Nuclear Weapon Testing - Blast and Shock Simulation
In the absence of atmospheric and underground nuclear testing to determine the survivability of structures, means must be found to simulate the phenomena associated with a nuclear explosion. For blast and shock this can be done either in a large-scale, open-air test employing chemical explosives or in a specially designed test facility which can also produce thermal fluxes comparable to those from a nuclear weapon. The air blast from a nuclear explosion is, however, different from that produced by conventional explosives. Because of the intense thermal pulse, the surface and near-surface air mass surrounding ground zero is heated rapidly. Within this heated region the blast wave travels more rapidly than it does in the cooler air above. As a result, blast waves reflected from the ground travel outwards and merge with the direct blast wave from the explosion. This produces a nearly vertical shock front called the Mach stem, which is more intense than that from the direct blast. To simulate the Mach stem with tests using high explosives, scientists employed helium-filled bags at ground level surrounding the high explosives used in the test. Because such tests can only be scaled and do not replicate the actual effects of a nuclear explosion, only scale models of test objects could normally be used.
More recently, U.S. attention has focused on a higher pressure regime than can be attained in open-air testing and on the construction of large simulators capable of re-producing simultaneously the blast and the thermal pulse from a nuclear detonation. These simulators typically employ a fuel-oxygen mixture, for example, liquid oxygen and finely powdered aluminum, and consist of long semicircular tubes. These simulators can even approximate the effects of soil type on blast wave propagation as well as the entraining of dust in the blast wave.
The actual combination of overpressure, dynamic pressure, lift, and diffraction effects on a target is exceedingly difficult to model analytically or to simulate numerically, particularly without actual data. Military interest in the effects of dynamic loading on systems is in the survivability of tracked and wheeled vehicles, towed vehicles, C 3 shelters, etc., in the pressure regime characteristic of nuclear weapons. Civilian interest is in the survivability of similar systems and structures subjected to storm winds. The two are not completely distinct interests because the dynamic pressure from strong hurricanes may be comparable to that from nuclear blasts. Military interest also focuses on shock loading, a dynamic process which differs from the nearly steady-state effects of storm winds. As a rule of thumb, a 30 kPa pressure threshold corresponding to a 60 m/s particle velocity in the shock, or a drag force equivalent to that produced by about 210 km/hr (130 mph) steady winds, distinguishes the military and civilian applications. A frequently used design objective for civil structures is survivability in 190 km/hr (120 mph) winds.
Technologies for simulation include not only the ability to produce strong shocks and air blasts but also those used to measure shock wave values, dynamic pressure in a dusty environment, and deflections or other motions of the test structure. Dust-loaded shock tubes are unique to NWE testing. Similarly, combining both blast and thermal pulse would be unique to the nuclear situation. Explosives which are diluted or mixed with inert materials such as dilute explosive tiles produce more uniform detonations that more closely resemble a nuclear detonation; such explosives would also be critical to NWE testing.
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