subsection Boundary velocity model
  set Tangential velocity boundary indicators = left, right, bottom
  set Prescribed velocity boundary indicators = top: function

  subsection Function
    set Variable names      = x,z,t
    set Function constants  = pi=3.1415926
    set Function expression = if(x>1+sin(0.5*pi*t), 1, -1); 0
  end
end
