Influence of muzzle gases on blood droplet backspatter
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Bloodstain pattern analysis (BPA) has been widely used for the forensic analysis of crime scenes, and recent attention has focused on patterns formed from blood drops in backspatter produced from a gunshot. However, much of the BPA literature lacks rigorous uncertainty estimates and is disconnected from the controlling physical processes resulting in bloodstain formation—in particular, fluid breakup and propagation mechanisms have only recently been addressed in literature. A detailed understanding of the droplet behavior in flight and the importance of competing processes is key to quantifying the uncertainty in a forensic bloodstain pattern analysis. Forces such as muzzle gases expelled from the barrel of the firearm can be very important for cases where the shooter is at close range. In this dissertation, a study of forward projected droplet tracks compared with blood stain patters as well as the influence of muzzle gases blood backspatter is presented. Droplets were tracked in three-dimensional space using digital in-line holography coupled with high-speed imaging at kHz rates for global visualization of the backspatter. The Sandia HOLOSAND code was used to identify and link droplets over multiple frames which produced position and size of the in 3D space. Calculating associated error and velocity of these tracks prepared them to be compared with physical stains through a forward propagation model. Muzzle gas interactions with blood backspatter were observed through shadowgraphy and critical distances for ligament and fully formed drop interaction were identified. In conjunction, blood stain patterns for various firearm distances were also observed to show obvious differences in stain patterns. Particle tracking velocimetry was applied to the shadowgraphy to analyze larger droplets entering particle breakup regimes during interactions with the muzzle gas flowfield. A retroreflective shadowgraphy technique was used to visualize the expansion of muzzle gases for various firearms. The gas expansion was captured and processed to extract the position of the leading edge and radial expansion over time. This data was then compared to a numerical model characterizing the expansion of gases leaving the firearm as self-similar vortex rings. Confirmation of an analytical theory allows for the generalization of muzzle gas interactions across a range of firearms and standoff distances, which may be key in analyzing bloodstain patterns in a range of scenarios.