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fix emit/face command

Syntax:

fix ID emit/face mix-ID face1 face2 ... keyword value(s) ... 

Examples:

fix in emit/face air all
fix in emit/face mymix xlo yhi n 1000 nevery 10 region circle
fix in emit/face air xlo subsonic 0.1 300
fix in emit/face air xhi subsonic 0.05 NULL 

Description:

Emit particles from one or more faces of the simulation box, continuously during a simulation. If invoked every timestep, this fix creates a continuous influx of particles thru the face(s).

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.

One or more faces of the simulation box can be specified via the face1, face2, etc arguments. The 6 possible faces can be specified as xlo, xhi, ylo, yhi, zlo, or zhi. Specifying all is the same as specifying all 6 individual faces.

On each insertion timestep, each grid cell with one or more of its faces touching a specified boundary face performs the following computations to add particles. The particles are added at the beginning of the SPARTA timestep.

The molecular flux across a grid cell face per unit time is given by equation 4.22 of (Bird94). The number of particles M to insert on a particular grid cell face is based on this flux and additional global, flow, and cell face 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 on the grid cell face. 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 face 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.

If the final particle velocity is not directed "into" the grid cell, then the velocity sampling procedure is repeated until it is. This insures that all added particles enter the simulation domain, 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 entering the simulation box.


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 exactly Np particles are added on each insertion timestep. This is done by dividing Np by the total number of grid cells that are adjacent to the specified box faces and adding an equal number of particles per grid cell.

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 in a grid cell 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 in a grid cell 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 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 boundary face. 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 cells will be candidates for particle insertion, unless they are entirely outside the bounding box that encloses the region. Particles those grid cells 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 boundary 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 adjacent to the boundary face(s) 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 boundary collision or chemistry model via the boundary or bound_modify or 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:

Particles cannot be emitted from periodic faces of the simulation box. Particles cannot be emitted from z faces of the simluation box for a 2d simulation.

A n setting of Np > 0 can only be used with a perspecies setting of no.

A warning will be issued if a specified face has an inward normal in a direction opposing the streaming velocity. Particles will still be emitted from that face, 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 that a thermal velocity 3 sigmas from the mean thermal velocity is large enough to overcome the outward streaming velocity and produce a net velocity into the simulation box.

Related commands:

mixture, create_particles, fix emit/face/file

Default:

The keyword defaults are n = 0, 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).