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### fix sph/pressure command

Syntax:

    fix ID group-ID sph/pressure style args

- ID, group-ID are documented in fix command
- sph/pressure = style name of this fix command
- style = absolut or Tait
- args = list of arguments for a particular style

        absolut args = NULL
        Tait args = B density0 gamma
          B = coefficient
          density0 = reference density
          gamma = isentropic exponent

Examples:

    fix pressure all sph/pressure absolut
    fix pressure all sph/pressure Tait 2000000. 1000. 7.

LIGGGHTS vs. LAMMPS Info:

This command is not available in LAMMPS.

Description:

The equation of state (EOS) for the SPH calculation is the link between the density field and the pressure field. A lot of different equations can be found in the literature.

The absolut style was the first implemented EOS. Based on "An initiation to SPH" from Lucas Braune and Thomas Lewiner this simple equation

is implemented, where ρa is the density of particle a.

In case of Tait style the rule

is applied. B denotes the pressure prefactor which is calculated by

where c0 is the speed of sound of the material. ρ0 is the reference density and γ is the isentropic exponent defined as cp/cv.

NOTE: Monaghan has found that the speed of sound could be artificially reduced. (Monaghan, 1994) Therefore, we can choose a greater time step. He argues that the minimum sound speed should be about ten times greater than the maximum expected flow speed. (Δρ < 1%)

Restart, fix_modify, output, run start/stop, minimize info:

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

None of the fix_modify options are relevant to this fix.

No global scalar or vector or per-atom quantities are stored by this fix for access by various output commands.

No parameter of this fix can be used with the start/stop keywords of the run command.

This fix is not invoked during energy minimization.

Restrictions:

One fix sph/density/summation (only dev-version) or sph/denstiy/continuity has to exist.

Related commands:

pair_style sph, fix sph/density/continuity

Default: none