Supersonic wind tunnel

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Engineers check an aircraft model before a test run in the Supersonic Wind Tunnel at Lewis Flight Propulsion Laboratory.
Engineers check an aircraft model before a test run in the Supersonic Wind Tunnel at Lewis Flight Propulsion Laboratory.

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 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 or liquefaction can occur. This means that a supersonic wind tunnel needs a drying or a pre-heating facility. A supersonic wind tunnel has a large power demand leading to only intermittent operation.

Contents

Optimistic estimate: Pressure ratio \leq the total pressure ratio over normal shock at M in test section:

\frac{P_t}{P_{amb}} \leq\left(\frac{P_{t_1}}{P_{t_2}}\right)_{M_1=M_m}

Examples:

Temperature in the test section:

\frac{T_m}{T_t}=\left(1+\frac{\gamma-1}{2}M_m^2\right)^{-1}

with Tt = 330K: Tm = 70K at Mm = 4

The Mach range is limited by reservoir temperature

The power required to run a supersonic windtunnel is enormous, of the order of 50 MW per square meter of test section. For this reason most wind tunnels operate intermittently using energy stored in high-pressure tanks. These windtunnels 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. Other problems operating a supersonic wind tunnel include:

  • enough supply of dry air
  • wall interference effects
  • fast instruments needed for intermittent measurements

Tunnels such as a Ludwieg tube have short test times (usually less than one second), relatively high Reynolds number, and low power requirements.

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