Supersonic wind tunnel

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 so that most are designed for intermittent instead of continuous operation.

Restrictions for supersonic tunnel operation

Minimum required pressure ratio

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: condensation

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 velocity range is limited by reservoir temperature

Power requirements

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. 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 a minimum pressure ratio)
• 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.

Further reading

• Pope, A.; Goin, K. (1978). High-speed Wind Tunnel Testing. Krieger. ISBN 088275727X. 

See also

• Wind tunnel
• Low speed wind tunnel
• High speed wind tunnel
• Hypersonic wind tunnel
• Ludwieg tube
• Shock tube

External links

• Supersonic wind tunnel test demonstration (Mach 2.5) with flat plate and wedge creating an oblique shock(Video)