Shock tube
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: ''For the pyrotechnic initiator, see Shock tube detonator'' The shock tube is an instrument used to replicate and direct blast waves at a sensor or a model in order to simulate actual explosions and their effects, usually on a smaller scale. Shock tubes (and related impulse facilities such as shock tunnels, expansion tubes, and expansion tunnels) can also be used to study aerodynamic flow under a wide range of temperatures and pressures that are difficult to obtain in other types of testing facilities. Shock tubes are also used to investigate compressible flow phenomena and gas phase
combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...
reactions. More recently, shock tubes have been used in biomedical research to study how biological specimens are affected by blast waves. A shock wave inside a shock tube may be generated by a small explosion (blast-driven) or by the buildup of high pressures which cause diaphragm(s) to burst and a shock wave to propagate down the shock tube (compressed-gas driven).


History

An early study of compression driven shock tubes was published in 1899 by French scientist Paul Vieille, though the apparatus was not called a shock tube until the 1940s. In the 1940s, interest revived and shock tubes were increasingly used to study the flow of fast moving gases over objects, the chemistry and physical dynamics of gas phase combustion reactions. In 1966, Duff and Blackwell described a type of shock tube driven by high explosives. These ranged in diameter from 0.6 to 2 m and in length from 3 m to 15 m. The tubes themselves were constructed of low-cost materials and produced shock waves with peak dynamic pressures of 7 MPa to 200 MPa and durations of a few hundred microseconds to several milliseconds. Both compression-driven and blast-driven shock tubes are currently used for scientific as well as military applications. Compressed-gas driven shock tubes are more easily obtained and maintained in laboratory conditions; however, the shape of the pressure wave is different from a blast wave in some important respects and may not be suitable for some applications. Blast-driven shock tubes generate pressure waves that are more realistic to free-field blast waves. However, they require facilities and expert personnel for handling high explosives. Also, in addition to the initial pressure wave, a jet effect caused by the expansion of compressed gases (compression-driven) or production of rapidly expanding gases (blast-driven) follows and may transfer momentum to a sample after the blast wave has passed. More recently, laboratory scale shock tubes driven by fuel-air mixtures have been developed that produce realistic blast waves and can be operated in more ordinary laboratory facilities. Because the molar volume of gas is much less, the jet effect is a fraction of that for compressed-gas driven shock tubes. To date, the smaller size and lower peak pressures generated by these shock tubes make them most useful for preliminary, nondestructive testing of materials, validation of measurement equipment such as high speed pressure transducers, and for biomedical research as well as military applications.


Operation

A simple shock tube is a tube, rectangular or circular in cross-section, usually constructed of metal, in which a gas at low pressure and a gas at high pressure are separated using some form of
diaphragm Diaphragm may refer to: Anatomy * Thoracic diaphragm, a thin sheet of muscle between the thorax and the abdomen * Pelvic diaphragm or pelvic floor, a pelvic structure * Urogenital diaphragm or triangular ligament, a pelvic structure Other * Diap ...
. See, for instance, texts by Soloukhin, Gaydon and Hurle, and Bradley. The diaphragm suddenly bursts open under predetermined conditions to produce a wave propagating through the low pressure section. The shock that eventually forms increases the temperature and pressure of the test gas and induces a flow in the direction of the shock wave. Observations can be made in the flow behind the incident front or take advantage of the longer testing times and vastly enhanced pressures and temperatures behind the reflected wave. The low-pressure gas, referred to as the driven gas, is subjected to the shock wave. The high pressure gas is known as the driver gas. The corresponding sections of the tube are likewise called the driver and driven sections. The driver gas is usually chosen to have a low
molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
, (e.g.,
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...
or
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
) for safety reasons, with high
speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as we ...
, but may be slightly diluted to 'tailor' interface conditions across the shock. To obtain the strongest shocks the pressure of the driven gas is well below atmospheric pressure (a partial
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...
is induced in the driven section before detonation). The test begins with the bursting of the diaphragm. Several methods are commonly used to burst the diaphragm. *A mechanically-driven plunger is sometimes used to pierce it or an explosive charge may be used to burst it. *Another method is to use diaphragms of plastic or metals to define specific bursting pressures. Plastics are used for the lowest burst pressures, aluminum and copper for somewhat higher levels and mild steel and stainless steel for the highest burst pressures. These diaphragms are frequently scored in a cross-shaped pattern to a calibrated depth to ensure that they rupture evenly, contouring the petals so that the full section of the tube remains open during the test time. *Yet another method of rupturing the diaphragm utilizes a mixture of combustible gases, with an initiator designed to produce a detonation within it, producing a sudden and sharp increase in what may or may not be a pressurized driver. This blast wave increases the temperature and pressure of the driven gas and induces a flow in the direction of the shock wave but at lower velocity than the lead wave. The bursting diaphragm produces a series of pressure waves, each increasing the
speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends strongly on temperature as we ...
behind them, so that they compress into a shock propagating through the driven gas. This
shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...
increases the temperature and pressure of the driven gas and induces a flow in the direction of the shock wave but at lower velocity than the lead wave. Simultaneously, a rarefaction wave, often referred to as the Prandtl-Meyer wave, travels back in to the driver gas. The interface, across which a limited degree of mixing occurs, separates driven and driver gases is referred to as the contact surface and follows, at a lower velocity, the lead wave. A 'Chemical Shock Tube' involves separating driver and driven gases by a pair of diaphragms designed to fail after pre-determined delays with an end 'dump tank' of greatly increased cross-section. This allows an extreme rapid reduction (quench) in temperature of the heated gases.


