A History of Research and a Review of Recent Developments
A History of Research and a Review of Recent Developments
A History of Research and a Review of Recent Developments
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Blast simulation 125<br />
layer between the front <strong>and</strong> the tube walls. In well-designed tubes the influence<br />
<strong>of</strong> both <strong>of</strong> the above effects can be reduced to a level that does not greatly<br />
impair the accuracy experiments.<br />
The properties <strong>of</strong> shock waves were measured by a number <strong>of</strong> techniques<br />
during the early years <strong>of</strong> shock tube development. A number <strong>of</strong> optical methods<br />
were used, based on the fact that the refractive index <strong>of</strong> a gas varies with its<br />
density according to the Gladstone Dale Law =1+Kρ, where is the refractive<br />
index, K the Gladstone Dale constant, <strong>and</strong> ρ the density. The measurement <strong>of</strong><br />
at high shock front speeds was <strong>of</strong>ten made by using short duration sparks as<br />
the light source <strong>and</strong> using shadow or Schlieren photography to examine density<br />
discontinuities. The density pr<strong>of</strong>ile was also found by interferometry, in which<br />
light from a point source was collimated <strong>and</strong> split at a half-silvered mirror.<br />
One half then passed through the shock tube before the beams were recombined,<br />
<strong>and</strong> with monochromatic illumination interference fringes could be detected.<br />
There was a linear relationship between fringe shift <strong>and</strong> density change. Highspeed<br />
rotating-mirror cameras were needed to achieve sufficient time resolution<br />
for very fast shocks.<br />
The advent <strong>of</strong> the piezo-electric pressure gauge enabled pressures rather<br />
than densities to be measured, as long as the gauges were mounted in the<br />
shock tube in ways that eliminated the effect on them <strong>of</strong> mechanical stresses<br />
transmitted through the walls <strong>of</strong> the shock tube. This is now the most usual<br />
way <strong>of</strong> measuring shock wave properties. The instantaneous rise in temperature<br />
across a shock front can also be used to investigate density changes, so there<br />
was some development in the use <strong>of</strong> resistance thermometers <strong>and</strong> hot-wire<br />
anemometers. The latter, however, generally have too slow a response time<br />
for shock tube investigations.<br />
We noted earlier that the strength <strong>of</strong> the shock in a tube can be increased by<br />
introducing combustion into the compression chamber. This method, which<br />
was the subject <strong>of</strong> much research in the 1950s, burns oxygen with the hydrogen<br />
to raise the temperature <strong>of</strong> the latter. The best mixture is 1 part by weight <strong>of</strong><br />
oxygen to 8 parts by weight <strong>of</strong> hydrogen, <strong>and</strong> this leads to a sound speed (c 3)<br />
about 1.7 times that <strong>of</strong> cold hydrogen. For this application the combustion<br />
chamber must be strongly constructed, <strong>and</strong> gun barrels are frequently used.<br />
Mach numbers <strong>of</strong> about 10 are obtainable with cold hydrogen, <strong>and</strong> up to 20<br />
with a combustion shock tube.<br />
The heating process tends to produce non-uniform conditions in the<br />
compression chamber <strong>and</strong> attenuates the shock wave as it travels down the<br />
expansion chamber. To eliminate this problem multiple diaphragms are<br />
sometimes used, in which an intermediate chamber is introduced at a lower<br />
pressure than the compression chamber, but at a higher pressure than the<br />
expansion chamber. When the first diaphragm is ruptured the shock wave<br />
travels down the intermediate chamber until it is reflected <strong>of</strong>f the second<br />
diaphragm. The reflected pressure is sufficient to rupture the second diaphragm,<br />
but the reflected wave leaves a hot, high pressure region to act as a compression