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NASA Langley's Hypersonic Facilities Complex, 1969

A hypersonic wind tunnel is designed to generate a hypersonic flow field in the working section. The speed of these tunnels vary from Mach 5 to 15. As with supersonic wind tunnels, these types of tunnels must run intermittently with very high pressure ratios when initializing. Since the temperature drops with the expanding flow, the air inside has the chance of becoming liquefied. For that reason, preheating is particularly critical (the nozzle may require cooling). High pressure and temperature ratios can be produced with a shock tube.

There are several technological problems in designing and constructing a hyper-velocity wind tunnel:

• supply of high temperatures and pressures for times long enough to perform a measurement
• reproduction of equilibrium conditions
• structural damage produced by over-heating
• fast instrumentation
• power requirements to run the tunnel

Simulations of a flow at 5.5 km/s, 45 km altitude would require tunnel temperatures of as much as 9000 K, and a pressure of 3 GPa.

One form of HWT is known as a Gun Tunnel or hot shot tunnel (up to M=27), which can be used for analysis of flows past ballistic missiles, space vehicles in atmospheric entry, and plasma physics or heat transfer at high temperatures. It runs intermittently, like other high speed tunnels, but has a very low running time (less than a second). The method of operation is based on a high temperature and pressurized gas (air or nitrogen) produced in an arc-chamber, and a near-vacuum in the remaining part of the tunnel. The arc-chamber can reach several MPa, while pressures in the vacuum chamber can be as low as 0.1 Pa. This means that the pressure ratios of these tunnels are in the order of 10 million. Also, the temperatures of the hot gas are up to 5000 K. The arc chamber is mounted in the gun barrel. The high pressure gas is separated by the vacuum by a diaphragm that breaks down as its resistance is exceeded.

Prior to a test run commencing, a membrane separates the compressed air from the gun barrel breach. A rifle (or similar) is used to rupture the membrane. Compressed air rushes into the breach of the gun barrel, forcing a small projectile to accelerate rapidly down the barrel. Although the projectile is prevented from leaving the barrel, the air in front of the projectile emerges at hypersonic velocity into the working section. Naturally the duration of the test is extremely brief, so high speed instrumentation is required to get any meaningful data.

See Also: Mach Hypersonic NASA

• Hot Shot Wind Tunnel at the Von Karman Institute for Fluid Dynamics