fix dragforce command

Warning

Warning

This command is deprecated, please use enable_one_way_coupling instead.

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, DiFelice, Zastawny or const_Cd

Schiller_Naumann style_values = fluid_viscosity viscosity-value fluid_density density-value
  fluid_viscosity = obligatory word
  viscosity-value = kinematic fluid density in length^2/time units
  fluid_density = obligatory word
  density-value = fluid density in mass/length^3 units
DiFelice style_values = fluid_viscosity viscosity-value fluid_density density-value
  fluid_viscosity = obligatory word
  viscosity-value = kinematic fluid density in length^2/time units
  fluid_density = obligatory word
  density-value = fluid density in mass/length^3 units
Zastawny style_values = fluid_viscosity viscosity-value fluid_density density-value
  fluid_viscosity = obligatory word
  viscosity-value = kinematic fluid density in length^2/time units
  fluid_density = obligatory word
  density-value = fluid density in mass/length^3 units
const_Cd style_values = Cd Cd-value fluid_viscosity viscosity-value fluid_density density-value
  Cd = obligatory word
  Cd-value = value of drag coefficient (dimensionless)
  fluid_viscosity = obligatory word
  viscosity-value = kinematic fluid density in length^2/time units
  fluid_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 fluid_viscosity 0.0002 fluid_density 1000 region water

Description

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

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

where A is the particle’s cross-sectional area, Urel is the relative velocity of particle and fluid. 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/fluid_viscosity

Re is the Reynolds Number, and keyword fluid_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.

In case DiFelice is used, then Cd and Re are calculated

Cd = (0.63+(4.8/sqrt(Re)))^2
Re = voidfraction*U*2*r/fluid_viscosity

Note that voidfraction is assumed to be 1. for all 1-way coupled simulations, for 4-way coupled simulations voidfraction is calculated in the CFD simulation. In case Zastawny is used, then Cd is calculated according to Zastawny et al. (2012) “Derivation of drag and lift force and torque coefficients for non-spherical particles in flows”. Please note that Zastawny can be applied only in combination with multi-sphere particles.

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

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 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.

Restrictions

none

Default

region = whole simulation box, fluid_density = 0, fluid_viscosity = 0