:orphan: .. index:: compute_surf .. _compute-surf: .. _compute-surf-command: #################### compute surf command #################### .. _compute-surf-kk-command: ####################### compute surf/kk command ####################### .. _compute-surf-syntax: ******* Syntax: ******* :: compute ID surf group-ID mix-ID value1 value2 ... keyword setting ... - ID is documented in :ref:`compute` command - surf = style name of this compute command - group-ID = group ID for which surface elements to perform calculation on - mix-ID = mixture ID for particles to perform calculation on - one or more values can be appended - value = *n* or *nwt* or *nflux* or *nflux_incident* or *mflux* or *mflux_incident* or *fx* or *fy* or *fz* or *tx* or *ty* or *tz* or *press* or *px* or *py* or *pz* or *shx* or *shy* or *shz* or *ke* :: 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 tx,ty,tz = components of torque on body center-of-mass 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 - zero or more keyword/setting pairs can be appended - keyword = *norm* or *com* :: norm arg = *flux* or *flow* for dividing flux quantities by area or not com args = Cx Cy Cz = coords of center-of-mass of body for calculating torque .. _compute-surf-examples: ********* 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\[2\] .. _compute-surf-descriptio: ************ 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 or are emitted from it. The values are summed for each group of species in the specified mixture. See the :ref:`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 :ref:`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 :ref:`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 :ref:`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. Additionally, surface elements can emit particles directly -- see the :ref:`fix_emit_surf` command doc page. The formulas below account for all these 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 :ref:`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 :ref:`dump surf` command. .. 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 :ref:`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 :ref:`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 :ref:`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 xyz components of force exerted on the surface element by particles in the group. These are computed as :: p_delta = mass \* (V_post - V_pre) Fx = - Sum_i (p_delta_x) / (dt / fnum) Fy = - Sum_i (p_delta_y) / (dt / fnum) Fz = - 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 *tx*, *ty*, *tz* values calculate the xyz components of torque Tq exerted on the entire body by particles in the group colliding with this surface element. Use of these values requires the the center-of-mass (COM) of the body be specified so the torque can be calculated around the COM. The COM can be the geometric center of a triangulated object, or it can reflect an uneven distribution of mass within the body. The torque components are computed as :: p_delta = mass \* (V_post - V_pre) force = p_delta / (dt / fnum) R = Xcollide - Xcom Tq = R x force 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 vector R is from the COM specified by the *com* keyword Xcollide = the collision point on the surface. The force exerted on then the surface element is the sum over all contributing p_delta, normalized by dt and fnum as defined before. .. note:: that if the surfaces defined in a simulation represent multiple objects each with their own COM, then you should use this command multiple times with different surface groups, if you want to calculate the torque on each object. 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 :ref:`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. .. _compute-surf-transparen-surface-elements: ***************************** Transparent surface elements: ***************************** This compute will tally information on particles that pass through transparent surface elements. The :ref:`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 :ref:`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:: 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 :ref:`compute reduce` command for additional ways to sum or average either normalized or un-normalized flux values produced by this compute. **Optional keywords** 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 *norm* keyword setting is *flux* (the default), then the flux formulas will be calculated as shown with the area A in the denominator. The *com* keyword is only used if torque is being computed by any of the *tx*, *ty*, *tz* values. The *Cx*, *Cy*, *Cz* settings are the coordinates of the center-of-mass of the body around which the torque will be calculated. .. _compute-surf-output-info: ************ 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 :ref:`Section 6.4` for an overview of SPARTA output options. The per-surf array values will be in the :ref:`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 :ref:`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 :ref:`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 :ref:`-suffix command-line switch` when you invoke SPARTA, or you can use the :ref:`suffix` command in your input script. See the :ref:`Accelerating SPARTA` section of the manual for more instructions on how to use the accelerated styles effectively. .. _compute-surf-restrictio: ************* Restrictions: ************* none .. _compute-surf-related-commands: ***************** Related commands: ***************** :ref:`fix ave/surf`, :ref:`dump surf`, :ref:`compute isurf/grid` .. _compute-surf-default: ******** Default: ******** The default for the norm keyword is flux.