text(Free Conv.; Steel Block In Water 
  The geometry is as for case 921, but the north wall no longer
  moves. Instead, the motion is induced by allowing for the
  effect of buoyancy, by way of the Boussinesq approximation.
  The PHOTON USE macro of case 921 can be activated by issuing the
  command 'use Q1EAR'; but not 'use Q1'. The reason is that the
  date of case 921 have been brought in by way of the compact
  #$921 command above.
moving=skip   ! to de-activate the moving wall
gravdir=2; Tref = WALLTEMP