mixture command

Syntax:

mixture ID species1 species2 ... keyword args ... 

• ID = user-defined name of the mixture
• species1, species2, ... = zero or more species IDs to include in the mixture
• zero or more keyword/arg pairs may be appended
• keyword = nrho or vstream or temp or frac or group or copy or delete

    nrho arg = density
      density = number density of entire mixture (# per length^3 units)
    vstream args = Vx Vy Vz
      Vx,Vy,Vz = streaming velocity of entire mixture (velocity units)
    temp arg = thermal
      thermal = temperature of entire mixture (temperature units)
    trot arg = Trot
      Trot = rotational temperature of entire mixture (temperature units)
    tvib arg = Tvib
      Tvib = vibrational temperature of entire mixture (temperature units)
    frac arg = fraction
      fraction = number fraction for each listed species (0 to 1)
    group arg = SELF or group-ID
      SELF = put each listed species (or all species if none listed) in its own group
      group-ID = put the listed species (or all species if none listed) in a group with this ID 
    copy arg = new-ID
      new-ID = ID of new mixture to create, as a copy of this one
    delete args = sp1 sp2 ...
      sp1,sp2,... = species to delete from the mixture 
  

Examples:

mixture air N O NO group lite
mixture air N O NO vstream 250.0 0.0 0.0 group species
mixture air N frac 0.8
mixture air O frac 0.2 copy myAir
mixture background N O 
mixture air delete N NO 

Description:

Define a gas mixture and its properties. A mixture can be referenced by its ID in several other SPARTA commands such as create_particles or per-grid computes. Any number of mixtures can be defined and used in a simulation.

A mixture is a collection of one or more particle species as defined by the species command. Each species belongs to a named group within the mixture so that particles of all species in the group can be acted on together by other commands. The mixture has both global attributes and per-species attributes. All attributes have default values unless they are explicitly specified.

The ID for a mixture is used to identify the mixture in other commands. Each mixture ID must be unique. The ID can only contain alphanumeric characters and underscores.

Note that the mixture command can be used multiple times with the same ID, to add species to the mixture, define groups within the mixture, or change its attributes. Also note that a species can belong to more than one mixture.

There are 2 default mixtures defined by SPARTA that always exist.

The first default mixture has an ID = "all", and contains all species that have been defined. When new species are created via the "species" command, they are automatically added to this mixture. This mixture has only a single group, also named "all", which all species belong to.

The second default mixture has an ID = "species", and also contains all species that have been defined. When new species are created via the "species" command, they are also automatically added to this mixture. This mixture defines one group per species, each with the species name, so that each species in the mixture belongs to its own group.

Zero or more species can be specified in the mixture command. If a listed species is not already in the mixture, due to a previous mixture command with the same ID, then that species is added to the mixture. As discussed below, it will be assigned to a default group and assigned default per-species attributes, unless the appropriate keywords are also specified.

Species can be specified which are already part of the mixture, to change their group assignment or their per-species proerties, as disussed below.

Zero species can be specified, if other keywords are used which alter group assignments or change global attributes of the mixture, as discussed below.

These keywords set global attributes of the mixture.

The nrho keyword sets a global attribute of the mixture, namely its density. For 3d simulations the units of the specified density are #/volume. For 2d, the units are effectively #/area, since the z-dimension thickness of the simulation box = 1.0.

The vstream keyword sets a global attribute of the mixture, namely the streaming velocity. Particles created using the mixture will use the specified Vx,Vy,Vz values.

The temp keyword sets a global attribute of the mixture, namely the thermal temperature of its particles. It must be a value >= zero. When particles are created, this value is used to sample a Gaussian velocity distribution, which is superposed on the streaming velocity, when each particle's velocity is initialized.

The trot keyword sets a global attribute of the mixture, namely the rotational temperature of its particles. It must be a value >= zero. When particles are created, this value is used to sample a Gaussian energy distribution to define each particle's rotational energy. If this keyword is not specified, the thermal temperature is used as the default.

The tvig keyword sets a global attribute of the mixture, namely the vibrational temperature of its particles. It must be a value >= zero. When particles are created, this value is used to sample a Gaussian energy distribution to define each particle's vibrational energy. If this keyword is not specified, the thermal temperature is used as the default.

This keyword sets per-species attributes of the mixture.

The frac keyword sets a per-species attribute for individual species in the mixture. Each species has a relative fractional density, such as 0.2, meaning one out of 5 particles is that species. The sum of this value across all species in the mixture must equal 1.0. The frac keyword sets this value for the listed species. If this value has never been set for M species out of the total N species in the mixture, then when a simulation is run, the frac value for each of the M species is set to (1 - sum)/M, where sum is the sum of the frac values for the N-M assigned species.

Each species in a mixture is assigned to exactly one group. The group keyword can be used to set or change these assignments. Every mixture has one or more named groups.

As described by the collide command, mixture groups are used when performing collisions so that collisions attempts, partners, and parameters can be treated on a per-group basis for accuracy and efficiency. Per-grid computes also use mixture groups to calculate per-grid quantities on a per-grid-cell, per-group basis, i.e. on subsets of particles within each grid cell.

If the group keyword is not used in a mixture command, no changes to group assignements are made for species that are already in the mixture. If one or more new species are specified, then all of them are assigned to a group with "default" as the group ID. Note that this means that mixtures defined with mixture commands that never use the group keyword will have just a single group.

If the group keyword is used, the group ID can be any string you choose. Similar to the mixture ID, it can only contain alphanumeric characters and underscores. Using SELF for the group ID has a special meaning as discussed below.

The operation of the group keyword depends on whether no species or some species are specified explicitly in the mixture command. It also depends on whether the group ID is SELF or a user-defined name. In each case, after the operation is done, any group IDs for the mixture that have no species assigned to them are deleted. This includes the "default" group if it was implicitly created by a previous mixture command.

• If no species are listed in the mixture command and the group ID is SELF, then every species already in the mixture is assigned to a group with its species ID as the group ID. I.e. there will now be one species per group.
• If one or more species are listed and the group ID is SELF, then each listed species is assigned to a group with its species ID as the group ID.
• If no species are listed and the group ID is not SELF, then all species already in the mixture are assigned to a group with the specified ID.
• If one or more species are listed and the group ID is not SELF, then the listed species are all assigned to a group with the specified ID.

These keywords operate on one or more mixtures.

The copy keyword creates a new mixture with new-ID which is an identical copy of the mixture with ID. Regardless of where the copy keyword appears in the command, the operation is delayed until all other keywords have been invoked.

This is useful if you wish to create a new mixture which is nearly the same as the current mixture. Subsequent mixture commands can be used to change the properties of the new mixture.

The delete keyword removes one or more species from the mixture, specified as sp1, sp2, etc. No other keywords can be used with delete. All arguments that follow it are assumed to be species IDs that are currently in the mixture. When using delete, no species can be defined before the keyword, i.e. species1, species2, etc cannot be defined in the comand syntax described above.

After the listed species are removed, any group IDs for the mixture that have no species assigned to them are also deleted.

Restrictions:

The streaming velocity and thermal temperature of the mixture cannot both be zero. A zero streaming velocity means a zero vector = (0,0,0).

The restrictions on use of the delete keyword are described above.

Related commands:

global, create_particles

Default:

The nrho, vstream, and temp defaults are those defined for the background gas density, as set by the global command. The trot and tvib defaults are to use the thermal temperature temp, either its default or the value specified by this command. The frac default is described above. The group keyword has no default; if it is not used, new species not already in the mixture are assigned to a group with a group ID = "default".