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### 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 (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 nparticles or mass or particlerate or massrate or insert_every or overlapcheck or all_in or random_distribute orvel 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)
        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 inseration rate (particles/time units)
        massrate values = mr
          mr = mass inseration 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)
        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 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 1001 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 controlls 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").

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 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.

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.

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, fix_deposit, fix_pour_dev, 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, random_distribute = exact, parallel = no