A History of Research and a Review of Recent Developments

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Pressure measurement and blast simulation
chamber input which drives the shock front down the expansion chamber
with a gain in strength. Multi-diaphragms result in a noticeable increase in
shock strengths. Each additional chamber is said by White to increase the
shock strength by a factor of 12 or 16 according to whether the gases used
have a constant (γ) of 1.67 or 1.4. These numerical increases are based on the
assumption that the times of rupture of the intermediate diaphragms are long
compared with the time for the reflected shock to travel an appreciable distance.
The strength of the shock is limited by the maximum speed at which gas can
escape from the compression chamber of a cylindrical shock tube (y m=2c 3(γ 3–1).
This limitation can be circumvented, and the compression chamber sound
speed (c 3) increased by using a compression chamber of non-uniform cross
section which converges in diameter as it approaches the expansion chamber.
Alpher and White [6.18] suggested that increases of up to 10% in Mach number
can be achieved by this method. The strengthening effects of convergence are
also attainable by tapered reductions in the diameter of the expansion chamber.
The final interesting development of the 1950s was the replacement of the
‘piston’ of gas in the compression chamber by electromagnetic forces. An
electromagnetic piston, having a velocity limited only by the speed of light,
was established by the introduction of two parallel electrodes, across which a
high voltage was suddenly applied. In argon gas at an ambient pressure of 1
cm of mercury a potential gradient of 100 volts/cm will generate Mach 20
shock fronts, and at very, very short durations it is even possible to achieve
Mach 200.
In using the shock tube to apply instantaneous loading to structures, there
is clearly a problem of scale as well as duration. This was very apparent in the
development of methods of simulating the effect of a nuclear explosion on
civil and military equipment. A very large diameter tube is required to
dynamically load a complete tank hull or ship superstructure, and the duration
of the positive period of the explosion is required to be much greater than that
from a conventional charge. These difficulties have led to a considerable research
interest in blast simulation methods since the 1960s, including the use of vertical
shock tubes.
By the mid-1980s several large air-blast simulators had been built in various
countries as part of the well-financed defence requirement to measure the
blast loads from nuclear explosions on full-sized military equipment such as
tanks, small aircraft and helicopters. Most of these were shock tubes in which
cross-sectional areas ranged from 20 to 180 m 2 , and total lengths were up to
100 m, or even 200 m. One of the first large air blast simulators to be built
was at the Atomic Weapons Research Establishment (as it was then called), at
Foulness Island, England, and has been described in a number of papers by
Leys [6.19]. It was completed during the 1970s, and the test section was 20
m 2 in area. It was originally built with an explosive driver developed from
two naval guns which fired into a 1.82 m diameter steel tunnel 36 m long.
This was linked by an asymmetric cone to a 2.43 m diameter section 44 m