temperatures, which are stored in a a fix property/atom.

The setting of compress_tags will trigger a periodic re-tagging of atom ids. This is usefull in case the simulation domain is used to model a periodic in and outflow of particles. If this switch is set, the global ids of the particles will remain in a certain range, and no “holes” in the arrays holding the global atom ids exist. IMPORTANT NOTE: This functionality may confuse external tools which perform post-processing based on atom IDs!

## 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 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

Keywords duration and extrude_length can not be used together. The insertion face cannot move.

## Related commands

fix_insert_pack, fix_insert_rate_region,

## 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, random_distribute = exact, parallel = no, compress_tags = no