Effect of secondary atomization on blood backspatter affected by muzzle gases

Abstract

Blood backspatter pattern analysis provides important evidence in firearm-related crime scenes. The mechanisms behind particular patterns have attracted significant recent attention in forensic sciences, in general, and in forensic-science-motivated fluid dynamics, in particular. However, investigations on the secondary atomization of blood drops in flight and its effects on trajectories and the corresponding blood stain distributions were scarce. The present work is especially concerned with the effect of secondary atomization on the blood backspatter interaction with muzzle gases at short-range shooting, where it can be very significant. A secondary breakup model is incorporated into the blood backspatter model accounting for interaction with a self-similar vortex ring formed by muzzle gases and moving with high speed in the direction opposite to that of the initial motion of blood drops. The behavior of blood drops of different sizes is investigated, and different scenarios are identified. The secondary atomization stems from high relative velocities of drops and air/muzzle gases and results in the formation of small blood droplets, which are swept easier by muzzle gases and even turned around toward a target. Overall, the secondary atomization in the presence of muzzle gases results in blood stains deposited on the floor closer to the target or even behind the target. It is revealed that in the cases of short-range shooting, the predicted blood stain locations on the floor without accounting for the secondary atomization could be misleading for realistic drop sizes observed experimentally.