In aeronautics, 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 process
Expansion tunnel
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 causing the two gases to mix with the acceleration and expand down the enclosed test section. Operation time is approximately 250 microseconds.
Shock tunnel
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.
Testing
During the expansion process, a variety of test are run to analyze the aerodynamic and thermal properties of the test vehicle. Skin friction The drag that is created when an object travels through a fluid, such as a liquid or gas Flow chemistry The analysis of reactions that take place during a continuous flow Durability The ability to withstand deterioration Turbulence The disordered movement of fluids Heat transfer The thermal energy transfer from one system to another Aero elastic The forces created by the movement of air and the manner in which air bends around the object Thermal protection The ability to withstand heat transfer, reducing the temperature Vibration The oscillation, or shaking, of the molecules
Testing instruments
Thin-film heat transfer gauge When the gauge is heated, the resistance changes; this causes a change in voltage, which is used to calculate the amount of heat transferred into an object Piezoelectric pressure transducer Under pressure, crystals became electrically charged, proportional to that of the pressure exerted Laser diode spectrograph Measures the properties of the refracted light, generated by the laser traveling through the turbulent gas around an object Force -moment balance Used to measure three or six components, three forces (lift, drag, and side) and three moments (pitch, roll, and yaw), to completely describe the conditions on the model. Forces on the model are detected by strain gauges located on the balance. Each gauge measures a force by the stretching of an electrical element or foil in the gauge. The stretching changes the resistance of the gauge which changes the measured electric current through the gauge according to Ohm's law. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.
Facilities
Hypervelocity Expansion Tube (HET) 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 its 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.