fix pour command

Syntax

fix ID group-ID pour N type seed keyword values ...
  • ID, group-ID are documented in fix command
  • pour = style name of this fix command
  • N = # of atoms to insert
  • type = atom type to assign to inserted atoms (offset for molecule insertion)
  • seed = random # seed (positive integer)
  • one or more keyword/value pairs may be appended to args
  • keyword = region or diam or dens or vol or rate or vel or mol or rigid or shake
region value = region-ID
  region-ID = ID of region to use as insertion volume
diam values = dstyle args
  dstyle = one or range or poly
    one args = D
      D = single diameter for inserted particles (distance units)
    range args = Dlo Dhi
      Dlo,Dhi = range of diameters for inserted particles (distance units)
    poly args = Npoly D1 P1 D2 P2 ...
      Npoly = # of (D,P) pairs
      D1,D2,... = diameter for subset of inserted particles (distance units)
      P1,P2,... = percentage of inserted particles with this diameter (0-1)
vol values = fraction Nattempt
  fraction = desired volume fraction for filling insertion volume
  Nattempt = max # of insertion attempts per atom
rate value = V
  V = z velocity (3d) or y velocity (2d) at which
      insertion volume moves (velocity units)
vel values (3d) = vxlo vxhi vylo vyhi vz
vel values (2d) = vxlo vxhi vy
  vxlo,vxhi = range of x velocities for inserted particles (velocity units)
  vylo,vyhi = range of y velocities for inserted particles (velocity units)
  vz = z velocity (3d) assigned to inserted particles (velocity units)
  vy = y velocity (2d) assigned to inserted particles (velocity units)
mol value = template-ID
  template-ID = ID of molecule template specified in a separate molecule command
molfrac values = f1 f2 ... fN
  f1 to fN = relative probability of creating each of N molecules in template-ID
rigid value = fix-ID
  fix-ID = ID of fix rigid/small command
shake value = fix-ID
  fix-ID = ID of fix shake command

Examples

fix 3 all pour 1000 2 29494 region myblock
fix 2 all pour 10000 1 19985583 region disk vol 0.33 100 rate 1.0 diam range 0.9 1.1
fix 2 all pour 10000 1 19985583 region disk diam poly 2 0.7 0.4 1.5 0.6
fix ins all pour 500 1 4767548 vol 0.8 10 region slab mol object rigid myRigid

Description

Insert finite-size particles or molecules into the simulation box every few timesteps within a specified region until N particles or molecules have been inserted. This is typically used to model the pouring of granular particles into a container under the influence of gravity. For the remainder of this doc page, a single inserted atom or molecule is referred to as a “particle”.

If inserted particles are individual atoms, they are assigned the specified atom type. If they are molecules, the type of each atom in the inserted molecule is specified in the file read by the molecule command, and those values are added to the specified atom type. E.g. if the file specifies atom types 1,2,3, and those are the atom types you want for inserted molecules, then specify type = 0. If you specify type = 2, the in the inserted molecule will have atom types 3,4,5.

All atoms in the inserted particle are assigned to two groups: the default group “all” and the group specified in the fix pour command (which can also be “all”).

This command must use the region keyword to define an insertion volume. The specified region must have been previously defined with a region command. It must be of type block or a z-axis cylinder and must be defined with side = in. The cylinder style of region can only be used with 3d simulations.

Individual atoms are inserted, unless the mol keyword is used. It specifies a template-ID previously defined using the molecule command, which reads a file that defines the molecule. The coordinates, atom types, center-of-mass, moments of inertia, etc, as well as any bond/angle/etc and special neighbor information for the molecule can be specified in the molecule file. See the molecule command for details. The only settings required to be in this file are the coordinates and types of atoms in the molecule.

If the molecule template contains more than one molecule, the relative probability of depositing each molecule can be specified by the molfrac keyword. N relative probablities, each from 0.0 to 1.0, are specified, where N is the number of molecules in the template. Each time a molecule is inserted, a random number is used to sample from the list of relative probabilities. The N values must sum to 1.0.

