fix nve/cfd_cn/bubble command

Syntax

fix ID group-ID nve/cfd_cn/bubble keyword value ...

  • ID, group-ID are documented in fix command

  • nve/cfd_cn/bubble = style name of this fix command

  • zero or more keyword/value pairs may be appended to args; the dim keyword must be used

  • keyword = update_radius_every update_radius_every value = update-every update-every = update bubble radius every this many time-steps

Examples

fix integr2 bubble nve/cfd_cn/bubble update_radius_every 1:pre

Description

Integrate bubble particles, including updates to density and radius of particles according to the ideal gas equation:

dp = density_mixture * dx_grav

where density_mixture is the assumed mixture density (see material properties below), and dx_grav is the vector depicting the position change from one time-step to another, projected onto the gravity vector (units m^2/s^2). Density is then updated according to

density += dp / RTinv

where RT is the ideal gas constant times the temperature (assumed to be constant). Keyword update_radius_every is used to define every how many time-steps the particle radius is updated. Since this is a cubic root operation, it is relatively expensive. However, not updating the value often enough will lead to numerical errors and instabilities. A value between 1 and 5 for update_radius_every is recommended.

The following material properties have to be defined, using multiple fix property/global commands:

fix id all property/global specificGasConstant scalar value
    (value=value specific gas constant)
fix id all property/global densityMixture scalar value
    (value=value mixture density bubble/liquid/solid)
fix id all property/global temperatureFluidMixture scalar value
    (value=value of Temperature for bubble/liquid mixture)
fix id all property/global radiusBubbleMin scalar value
    (value=minimum bubble radius to be resolved in the simulation)

Warning

You have to use atom styles beginning from 1, e.g. 1,2,3,…

Restart, fix_modify, output, run start/stop

No information about this fix is written to binary restart files. No fix_modify option applies to this fix. This fix computes a a global 3-vector of the total buoyancy force, which can be accessed by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command.

Restrictions

This fix requires to use a fix gravity. The gravity vector has to be specified in the same axis as in this fix (via the dim keyword).

Default

update_radius_every = 1