Syntax:
compute ID surf group-ID mix-ID value1 value2 ... keyword setting ...
n = count of particles hitting surface element nwt = weighted count of particles hitting surface element nflux = net flux of particles through surface element nflux_incident = incident flux of particles on surface element mflux = net flux of mass through surface element mflux_incident = incident flux of mass on surface element fx,fy,fz = components of force on surface element press = magnitude of normal pressure on surface element px,py,pz = components of normal pressure on surface element shx,shy,shz = components of shear stress on surface element ke = flux of particle kinetic energy on surface element erot = flux of particle rotational energy on surface element evib = flux of particle vibrational energy on surface element echem = flux of particle chemical catalytic energy on surface element etot = flux of particle total energy on surface element
norm arg = flux or flow for dividing flux quantities by area or not
Examples:
compute 1 surf all all n press etot compute mine surf sphere species press shx shy shz compute 2 surf all all mflux ke erot norm flow
These commands will dump time averages for each species and each surface element to a dump file every 1000 steps:
compute 1 surf all species n press shx shy shz fix 1 ave/surf all 10 100 1000 c_1[*] dump 1 surf all 1000 tmp.surf id f_1[*]
These commands will time-average the force on each surface element then sum them across element to compute drag (fx) and lift (fy) on the body:
compute 1 surf all all fx fy fix 1 ave/surf all 10 100 1000 c_1[*] compute 2 reduce sum f_1[1] f_1[2] stats 1000 stats_style step cpu np c_2[1] c_2[2]
Description:
Define a computation that calculates one or more values for each explicit surface element in a surface element group, based on the particles that collide with that element. The values are summed for each group of species in the specified mixture. See the mixture command for how a set of species can be partitioned into groups. Only surface elements in the surface group specified by group-ID are included in the calculations. See the group surf command for info on how surface elements can be assigned to surface groups.
This command can only be used for simulations with explicit surface elements. See the similar compute isurf/grid command for use with simulations with implicit surface elements.
Explicit surface elements are triangles for 3d simulations and line segments for 2d simulations. Unlike implicit surface elements, each explicit triangle or line segment may span multiple grid cells. See Section 4.9 of the manual for details.
Note that when a particle collides with a surface element, it can bounce off (possibly as a different species), be captured by the surface (vanish), or a 2nd particle can also be emitted. The formulas below account for all the possible outcomes. For example, the kinetic energy flux ke onto a suface element for a single collision includes a positive contribution from the incoming particle and negative contributions from 0, 1, or 2 outgoing particles. The exception is the n and nwt values which simply tally counts of particles colliding with the surface element.
If the explicit surface element is transparent, the particle will pass through the surface unaltered. See the transparent keyword for the read_surf command. The count of particles going through the surfacce as well as their mass or energy fluxes can still be tallied by this compute. See details on transparent surface elements below.
Also note that all values for a collision are tallied based on the species group of the incident particle. Quantities associated with outgoing particles are part of the same tally, even if they are in different species groups.
The results of this compute can be used by different commands in different ways. The values for a single timestep can be output by the dump surf command.
The values over many sampling timesteps can be averaged by the fix ave/surf command. It does its averaging as if the particles striking the surface element at each sampling timestep were combined together into one large set to compute the formulas below. The answer is then divided by the number of sampling timesteps if it is not otherwise normalized by the number of particles. Note that in general this is a different normalization than taking the values produced by the formulas below for a single timestep, summing them over the sampling timesteps, and then dividing by the number of sampling steps. However for the current values listed below, the two normalization methods are the same.
NOTE: If particle weighting is enabled via the global weight command, then all of the values below are scaled by the weight assigned to the grid cell in which the particle collision with the surface element occurs. The only exception is the the n value, which is NOT scaled by the weight; it is a simple count of particle collisions with the surface element.
The n value counts the number of particles in the group striking the surface element.
The nwt value counts the number of particles in the group striking the surface element and weights the count by the weight assigned to the grid cell in which the particle collision with the surface element occurs. The nwt quantity will only be different than n if particle weighting is enabled via the global weight command.
The nflux and nflux_incident values calculate the net and incident number flux imparted to the surface element by particles in the group respectively. Incident flux sums over all the impacting particles, while net flux subtracts out reflected particles and includes effects from surface chemistry such as particle deletion. These are computed as
Nflux = N / (A * dt / fnum)
where N is the number of all contributing particles, normalized by A = the area of the surface element, dt = the timestep, and fnum = the real/simulated particle ratio set by the global fnum command.
If the optional norm key is set to flow, then the area A is not included in the Nflux formula. The Nflux quantity becomes effectively a particle flow rate (count per time). See discussion of the norm keyword below.
