A History of Research and a Review of Recent Developments

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