Syntax:
fix ID emit/surf mix-ID group-ID keyword value ...
n value = Np = number of particles to create
Np can be a variable (see below)
normal value = yes or no = emit normal to surface elements or with streaming velocity
nevery value = Nstep = add particles every this many timesteps
perspecies value = yes or no
region value = region-ID
subsonic values = Psub Tsub
Psub = pressure setting at inflow boundary (pressure units)
Tsub = temperature setting at inflow boundary, can be NULL (temperature units)
Examples:
fix in emit/surf air all fix in emit/face mymix myPatch region circle normal yes fix in emit/surf air all subsonic 0.1 300 fix in emit/surf air all subsonic 0.05 NULL
Description:
Emit particles from a group of surface elements, continuously during a simulation. If invoked every timestep, this fix creates a continuous outflux of particles from the surface elements in the group.
The properties of the added particles are determined by the mixture with ID mix-ID. This sets the number and species of added particles, as well as their streaming velocity, thermal temperature, and internal energy modes. The details are explained below.
Which surface elements emit particles is specified by the group-ID for a surface group, which defines a set of surface elements. The group surf is used to define surface groups.
On each insertion timestep, each grid cell that overlaps with one or more emitting surface elements performs the following computations to add particles for each grid cell/surface element pairing. The particles are added at the beginning of the SPARTA timestep.
The molecular flux emitted from a surface element per unit time is given by equation 4.22 of (Bird94). The number of particles M to insert on the portion of a surface element that is contained within a grid cell is based on this flux and additional global, flow, and surface element properties:
The flow properties are defined for the specified mixture via the mixture command.
If M has a fractional value, e.g. 12.5, then 12 particles are added, and a 13th depending on the value of a random number. Each particle is added at a random location within the portion of the surface element that overlaps with the grid cell. The particle species is chosen randomly in accord with the frac settings of the collection of species in the mixture, as set by the mixture command.
IMPORTANT NOTE: The preceeding calculation is actually done using surface element areas associated with weighted cell volumes. Grid cells can be weighted using the global weight command.
The velocity of the particle is set to the sum of the streaming velocity and a thermal velocity sampled from the thermal temperature. The internal energy modes of the particle are determined by the trot and tvib settings of the mixture and the rotate and vibrate options of the collide_modify command. Note that if the collide command has not been specified (free molecular flow), then no rotational or vibrational energy will be assigned to created particles. See the discussion of the normal keyword below for a way to change the velocity assignment to be oriented in the direction normal to the surface element, rather than in the direction of the streaming velocity.
If the final particle velocity is not directed "out of" the surface element, then the velocity sampling procedure is repeated until it is. This insures that all added particles emit from the surface element, as desired.
The first timestep that added particles are advected, they move for a random fraction of the timestep. This insures a continuous flow field of particles emitting from each surface element.
The n keyword can alter how many particles are added, which can be useful for debugging purposes. If Np is set to 0, then the number of added particles is a function of fnum, nrho, and other mixture settings, as described above.
If Np is set to a value > 0, then the fnum and nrho settings are ignored, and roughly Np particles are added on each insertion timestep. For each grid cell/surface element pair, its target number of emitted particles is set to its fraction of the total emission area (for all grid cell/surface element pairs), multiplied by Np. If that results in a fractional value, then an extra particle is emitted depending on the value of a random number, as explained above.
The Np value can be also be specified as an equal-style variable. If the value is a variable, it should be specified as v_name, where name is the variable name. In this case, the variable will be evaluated on each emission timestep, and its value used as Np on that step to determine the target number of emitted particles for each grid cell/surface element pair, the same as described in the preceeding paragraph.
Equal-style variables can specify formulas with various mathematical functions, and include stats_style command keywords for the simulation box parameters and timestep and elapsed time. Thus it is easy to specify a time-dependent value of Np.
The normal keyword can be used to alter how velocities are set for added particles. If normal is set to no, then a particle's velocity is set as described above, using the mixture's streaming velocity superposed with a thermal velocity sampled from the temperature of the mixture. Note that the same streaming velocity is used for all emitting surface elements, regardless of their orientation with respect to the streaming velocity. If normal is set to yes, then each surface element is assigned its own "streaming" velocity in the following manner. The streaming velocity points in the direction of the outward normal of the surface element, and its magnitude is set to the magnitude of the mixture's streaming velocity. A velocity is then assigned to the particle in the same manner as before. It is assigned the outward streaming velocity superposed with a thermal velocity sampled from the temperature of the mixture. The effect is that particles effectively stream outward from each emitting surface element.