Applications

In addition to measurements of rates of
chemical kinetics Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with chemical thermodynamics, which deals with the direction in wh ...
shock tubes have been used to measure dissociation energies and molecular relaxation ratesStrehlow, 1967, Illinois University, Dept.Aero.and Astro. AAE Rept.76-2.Nettleton, 1977, Comb.and Flame, 28,3. and 2000, Shock Waves, 12,3. they have been used in aerodynamic tests. The fluid flow in the driven gas can be used much as a
wind tunnel Wind tunnels are large tubes with air blowing through them which are used to replicate the interaction between air and an object flying through the air or moving along the ground. Researchers use wind tunnels to learn more about how an aircraft ...
, allowing higher temperatures and pressures therein replicating conditions in the
turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating ...
sections of
jet engine A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, Pump-jet, water jet, and hybrid propulsion, the term ...
s. However, test times are limited to a few milliseconds, either by the arrival of the contact surface or the reflected shock wave. They have been further developed into
shock tunnel 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 wit ...
s, with an added
nozzle A nozzle is a device designed to control the direction or characteristics of a fluid flow (specially to increase velocity) as it exits (or enters) an enclosed chamber or pipe. A nozzle is often a pipe or tube of varying cross sectional area, a ...
and dump tank. The resultant high temperature
hypersonic flow In aerodynamics, a hypersonic speed is one that exceeds 5 times the speed of sound, often stated as starting at speeds of Mach 5 and above. The precise Mach number at which a craft can be said to be flying at hypersonic speed varies, since i ...
can be used to simulate
atmospheric re-entry Atmospheric entry is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: ''uncontrolled entry'', such as the entr ...
of
spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
or hypersonic craft, again with limited testing times. Shock tubes have been developed in a wide range of sizes. The size and method of producing the shock wave determine the peak and duration of the pressure wave it produces. Thus, shock tubes can be used as a tool used to both create and direct blast waves at a sensor or an object in order to imitate actual explosions and the damage that they cause on a smaller scale, provided that such explosions do not involve elevated temperatures and
shrapnel Shrapnel may refer to: Military * Shrapnel shell, explosive artillery munitions, generally for anti-personnel use * Shrapnel (fragment), a hard loose material Popular culture * ''Shrapnel'' (Radical Comics) * ''Shrapnel'', a game by Adam C ...
or flying debris. Results from shock tube experiments can be used to develop and validate numerical model of the response of a material or object to an ambient blast wave without shrapnel or flying debris. Shock tubes can be used to experimentally determine which materials and designs would be best suited to the job of attenuating ambient blast waves without shrapnel or flying debris. The results can then be incorporated into designs to protect structures and people that might be exposed to an ambient blast wave without shrapnel or flying debris. Shock tubes are also used in biomedical research to find out how biological tissues are affected by blast waves. There are alternatives to the classical shock tube; for laboratory experiments at very high pressure,
shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...
s can also be created using high-intensity short-pulse lasers.


See also

*
Hypersonic wind tunnel A hypersonic wind tunnel is designed to generate a hypersonic flow field in the working section, thus simulating the typical flow features of this flow regime - including compression shocks and pronounced boundary layer effects, entropy layer an ...
*
Light-gas gun The light-gas gun is an apparatus for physics experiments. It is a highly specialized gun designed to generate extremely high velocities. It is usually used to study high-speed impact phenomena (hypervelocity research), such as the formation of ...
* Ludwieg tube *
Expansion fan Expansion may refer to: Arts, entertainment and media * ''L'Expansion'', a French monthly business magazine * ''Expansion'' (album), by American jazz pianist Dave Burrell, released in 2004 * ''Expansions'' (McCoy Tyner album), 1970 * ''Expansio ...
* Shock tube detonator *
Shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...
* Supersonic wind tunnel


References


External links


RPI Shock Tube page





High Pressure shock tube group
{{Authority control Laboratory equipment Aerodynamics Engineering equipment Shock waves