fix dragforce command

Syntax

fix ID group-ID dragforce style_keyword style_values keyword value ...
  • ID, group-ID are documented in fix command
  • dragforce = style name of this fix command
  • style_keyword = Schiller_Naumann or const_Cd
Schiller_Naumann style_values = viscosity viscosity-value density density-value
  viscosity = obligatory word
  viscosity-value = kinematic fluid density in length^2/time units
  density = obligatory word
  density-value = fluid density in mass/length^3 units
const_Cd style_values = Cd Cd-value viscosity viscosity-value density density-value
  Cd = obligatory word
  Cd-value = value of drag coefficient (dimensionless)
  viscosity = obligatory word
  viscosity-value = kinematic fluid density in length^2/time units
  density = obligatory word
  density-value = fluid density in mass/length^3 units
  • zero or more keyword/value pairs may be appended to args
  • keyword = U_fluid or region
U_fluid value = ux uy uz
  ux, uy, uz = fluid velocity vector in length/time units
region value = region-ID
  region-ID = ID of region atoms must be in to have added drag force

Examples

fix drag all dragforce Schiller_Naumann viscosity 0.0002 density 1000 region water

Description

Add a drag force for each atom in the group, which reads:

Fd = 0.5*density*Urel^2*Cd*A*

where A is the particle’s cross-sectional area, Urel is the relative velocity of particle and fluid. density is the fluid density. If const_Cd is used, then the value of Cd is specified by the user. In case Schiller_Naumann is used, then Cd reads

Cd = max(0.44, 24/Re*(1.+0.15*Re^0.687)
Re = U*2*r/viscosity

Re is the Reynolds Number, and keyword viscosity is the kinematic viscosity of the fluid. R is the particle radius. Keyword U_fluid is used to define the fluid velocity, which is assumed to be constant.

If the region keyword is used, the atom must also be in the specified geometric region in order to have buoyancy force added to it.

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

No information about this fix is written to binary restart files. No fix_modify option applies to this fix. This fix computes no output, 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.

The forces due to this fix are imposed during an energy minimization, invoked by the minimize command.

Restrictions

none

Default

region = whole simulation box, density = 0, viscosity = 0