We have been examining a surface burst explosion, in which the explosive charge is assumed to be hemispherical and placed on flat, hard, smooth ground surface, so that the blast wave will also be hemispherical. In the Free Air configuration, the blast wave is assumed to be spherical. (
AirBlast has separate databases for the surface burst and free air configurations, both based on experimental measurements of blast wave properties; it is not assumed that a surface burst explosion is equivalent to a free air explosion of twice the size).

AirBlast also provides databases for 'height-of-burst' configurations. In these configurations a spherical charge is exploded at a finite height above a flat, hard, smooth ground surface. The spherical free air blast wave is reflected at the ground surface. The reflection moves away from the point of first contact between the blast wave and the ground, directly under the charge at ground zero, and then along the ground until it reaches a distance roughly equal to the charge height above ground, where it undergoes a transition from the 'regular' reflection (RR) to Mach reflection (MR). Mach reflection is characterized by a triple point at which the original free-air shock, the reflected shock and a new shock, the Mach stem, meet. The triple point trajectory starts on the ground at the RR-MR transition point and rises above the ground.

Sample results for a height-of-burst configuration are shown below:

abgraph6.jpg (15421 bytes)  abgraph7.jpg (46505 bytes)
Peak static overpressure vs ground radius for a 2000 kg of ANFO detonated 30m above the ground surface, using linear axes (first plot) and log-log axes. Note that logarithmic axes are only able to plot values greater than zero and that in height-of-burst configurations AirBlast calculates data at ground range zero (ground zero), and that therefore the first data point (0,3.529) plotted in the first figure, is missing from the second.

In the plots above, RR-MR transition is seen as an increase in the overpressure ratio around ground radius 30 m. The increase begins in the RR region before transition, where peak pressures are calculated using the free-air database and two-shock reflection theory. After transition, in the MR region, results are from databases of experimental measurements made in actual height-of-burst explosions.

Dynamic Pressures in the example above look like:

abgraph8.jpg (15327 bytes)   abgraph9.jpg (50186 bytes)
Peak dynamic pressure vs ground radius for the same explosion (height of burst 30 m). Dynamic pressures are generally lower than, and decrease with distance more rapidly than, static pressures (cf. plots at top of page), but more interesting perhaps is the fact that dynamic pressure is zero at ground zero, at which point static pressure is a maximum. This is because the air at the ground surface at ground zero remains at rest. Note that the point (0,0) cannot be plotted on the log axes.



square.gif (821 bytes) Congratulations on making it this far. We went through considerable detail towards the end, mainly to stress the fact that AirBlast can provide a large amount of complex information in an understandable way. We scratched the surface.