text: A supersonic wind tunnel is a wind tunnel that produces supersonic speeds (1.2<M<5)
  The Mach number and flow are determined by the nozzle geometry. The Reynolds number is varied by changing the density level (pressure in the settling chamber). Therefore, a high pressure ratio is required (for a supersonic regime at M=4, this ratio is of the order of 10). Apart from that, condensation of moisture or even gas liquefaction can occur if the static temperature becomes cold enough. This means that a supersonic wind tunnel usually needs a drying or a pre-heating facility.
  A supersonic wind tunnel has a large power demand, so most are designed for intermittent instead of continuous operation.
  The first supersonic wind tunnel (with a cross section of 2 cm) was built in National Physical Laboratory in England, and started working in 1922.