The mflux and mflux_incident values calculate the net and incident mass flux imparted to the surface element by particles in the group respectively. These are computed as
Mflux = Sum_i (mass_i) / (A * dt / fnum)
where the sum is over all contributing particle masses, normalized by the area of the surface element, dt and fnum as defined before.
If the optional norm key is set to flow, then the area A is not included in the Nflux formula. Then Mflux quantity becomes effectively a mass flow rate (mass per time). See discussion of the norm keyword below.
The fx, fy, fz values calculate the components of force extered on the surface element by particles in the group, with respect to the x, y, z coordinate axes. These are computed as
p_delta = mass * (V_post - V_pre) Px = - Sum_i (p_delta_x) / (dt / fnum) Py = - Sum_i (p_delta_y) / (dt / fnum) Pz = - Sum_i (p_delta_z) / (dt / fnum)
where p_delta is the change in momentum of a particle, whose velocity changes from V_pre to V_post when colliding with the surface element. The force exerted on the surface element is the sum over all contributing p_delta, normalized by dt and fnum as defined before.
The press value calculates the pressure P exerted on the surface element in the normal direction by particles in the group, such that outward pressure is positive. This is computed as
p_delta = mass * (V_post - V_pre) P = Sum_i (p_delta_i dot N) / (A * dt / fnum)
where p_delta, V_pre, V_post, dt, fnum are defined as before. The pressure exerted on the surface element is the sum over all contributing p_delta dotted into the outward normal N of the surface element, also normalized by A = the area of the surface element.
The px, py, pz values calculate the normal pressure Px, Py, Pz extered on the surface element in the direction of its normal by particles in the group, with respect to the x, y, z coordinate axes. These are computed as
p_delta = mass * (V_post - V_pre) p_delta_n = (p_delta dot N) N Px = - Sum_i (p_delta_n_x) / (A * dt / fnum) Py = - Sum_i (p_delta_n_y) / (A * dt / fnum) Pz = - Sum_i (p_delta_n_z) / (A * dt / fnum)
where p_delta, V_pre, V_post, N, A, and dt are defined as before. P_delta_n is the normal component of the change in momentum vector p_delta of a particle. P_delta_n_x (and y,z) are its x, y, z components.
The shx, shy, shz values calculate the shear pressure Sx, Sy, Sz extered on the surface element in the tangential direction to its normal by particles in the group, with respect to the x, y, z coordinate axes. These are computed as
p_delta = mass * (V_post - V_pre) p_delta_t = p_delta - (p_delta dot N) N Sx = - Sum_i (p_delta_t_x) / (A * dt / fnum) Sy = - Sum_i (p_delta_t_y) / (A * dt / fnum) Sz = - Sum_i (p_delta_t_z) / (A * dt / fnum)
where p_delta, V_pre, V_post, N, A, and dt are defined as before. P_delta_t is the tangential component of the change in momentum vector p_delta of a particle. P_delta_t_x (and y,z) are its x, y, z components.
The ke value calculates the kinetic energy flux Eflux imparted to the surface element by particles in the group, such that energy lost by a particle is a positive flux. This is computed as
e_delta = 1/2 mass (V_post^2 - V_pre^2) Eflux = - Sum_i (e_delta) / (A * dt / fnum)
where e_delta is the kinetic energy change in a particle, whose velocity changes from V_pre to V_post when colliding with the surface element. The energy flux imparted to the surface element is the sum over all contributing e_delta, normalized by A = the area of the surface element and dt = the timestep and fnum = the real/simulated particle ratio set by the global fnum command.
If the optional norm key is set to flow, then the area A is not included in the Eflux formula. Then Eflux quantity becomes effectively an energy flow rate (energy per time). See discussion of the norm keyword below.
The erot value calculates the rotational energy flux Eflux imparted to the surface element by particles in the group, such that energy lost by a particle is a positive flux. This is computed as
e_delta = Erot_post - Erot_pre Eflux = - Sum_i (e_delta) / (A * dt / fnum)
where e_delta is the rotational energy change in a particle, whose internal rotational energy changes from Erot_pre to Erot_post when colliding with the surface element. The flux equation is the same as for the ke value.
If the optional norm key is set to flow, then the area A is not included in the Eflux formula. Then Eflux quantity becomes effectively an energy flow rate (energy per time). See discussion of the norm keyword below.
The evib value calculates the vibrational energy flux Eflux imparted to the surface element by particles in the group, such that energy lost by a particle is a positive flux. This is computed as
e_delta = Evib_post - Evib_pre Eflux = - Sum_i (e_delta) / (A * dt / fnum)
where e_delta is the vibrational energy change in a particle, whose internal vibrational energy changes from Evib_pre to Evib_post when colliding with the surface element. The flux equation is the same as for the ke value.