The nevery keyword determines how often particles are added. If Nstep > 1, this may give a non-continuous, clumpy distribution in the inlet flow field.
The perspecies keyword determines how the species of each added particle is randomly determined. This has an effect on the statistical properties of added particles.
If perspecies is set to yes, then a target insertion number M for a grid cell/surface element pair is calculated for each species, which is a function of the relative number fraction of the species, as set by the mixture nfrac command. If M has a fractional value, e.g. 12.5, then 12 particles of that species will always be added, and a 13th depending on the value of a random number.
If perspecies is set to no, then a single target insertion number M for a grid cell/surface element pair is calculated for all the species. Each time a particle is added, a random number is used to choose the species of the particle, based on the relative number fractions of all the species in the mixture. As before, if M has a fractional value, e.g. 12.5, then 12 particles will always be added, and a 13th depending on the value of a random number.
Here is a simple example that illustrates the difference between the two options. Assume a mixture with 2 species, each with a relative number fraction of 0.5. Assume a particular grid cell/surface element pair adds 10 particles from that mixture. If perspecies is set to yes, then exactly 5 particles of each species will be added on every timestep insertions take place. If perspecies is set to no, then exactly 10 particles will be added every time and on average there will be 5 particles of each of the two species. But on one timestep it might be 6 of the first and 4 of the second. On another timestep it might be 3 of the first and 7 of the second.
If the region keyword is used, then a particle will only added if its position is within the specified region-ID. This can be used to only allow particle insertion on a subset of the collective area of the specified group of surface elements. Note that the side option for the region command can be used to define whether the inside or outside of the geometric region is considered to be "in" the region.
IMPORTANT NOTE: If the region and n keywords are used together, less than N particles may be added on an insertion timestep. This is because grid cell/suface element pairs will be candidates for particle insertion, unless the grid cell is entirely outside the bounding box that encloses the region. Particles those grid cell/surface element pairs will attempt to add are included in the count for N, even if some or all of the particle insertions are rejected due to not being inside the region.
The subsonic keyword uses the method of Fang and Liou (Fang02) to determine the number of particles to insert in each grid cell on the emitting face(s). They used the method of characteristics to calculate the mean properties of the incoming molecular flux, so that the prescribed pressure condition is achieved. These properties are then applied to calculate the molecular flux across a grid cell face per unit time, as given by equation 4.22 of (Bird94).
This keyword allows specification of both the pressure and temperature at the surface or just the pressure (by specifying the temperature as NULL). If specified, the temperature must be > 0.0. Currently, instantaneous values for the density, temperature, and stream velocity of particles in the cells containing the surface elements are computed and used to determine the properties of inserted particles on each timestep.
IMPORTANT NOTE: Caution must be exercised when using the subsonic boundary condition without specifying an inlet temperature. In this case the code tries to estimate the temperature of the flow from the properties of the particles in the domain. If the domain contains few particles per cell it may lead to spurious results. This boundary condition is meant more for an outlet than an inlet boundary condition, and performs well in cases where the cells are adequately populated.
IMPORTANT NOTE: When using this keyword, you should also use an appropriate surface collision or chemistry model via the surf_collide or surf_react commands, so that particles hitting the surface disappear as if they were exiting the simulation domain. That is necessary to produce the correct subsonic conditions that the particle insertions due to this command are trying to achieve.
Restart, output info:
No information about this fix is written to binary restart files.
This fix computes a global vector of length 2 which can be accessed by various output commands. The first element of the vector is the total number of particles added on the most recent insertion step. The second element is the cummulative total number added since the beginning of the run. The 2nd value is initialized to zero each time a run is performed.
Restrictions:
A n setting of Np > 0 or Np as a variable can only be used with a perspecies setting of no.
If normal is set to no, which is the default, then unlike the fix emit/face command, no warning is issued if a surface element has an inward normal in a direction opposing the streaming velocity, as defined by the mixture.
For that surface element, particles will still be emitted, so long as a small fraction have a thermal velocity large enough to overcome the outward streaming velocity, so that their net velocity is inward. The threshold for this is the thermal velocity for particles 3*sigma from the mean thermal velocity.
Related commands:
mixture, create_particles, fix emit/face
Default:
The keyword defaults are n = 0, normal = no, nevery = 1, perspecies = yes, region = none, no subsonic settings.
(Bird94) G. A. Bird, Molecular Gas Dynamics and the Direct Simulation of Gas Flows, Clarendon Press, Oxford (1994).
(Fang02) Y. Fang and W. W. Liou, Microfluid Flow Computations Using a Parallel DSMC Code, AIAA 2002-1057. (2002).