ID 25

Atomization of Blood Spatter Resulting from a Gunshot

 

Abstract:

A theoretical model for predicting and interpreting the final locations, stain area, impact angle, and number distribution of blood droplets spattered from a sharp and blunt bullet gunshot wound is proposed. The Rayleigh-Taylor instability is shown to be the process generating a spatter of blood which allows for the determination of blood droplet sizes and initial velocities. The models for each bullet predict the atomization process as a system of two-interpenetrating continua travelling through air. Trajectories of these spattered droplets are then predicted accounting for all relevant forces, namely, gravity and drag forces, including the collective effect of droplets interacting with one another in flight which diminishes the drag force acting on the trailing droplets. Both gunshot scenarios were tested experimentally and the proposed theoretical/numerical models revealed satisfactory agreement. Moreover, an analysis of high-speed videos of blood spatter due to a gunshot was conducted to elucidate basic physical mechanisms of the spatter process and thus validate the model. Using particle image velocimetry and particle analysis software, the investigation revealed that blood spatter in the direction of bullet motion results in droplets travelling twice as fast compared to droplets travelling in the opposite direction. Both types of spatter contain droplets of roughly the same size. It was also found that droplets within the spatter can move with an acceleration due to the aerodynamic wake of preceding droplets spattered at earlier time moments, which, corroborated the mechanism of the collective effect diminishing the aerodynamic drag on the trailing droplets.