Supersonic wind tunnel

Minimum required pressure ratio

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

P t P a m b ≤ ( P t 1 P t 2 ) M 1 = M m

Examples:

Temperature effects: condensation

Temperature in the test section:

T m T t = ( 1 + γ − 1 2 M m 2 ) − 1

with T t = 330 K: T m = 70 K at M m = 4

The velocity range is limited by reservoir temperature

Power requirements

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:

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