Expansion tunnel

Expansion and shock tunnels are aerodynamic testing facilities with a specific interest in high speeds and high temperature testing. Shock tunnels use steady flow nozzle expansion whereas expansion tunnels use unsteady expansion with higher enthalpy, or thermal energy. In both cases the gases are compressed and heated until the gases are released, expanding rapidly down the expansion chamber. The tunnels reach speeds from Mach 3 to Mach 30 to create testing conditions that simulate hypersonic to re-entry flight. These tunnels are used by military and government agencies to test hypersonic vehicles that undergo a variety of natural phenomenon that occur during hypersonic flight.

Expansion tunnels use a dual-diaphragm system where the diaphragms act as rupture discs, or a pressure relief. The tunnel is separated into three sections: drive, driven, and acceleration. The drive section is filled with high pressure helium gas. The driven section is filled with a lower pressure desired test gas, such as carbon dioxide, helium, nitrogen, or oxygen.. The acceleration section is filled with an even lower pressurized test gas. Each section is divided by a diaphragm, which is meant to be ruptured in sequence causing the first diaphragm to rupture, mixing and expanding the drive and the driven. When the shock wave hits the second diaphragm, it ruptures casing the two gases to mix with the acceleration and expand down the enclosed test section. Operation time is approximately 250 microseconds.

Reflected shock tunnels heat and pressurize a stagnant gas by using shockwaves that are redirected back into the center; this excites the gases and produces movement, heat, and pressure. The gases are then released and expanded through the nozzle and into the test chamber. Operation time is approximately 20 milliseconds.

During the expansion process, a variety of test are run to analyze the aerodynamic and thermal properties of the test vehicle.

The HET is one of the shock tunnels in the Caltech Hypersonics group at the California Institute of Technology directed by Professor Joanna Austin. It operates similarly to a shock tube where a shock formed by the primary diaphragm heats up the test gas. The novel part of this facility is when it's test gas is further accelerated by an expansion shock that forms when the primary shock interacts with a second downstream diaphragm. It is a 150mm inner diameter facility with the capability to reach Mach 4-8, and was built in 2005.

NASA's Hypersonic Pulse Facility (HYPULSE) is operated by the General Applied Science Laboratory (GASL) in New York. The HYPULSE facility was developed for the testing of re-entry vehicles and air-breathing engines. The specifications of the HYPULSE include a diameter of 7 feet and a 19 foot length. This facility was upgraded to have two modes, Reflected Shock Tunnel (RST) and Shock-Expansion Tunnel (SET). HYPULSE-RST generates speeds from Mach 5 to 10, whereas the HYPULSE-SET produces speeds from Mach 12 to 25.

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