MODELING AND SIMULATION OF AIR BLAST EFFECTS

Abstract

The fragment trajectory routine currently implemented in the hydrodynamic code for blast and fragmentation simulation by the Institute of High Performance Computing (IHPC) has been found to be inconsistent with the equations of projectile motion under drag and gravity. Its trajectory predictions for a spherical fragment in still air showed significant difference with a 4th order Runge-Kutta (RK4) implementation of the equations of motion. The latter has been validated with literature results under the conditions of still air, constant drag coefficient and drag area, and showed excellent agreement with a modified implementation of the hydrocode that is consistent with the equations of motion. Under blast conditions, the difference between the original (IHPC) and modified hydrocode implementations increased further due to the omission of air velocity in the drag model of the former. Hence, the modified hydrocode implementation should be used in place of the former to predict fragment trajectory under blast conditions. Based on the blast wave data predicted by the hydrocode, the difference in fragment trajectory under blast and still air conditions is ascertained to be largely the result of blast wave – fragment interactions in the first 0.01 s after detonation. Air density and air velocity changes in the first 0.01 s greatly affect the drag force, and consequently, the fragment’s trajectory, range and kinetic energy under blast conditions. After t = 0.01 s, the blast wave is likely to have little effect on the fragment trajectory of the heaviest 5% of fragments either because the fragment has overtaken the blast wave and or that the blast wave has weakened considerably such that air density and velocity are close to that of still air.