Hydrodynamics of back spatter by blunt bullet gunshot with a link to bloodstain pattern analysis

Abstract

A theoretical model describing the blood spatter pattern resulting from a blunt bullet gunshot is proposed. The predictions are compared to experimental data acquired in the present work. This hydrodynamic problem belongs to the class of the impact hydrodynamics with the pressure impulse generating the blood flow. At the free surface, the latter is directed outwards and accelerated toward the surrounding air. As a result, the Rayleigh-Taylor instability of the flow of blood occurs, which is responsible for the formation of blood drops of different sizes and initial velocities. Thus, the initial diameter, velocity, and acceleration of the atomized blood drops can be determined. Then, the equations of motion are solved, describing drop trajectories in air accounting for gravity, and air drag. Also considered are the drop-drop interactions through air, which diminish air drag on the subsequent drops. Accordingly, deposition of two-phase (blood-drop and air) jets on a vertical cardstock sheet located between the shooter and the target (and perforated by the bullet) is predicted and compared with experimental data. The experimental data were acquired with a porous polyurethane foam sheet target impregnated with swine blood, and the blood drops were collected on a vertical cardstock sheet which was perforated by the blunt bullet. The highly porous target possesses a low hydraulic resistance and therefore resembles a pool of blood shot by a blunt bullet normally to its free surface. The back spatter pattern was predicted numerically and compared to the experimental data for the number of drops, their area, the total stain area, and the final impact angle as functions of radial location from the bullet hole in the cardstock sheet (the collection screen). Comparisons of the predicted results with the experimental data revealed satisfactory agreement. The predictions also allow one to find the impact Weber number on the collection screen, which is necessary to predict stain shapes and sizes. A theoretical model describing the blood spatter pattern resulting from a blunt bullet gunshot is proposed. The predictions are compared to experimental data acquired in the present work. This hydrodynamic problem belongs to the class of the impact hydrodynamics with the pressure impulse generating the blood flow. At the free surface, the latter is directed outwards and accelerated toward the surrounding air. As a result, the Rayleigh-Taylor instability of the flow of blood occurs, which is responsible for the formation of blood drops of different sizes and initial velocities. Thus, the initial diameter, velocity, and acceleration of the atomized blood drops can be determined. Then, the equations of motion are solved, describing drop trajectories in air accounting for gravity, and air drag. Also considered are the drop-drop interactions through air, which diminish air drag on the subsequent drops. Accordingly, deposition of two-phase (blood-drop and air) jets on a vertical cardstock sheet located between the shooter and the t...

Automatic Classification of Bloodstain Patterns Caused by Gunshot and Blunt Impact at Various Distances

“…This latter fact can be associated with the higher velocity and kinetic energy imparted by the bullet to the blood . Although gunshot spatters tend to generate more bloodstains than impact spatters, the number of bloodstains is not by itself discriminative enough for classification (see Fig.…”

“…Figure illustrates why we vary r 0 among spatter patterns. If the gravity and air resistance are ignored, the trajectory of drops is contained in a cone , whose base is roughly circular on the target surface. Moving the blood source toward the target surface, the area of the base progressively becomes smaller.…”

Theoretical and experimental investigation of forward spatter of blood from a gunshot

“…The formation of blood spatter due to a bullet impact is a short-term event in which the viscous forces are negligibly small compared to the inertial ones because the Reynolds number is of the order [13]. Blood spatter formation in forward spatter involves a cascade of instability phenomena, and in this sense is seemingly more involved than in the case of backward spatter, where at least the first group of spattered drops is formed due to the Rayleigh-Taylor instability associated with acceleration of denser blood toward lighter surrounding air [13,14]. It was also found recently that the maximum velocity of forward spattered drops is approximately two times higher than that of backwards spattered ones partially due to the entrainment effect of the bullet traveling in the same direction as the forward blood spatter [15].…”

“…In the present work, the forward spatter pattern and the formation of blood drops which created that pattern are linked through physics-based modeling. In contrast with the Rayleigh-Taylor instability mechanism of blood drop formation previously proposed for back spatter due to a gunshot [13,14], chaotic disintegration characteristic of debris formation in terminal ballistics [16,19,20], which encompasses multiple instability cycles in the framework of percolation theory, is in focus. That means that in contrast with the back spatter attributed to a straightforward Rayleigh-Taylor instability of blood at first contact with the bullet, forward spatter follows the penetration of the bullet through a body, which is accompanied by a cascade of multiple instabilities and rupture phenomena, characteristic of the problems of terminal ballistics.…”

“…The physical mechanism of blood drop formation and acceleration in the region of origin in the case of backspatter has recently been discussed and attributed to the Rayleigh-Taylor instability [13,14]. An understanding of the formation of blood drops is an important prerequisite for predicting their trajectories because their initial size, quantity, and corresponding velocities influence the trajectories.…”