If the optional norm key is set to flow, then the area A is not included in the Eflux formula. Then Eflux quantity becomes effectively an energy flow rate (energy per time). See discussion of the norm keyword below.
The echem value calculates the chemical catalytic energy flux Eflux imparted to the surface element by particles in the group, such that energy lost by a particles recombining is a positive flux. This is computed as
Eflux = - Sum_i (e_recomb) / (A * dt / fnum)
where e_recomb is the catalytic chemical energy of a particle pair (positive for an exothermic recombination reaction). The flux equation is the same as for the ke value. This option applies only to the prob style of surface reations. A value of 0.0 will be returned for other styles of surface reactions, e.g. global and adsorb.
The etot value calculates the total energy flux imparted to the surface element by particles in the group, such that energy lost by a particle is a positive flux. This is simply the sum of kinetic, rotational, and vibrational energies. Thus the total energy flux is the sum of what is computed by the ke, erot, and evib values.
If the optional norm key is set to flow, then the area A is not included in the etot formula. Then etot quantity becomes effectively an energy flow rate (energy per time). See discussion of the norm keyword below.
Transparent surface elements:
This compute will tally information on particles that pass through transparent surface elements. The Section 6.15 doc page provides an overview of transparent surfaces and how to create them.
The n and nwt value are calculated the same for transparent surfaces as for non-transparent. I.e. they are the count and weighted count of particles passing through the surface.
The nflux, mflux, ke, erot. evib, echem, and etot values are fluxes. For transparent surfaces, they are calculated only for the incident particle as if it had struck the surface. The outgoing particle is ignored. This means the tally quantity is the flux of particles onto the outward face of the surface. No tallying is done for particles hitting the inward face of the transparent surface. See Section 6.15 for how to do tallying in both directions.
All the other values are calculated as described above. This means they will be zero, since the incident and outgoing particle have the same mass and velocity.
IMPORTANT NOTE:
Transparent surface elements can intersect standard non-transparent surface elements. For example, to model flow around a spherical object, the sphere would be defined by the usual non-transparent triangles which interact with flow particles. A plane of transparent surface elements normal to the flow direction could be defined which cut through the sphere. In this case some or all of the transparent triangles will be partially or wholly inside the sphere. SPARTA does not attempt to calculate the portion of a tranparent triangle (or line segment in 2d) which is inside the flow volume. The "area" specified in all the formulas above will be the area of the entire transparent triangle (or line segment in 2d), which may or may not be what you want.
See the optional norm keyword (below) to calculate flux values un-normalized by the surface element area. Also see the "sum-area" and "ave-area" modes of the compute reduce command for additional ways to sum or average either normalized or un-normalized flux values produced by this compute.
Optional norm keyword:
If the norm keyword is used with a setting of flow, then the formulas above for all flux values will not use the surface element area A in the denominator. Specifically these values are nflux, mflux, ke, erot, evib, etot.
The formulas thus compute the aggregate mass or energy flow to the surface (e.g. mass per time), not the flux (e.g. mass per area per time).
If the setting is flux (the default), then the flux formulas will be calculated as shown with the area A in the denominator.
Output info:
This compute calculates a per-surf array, with the number of columns equal to the number of values times the number of groups. The ordering of columns is first by values, then by groups. I.e. if the n and u values were specified as keywords, then the first two columns would be n and u for the first group, the 3rd and 4th columns would be n and u for the second group, etc.
Surface elements not in the specified group-ID will output zeroes for all their values.
The array can be accessed by any command that uses per-surf values from a compute as input. See Section 6.4 for an overview of SPARTA output options.
The per-surf array values will be in the units appropriate to the individual values as described above. N is unitless. Press, px, py, pz, shx, shy, shz are in in pressure units. Ke, erot, evib, echem, and etot are in energy/area-time units for 3d simulations and energy/length-time units for 2d simulations.
Styles with a kk suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed in the Accelerating SPARTA section of the manual. The accelerated styles take the same arguments and should produce the same results, except for different random number, round-off and precision issues.
These accelerated styles are part of the KOKKOS package. They are only enabled if SPARTA was built with that package. See the Making SPARTA section for more info.
You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the -suffix command-line switch when you invoke SPARTA, or you can use the suffix command in your input script.
See the Accelerating SPARTA section of the manual for more instructions on how to use the accelerated styles effectively.
Restrictions: none
Related commands:
fix ave/surf, dump surf, compute isurf/grid
Default:
The default for the norm keyword is flux.