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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.

## Restrictions for supersonic tunnel operationEdit

### Minimum required pressure ratioEdit

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 effects: condensationEdit

Temperature in the test section:

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

with $T_t$ = 330K: $T_m$ = 70K at $M_m$ = 4

The Mach range is limited by reservoir temperature

## Power requirementsEdit

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:

• adequate supply of dry air
• wall interference effects
• high-quality instruments capable of rapid measurements due to short run times on 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.