text: The power required to run a supersonic wind tunnel is enormous, of the order of 50 MW per square meter of test section cross-sectional area. For this reason most wind tunnels operate intermittently using energy stored in high-pressure tanks. These wind tunnels are also called intermittent supersonic blowdown wind tunnels (of which a schematic preview is given below). Another way of achieving the huge power output is with the use of a vacuum storage tank.   These tunnels are called indraft supersonic wind tunnels, and are seldom used because they are restricted to low Reynolds numbers.  Some large countries have built major supersonic tunnels that run continuously; one is shown in the photo.  
  Other problems operating a supersonic wind tunnel include:
  *starting and unstart of the test section (related to maintaining at least a minimum pressure ratio)
  *adequate supply of dry air
  *wall interference effects due to shock wave reflection and (sometimes) blockage
  *high-quality instruments capable of rapid measurements due to short run times in intermittent tunnels
  Tunnels such as a Ludwieg tube have short test times (usually less than one second), relatively high Reynolds number, and low power requirements.