2 1 GfsSimulation GfsBox GfsGEdge {
  # we need to shift the origin of the reference box to (0,0.5)
  y = 0.5
} {

  # Stop the simulation at t = 300 if convergence has not been reached before
  Time { end = 300 }

  # Use an initial refinement of 6 levels (i.e. 2^6=64x64)
  Refine 6

  # The mesh is stretched by a factor 0.5 (compressed) in the y direction.
  MetricStretch {} { sy = 0.5 }

  # Set a viscosity source term on the velocity vector with x-component U
  # The Reynolds number is Re = L*U/Nu = 1*1/1e-3 = 1000
  SourceDiffusion {} U 1e-3
  SourceDiffusion {} V 1e-3

  # Stops the simulation if the maximum of the absolute value of the
  # difference between the current U field and the U field 10 timesteps
  # before is smaller than 1e-4.
  #
  # Stores this difference in the DU field (this can be used for
  # monitoring the convergence of the simulation).
  EventStop { istep = 10 } U 1e-4 DU

  OutputScalarNorm { istep = 10 } du { v = DU }

  # Pipes a bitmap PPM image representation of the velocity field at the end of the simulation
  # into the ImageMagick converter "convert" to create the
  # corresponding EPS file
  OutputPPM { start = end } { convert -colors 256 ppm:- -resize 128x128! velocity.eps } {
    v = Velocity
  }

  # At the end of the simulation, computes the values of the variables
  # at the locations defined in files xprofile, yprofile and stores the
  # results in files xprof, yprof
  OutputLocation { start = end } xprof xprofile
  OutputLocation { start = end } yprof yprofile

  OutputSimulation { start = end } end.gfs

  # At the end of the simulation calls the script generating the EPS
  # files using gnuplot and files: xprof, yprof, xprof.ghia, yprof.ghia
  EventScript { start = end } {
    gnuplot <<EOF
    set term postscript eps lw 3 solid 20
    set output 'xprof.eps'
    set xlabel 'Y'
    set ylabel 'U'
    plot [-0.5:0.5]'xprof.ghia' u 1:2 title "Ghia et al." w p ps 2 pt 9, 'xprof' u 3:7 w l title "Gerris"
    set output 'yprof.eps'
    set xlabel 'X'
    set ylabel 'V'
    plot [-0.5:0.5]'yprof.ghia' u 1:2 title "Ghia et al." w p ps 2 pt 9, 'yprof' u 2:8 w l title "Gerris"
EOF
  }
}
GfsBox {

  # Dirichlet boundary conditions for both components of the velocity on all sides:
  # - non-slip (U = V = 0) on right, left and bottom boundaries
  # - tangential velocity equal to 1 (U = 1) on top boundary

  top = Boundary {
    BcDirichlet U 1
    BcDirichlet V 0
  }
  right = Boundary {
    BcDirichlet U 0
    BcDirichlet V 0
  }
  left = Boundary {
    BcDirichlet U 0
    BcDirichlet V 0
  }
}
GfsBox {
  bottom = Boundary {
    BcDirichlet U 0
    BcDirichlet V 0
  }
  right = Boundary {
    BcDirichlet U 0
    BcDirichlet V 0
  }
  left = Boundary {
    BcDirichlet U 0
    BcDirichlet V 0
  }
}
2 1 top