If you wish to insert molecules via the mol keyword, that will be treated as rigid bodies, use the rigid keyword, specifying as its value the ID of a separate fix rigid/small command which also appears in your input script.

If you wish to insert molecules via the mol keyword, that will have their bonds or angles constrained via SHAKE, use the shake keyword, specifying as its value the ID of a separate fix shake command which also appears in your input script.

Each timestep particles are inserted, they are placed randomly inside the insertion volume so as to mimic a stream of poured particles. If they are molecules they are also oriented randomly. Each atom in the particle is tested for overlaps with existing particles, including effects due to periodic boundary conditions if applicable. If an overlap is detected, another random insertion attempt is made; see the vol keyword discussion below. The larger the volume of the insertion region, the more particles that can be inserted at any one timestep. Particles are inserted again after enough time has elapsed that the previously inserted particles fall out of the insertion volume under the influence of gravity. Insertions continue every so many timesteps until the desired # of particles has been inserted.

All other keywords are optional with defaults as shown below.

The diam option is only used when inserting atoms and specifes the diameters of inserted particles. There are 3 styles: one, range, or poly. For one, all particles will have diameter D. For range, the diameter of each particle will be chosen randomly and uniformly between the specified Dlo and Dhi bounds. For poly, a series of Npoly diameters is specified. For each diameter a percentage value from 0.0 to 1.0 is also specified. The Npoly percentages must sum to 1.0. For the example shown above with “diam 2 0.7 0.4 1.5 0.6”, all inserted particles will have a diameter of 0.7 or 1.5. 40% of the particles will be small; 60% will be large.

Note that for molecule insertion, the diameters of individual atoms in the molecule can be specified in the file read by the molecule command. If not specified, the diameter of each atom in the molecule has a default diameter of 1.0.

The dens and vel options enable inserted particles to have a range of densities or xy velocities. The specific values for a particular inserted particle will be chosen randomly and uniformly between the specified bounds. The vz or vy value for option vel assigns a z-velocity (3d) or y-velocity (2d) to each inserted particle.

The vol option specifies what volume fraction of the insertion volume will be filled with particles. For particles with a size specified by the diam range keyword, they are assumed to all be of maximum diamter Dhi for purposes of computing their contribution to the volume fraction.

The higher the volume fraction value, the more particles are inserted each timestep. Since inserted particles cannot overlap, the maximum volume fraction should be no higher than about 0.6. Each timestep particles are inserted, LAMMPS will make up to a total of M tries to insert the new particles without overlaps, where M = # of inserted particles * Nattempt. If LAMMPS is unsuccessful at completing all insertions, it prints a warning.

The rate option moves the insertion volume in the z direction (3d) or y direction (2d). This enables pouring particles from a successively higher height over time.

Note

If you are monitoring the temperature of a system where the particle count is changing due to adding particles, you typically should use the compute_modify dynamic yes command for the temperature compute you are using.

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

No information about this fix is written to binary restart files. This means you must be careful when restarting a pouring simulation, when the restart file was written in the middle of the pouring operation. Specifically, you should use a new fix pour command in the input script for the restarted simulation that continues the operation. You will need to adjust the arguments of the original fix pour command to do this.

Also note that because the state of the random number generator is not saved in restart files, you cannot do “exact” restarts with this fix, where the simulation continues on the same as if no restart had taken place. However, in a statistical sense, a restarted simulation should produce the same behavior if you adjust the fix pour parameters appropriately.

None of the fix_modify options are relevant to this fix. No global or per-atom quantities are stored by this fix for access by various output commands. 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

This fix is part of the GRANULAR package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

For 3d simulations, a gravity fix in the -z direction must be defined for use in conjunction with this fix. For 2d simulations, gravity must be defined in the -y direction.

The specified insertion region cannot be a “dynamic” region, as defined by the region command.

Default

Insertions are performed for individual particles, i.e. no mol setting is defined. If the mol keyword is used, the default for molfrac is an equal probabilities for all molecules in the template. Additional option defaults are diam = one 1.0, dens = 1.0 1.0, vol = 0.25 50, rate = 0.0, vel = 0.0 0.0 0.0 0.0 0.0.