SPARTA WWW Site - SPARTA Documentation - SPARTA Commands

fix ave/surf command

Syntax:

fix ID ave/surf group-ID Nevery Nrepeat Nfreq value1 value2 ... keyword args ... 

Examples:

fix 1 ave/surf all 1 100 100 c_surf ave running
fix 1 ave/surf leftcircle 10 20 1000 c_mine[2]
fix 1 ave/surf leftcircle 10 20 1000 c_mine[*] 

Description:

Use one or more per-surf vectors as inputs every few timesteps, and average them surface element by surface element by over longer timescales, applying appropriate normalization factors. The resulting per-surf averages can be used by other output commands such as the dump surf command. Only surface elements in the surface group specified by group-ID are included in the averaging. See the group surf command for info on how surface elements can be assigned to surface groups.

Each input value can be the result of a compute or fix. The compute or fix must produce a per-surf vector or array, not a global or per-particle or per-grid quantity. If you wish to time-average global quantities from a compute or fix then see the fix ave/time command. To time-average per-grid quantities, see the fix ave/grid command.

Each per-surf value of each input vector is averaged independently.

Computes that produce per-surf vectors or arrays are those which have the word surf in their style name. See the doc pages for individual fixes to determine which ones produce per-surf vectors or arrays.

Note that for values from a compute or fix, the bracketed index I can be specified using a wildcard asterisk with the index to effectively specify multiple values. This takes the form "*" or "*n" or "n*" or "m*n". If N = the size of the vector (for mode = scalar) or the number of columns in the array (for mode = vector), then an asterisk with no numeric values means all indices from 1 to N. A leading asterisk means all indices from 1 to n (inclusive). A trailing asterisk means all indices from n to N (inclusive). A middle asterisk means all indices from m to n (inclusive).

Using a wildcard is the same as if the individual columns of the array had been listed one by one. E.g. these 2 fix ave/surf commands are equivalent, since the compute surf command creates a per-surf array with 4 columns:

compute mySurf all all n fx fy fz
fix 1 all ave/surf 10 20 1000 c_mySurf[*]
fix 1 all ave/surf 10 20 1000 c_mySurf[1] c_mySurf[2] &
                              c_mySurf[3] c_mySurf[4] 

The Nevery, Nrepeat, and Nfreq arguments specify on what timesteps the input values will be used in order to contribute to the average. The final averaged quantities are generated on timesteps that are a multiple of Nfreq. The average is over Nrepeat quantities, computed in the preceding portion of the simulation every Nevery timesteps. Nfreq must be a multiple of Nevery and Nevery must be non-zero even if Nrepeat is 1. Also, the timesteps contributing to the average value cannot overlap, i.e. Nfreq > (Nrepeat-1)*Nevery is required.

For example, if Nevery=2, Nrepeat=6, and Nfreq=100, then values on timesteps 90,92,94,96,98,100 will be used to compute the final average on timestep 100. Similarly for timesteps 190,192,194,196,198,200 on timestep 200, etc.


If a value begins with "c_", a compute ID must follow which has been previously defined in the input script. If no bracketed term is appended, and the compute calculates a per-surf vector, then the per-surf vector is used. If c_ID[I] is used, then I must be in the range from 1-M, which will use the Ith column of the M-column per-surf array calculated by the compute. See the discussion above for how I can be specified with a wildcard asterisk to effectively specify multiple values.

Users can also write code for their own compute styles and add them to SPARTA.

If a value begins with "f_", a fix ID must follow which has been previously defined in the input script. If no bracketed term is appended, and the fix calculates a per-surf vector, then the per-surf vector is used. If f_ID[I] is used, then I must be in the range from 1-M, which will use the Ith column of the M-column per-surf array calculated by the fix. See the discussion above for how I can be specified with a wildcard asterisk to effectively specify multiple values.

Note that some fixes only produce their values on certain timesteps, which must be compatible with Nevery, else an error will result. Users can also write code for their own fix styles and add them to SPARTA.


For averaging of a value that comes from a compute or fix, normalization is performed as follows. If the compute or fix is summing over particles to calculate a per-surf quantity (e.g. pressure or energy flux), this takes the form of a numerator divided by a denominator. For example, see the formulas discussed on the compute surf doc page, where the denominator is 1 (for keyword n), area times dt (timestep) for the other quantities (press, shx, ke, etc). When this command averages over a series of timesteps, the numerator and denominator are summed separately. This means the numerator/denominator division only takes place when this fix produces output, every Nfreq timesteps.


Additional optional keywords also affect the operation of this fix.

The ave keyword determines what happens to the accumulation of statistics every Nfreq timesteps.

If the ave setting is one, then the values produced on timesteps that are multiples of Nfreq are independent of each other. Normalization as described above is performed, and all tallies are zeroed before accumulating over the next Nfreq steps.

If the ave setting is running, then tallies are never zeroed. Thus the output at any Nfreq timestep is normalized over all previously accumulated samples since the fix was defined. The tallies can only be zeroed by deleting the fix via the unfix command, or by re-defining the fix, or by re-specifying it.


Restart, output info:

No information about this fix is written to binary restart files.

This fix produces a per-surf vector or array which can be accessed by various output commands. A vector is produced if only a single quantity is averaged by this fix. If two or more quantities are averaged, then an array of values is produced, where the number of columns is the number of quantities averaged. The per-surf values can only be accessed on timesteps that are multiples of Nfreq since that is when averaging is performed.

Surface elements not in the specified group-ID will output zeroes for all their values.

Restrictions: none

Related commands:

compute, "fix ave/time

Default:

The option defaults are ave = one.