Fluid Mechanics of Blood Motion Resulting from Common Bloodletting Events
Comiskey, Patrick M
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Blood motion resulting from a gunshot is a complex phenomenon which requires fluid mechanics to properly understand. The atomization of blood is a result of fluid instabilities due to the impact of a bullet and the resultant flight of blood droplets must include the effects of gravity, air drag, and the collective effect. This effect is the drag reduction present in groups of droplets where the leading droplets take the bulk of the drag force whereas the ones behind do not. As a result, those blood droplets travel faster, eventually becoming the leading droplets, and the group can travel further. Through particle image velocimetry, this effect is seen to occur in blood spatter due to a gunshot when analyzing high-speed videos for simulated crime scenes. Standard spatter angles, drop sizes, and velocity histories were also analyzed from the videos. The atomization of backward spattered blood is attributed to the Rayleigh-Taylor instability which arises when a dense fluid is accelerated towards a lighter one. This atomization model is produced for an idealized sharp and blunt bullet, and the trajectories of blood are issued into quiescent air. The atomization of forward spattered blood is shown to require a more complex analysis as a cascade of instabilities is present. Therefore, the chaotic disintegration of blood is considered in the framework of percolation theory and is generalized for a bullet of arbitrary shape. Both atomization models were compared with experiments and agreement was good. The interaction of muzzle gases arising from the muzzle of a gun due to the chemical propulsion of the bullet on backward spattered blood droplets is considered. It is shown that the important components are the propellant gases which forms a turbulent vortex ring, the self-similar motion of which is solved and the interaction and effect on backward spattered blood droplets is shown. The gunpowder particle concentration swept by the turbulent vortex ring is also solved aiding in the understanding of the distribution of gunshot residue. Finally, the motion of intact jets of blood is considered and the rheological properties of blood are discussed in the context of forensic science.