title: Experimental validation of a numerical model for predicting the trajectory of blood drops in typical crime scene conditions, including droplet deformation and breakup, with a study of the effect of indoor air currents and wind on typical spatter drop trajectories.
abstract: Bloodstain Pattern Analysis (BPA) provides information about events during an assault, e.g. location of participants, weapon type and number of blows. To extract the maximum information from spatter stains, the size, velocity and direction of the drop that produces each stain, and forces acting during flight, must be known. A numerical scheme for accurate modeling of blood drop flight, in typical crime scene conditions, including droplet oscillation, deformation and in-flight disintegration, was developed and validated against analytical and experimental data including passive blood drop oscillations, deformation at terminal velocity, cast-off and impact drop deformation and breakup features. 4th order Runge-Kutta timestepping was used with the Taylor Analogy Breakup (TAB) model and Pilch and Erdman's (1987) expression for breakup time. Experimental data for terminal velocities, oscillations, and deformation was obtained via digital high-speed imaging. A single model was found to describe drop behavior accurately in passive, cast off and impact scenarios. Terminal velocities of typical passive drops falling up to 8m, distances and times required to reach them were predicted within 5%. Initial oscillations of passive blood drops with diameters of 1mm<d<6mm falling up to 1.5m were studied. Predictions of oscillating passive drop aspect ratio were within 1.6% of experiment. Under typical crime scene conditions, the velocity of the drop within the first 1.5m of fall is affected little by drag, oscillation or deformation. Blood drops with diameter 0.4-4mm and velocity 1-15m/s cast-off from a rotating disk showed low deformation levels (Weber number<3). Drops formed by blunt impact 0.1-2mm in diameter at velocities of 14-25m/s were highly deformed (aspect ratios down to 0.4) and the larger impact blood drops (∼1-1.5mm in diameter) broke up at critical Weber numbers of 12-14. Most break-ups occurred within 10-20cm of the impact point. The model predicted deformation levels of cast-off and impact blood drops within 5% of experiment. Under typical crime scene conditions, few cast-off drops will break up in flight. However some impact-generated drops were seen to break up, some by the vibration, others by bag breakup. The validated model can be used to gain deep understanding of the processes leading to spatter stains, and can be used to answer questions about proposed scenarios, e.g. how far blood drops may travel, or how stain patterns are affected by winds and draughts.
authors.affiliation: Department of Mechanical Engineering, University of Canterbury, Private Bag 4800, Christchurch 8041, New Zealand.
authors.forename: N
authors.initials: N
authors.lastname: Kabaliuk
authors.affiliation: Department of Mechanical Engineering, University of Canterbury, Private Bag 4800, Christchurch 8041, New Zealand. Electronic address: mark.jermy@canterbury.ac.nz.
authors.forename: M C
authors.initials: MC
authors.lastname: Jermy
authors.affiliation: Christchurch Science Centre, Institute of Environmental Sciences and Research, 27 Creyke Road, Christchurch 8041, New Zealand; Department of Sport and Exercise Science, Faculty of Science, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.
authors.forename: E
authors.initials: E
authors.lastname: Williams
authors.affiliation: Minnesota Bureau of Criminal Apprehension, 1430 Maryland Ave. East, St. Paul, MN 55106, United States.
authors.forename: T L
authors.initials: TL
authors.lastname: Laber
authors.affiliation: Christchurch Science Centre, Institute of Environmental Sciences and Research, 27 Creyke Road, Christchurch 8041, New Zealand.
authors.forename: M C
authors.initials: MC
authors.lastname: Taylor
mesh_terms: D000392:Air Movements; D055592:Biophysical Phenomena; D001801:Blood Stains; D005554:Forensic Medicine / Q000379:methods; D006801:Humans; D015233:Models, Statistical
keywords: Bloodstain pattern analysis; Drag; Droplet breakup; Droplet oscillation; Numerical modeling; Trajectory