Blood drop size in passive dripping from weapons

Empirical investigation of passive blood drop trajectory and first point of contact on inclined surfaces

The first point of contact between a spherical blood drop and a surface is related to the angle between the trajectory of the blood drop and the surface being struck. This angle is often referred to as the impact angle which can be estimated by knowing the width and length of the resultant elliptical bloodstain. Most software programs dedicated to area of origin analysis indicate the location of the backtracked bloodstain trajectory to be at the geometric centre or at the tip of the bloodstain ellipse. However, it is unknown how the first point of contact and the blood drop trajectory (here defined as the locus of the centre of mass of the drop as it travels) are related empirically. Thus, this study aims to look at how the initial point of contact and the trajectory at the impact of a blood drop relates to the formed bloodstain ellipse. Two volumes of blood (0.013 ml and 0.071 ml) were dropped from a height of 10 cm and 40 cm onto an inclined surface at 0°, 15°, 30°, 45°, 60°, and 75°. The transition from a spherical blood drop to an elliptically shaped bloodstain was recorded using a high-speed camera for all tests. A total of 72 ellipses were analyzed to determine the location of the first point of contact and trajectory point of the blood drop and how they relate to the formed elliptical bloodstain. A relationship was found between the first point of contact and the bloodstain trajectory which was dependent on the impact angle. However, there were clear deviations from theoretical assumptions due to blood drop oscillations, the effects of gravity, and the natural fluid characteristics of blood. The results of this study may assist bloodstain pattern analysts and software developers by more accurately applying the location of the blood drop trajectory based on empirical data.

Evaluation of the pattern of spray released from a moving multicopter

BACKGROUND

Multicopters are used for releasing particulates seeds, fertilizer and spray. Their low cost and high manoeuvrability make them attractive for spraying in steep terrain and areas where overspray is undesirable. This article describes a model of multicopter wake and its influence on particulate dispersion, which is computationally economical compared to many computational fluid dynamics (CFD) approaches, yet retains reasonable accuracy.

RESULTS

A model was successfully implemented in OpenFOAM. It features source terms for the rotor wash, Lagrangian particle tracking, an evaporation model, and a porous medium approach to model the effect of the ground vegetation. Predictions were validated against the field tests of Richardson et al. which used a DJI Agras MG-1 multicopter in three different flights with airspeeds of 3.2-4.9 m s^-1 , ground speeds of 2.1-2.9 m s^-1 and cross-wind speeds of 0.04-2.2 m s^-1 . The effective swath width (30% line separation) was predicted to within one standard deviation. Sensitivity to a rotor rotational speed, flight height, flight velocity, multicopter roll and yaw angles, surface roughness length, plant height and leaf density was checked.

CONCLUSION

In all flight trials, the modelled swath was closest to the experimentally obtained swath when the surface roughness of the fetch was equal to 0.5 m (bushes) and the rotational speed of all rotors was equal to 2475 rpm with 0.75R (0.2 m) tall plant canopy (grass) introduced to the model. The model showed acceptable validity for flight velocities of ≤2.8-5 m s^-1 when flight parameters can be approximately estimated. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Whole bovine blood use in forensic research: Sample preparation and storage considerations

Mammalian whole blood sources are often used for forensic research and training when human samples cannot be sourced. While porcine, ovine and equine blood have been shown to be viable alternatives to whole human blood for forensic purposes, procurement can still pose a problem, especially for smaller and remote institutions. This work explores the use of whole bovine blood for basic bloodstain simulation. Sample preparation through the addition of ACD-A anticoagulant was optimized and storability was explored. Viscosity, surface tension, density, and packed cell volume, four fluid properties relevant to bloodstain pattern analysis, were monitored over four days and in two temperature conditions. Linear mixed models accounting for variation in the donor demonstrated that these fluid properties of the bovine blood changed predictably over time and with temperature. Whole bovine blood with 12.5% v/v ACD-A was found to be viable for use in basic bloodstain simulation at ambient and physiological temperature.

Storability of porcine blood in forensics: How far should we go?

Previous studies on the storability of porcine blood for bloodstain pattern analysis (BPA) focused on abattoir blood only and did not include measurements of viscoelasticity. Although known to provoke echinocyte formation, EDTA is widely used for BPA issues. We compared ageing samples taken from live pigs with abattoir blood and detected considerable differences in hematocrit (HCT), total protein and shear viscosity that even worsened with time. Upon storage, high shear viscosity continuously increased, resulting in a partial loss of the typical shear thinning property of blood. Furthermore, we explored CPDA-1, the gold standard in preserving red blood cells (RBCs), for storage of forensic samples. We found it to be a superior choice for anticoagulation, as the rise of high shear viscosity was attenuated compared to EDTA. When performing oscillation measurements, we found a sudden change of viscoelasticity of blood after 22 days, providing a cut-off for storage time. To highlight the importance of hematological and hemorheological changes upon cold storage, we performed simple drip pattern experiments. These tests revealed a tendency to smaller stain diameters and higher numbers of satellite spatter. While this contradicts expectations from elevated viscosity values, we associate this trend to microscopic inhomogeneities due to storage. We recommend CPDA-1 for prolonged storage of BPA samples and suggest the use of comprehensive test protocols including viscoelasticity for determination of the maximum shelf life of pig blood.

Experimental and theoretical studies into the release of blood droplets from weapon tips

The formation and release under gravity of blood droplets from simulated weapon tips has been investigated experimentally and the results analysed and interpreted using established theoretical models for detached pendent drops. Droplets were produced from a series of conical nozzles, manufactured with a range of cone angles and including a set of un-bored conical tips, under conditions where the tip was either non-wetted or pre-wetted with blood. For the former, radius-limited case, detached droplet volumes were found to agree well with the predictions of both the pendent drop weight and drop shape models. For pre-wetted tips, droplet volumes were found to increase with increasing cone angle and to be independent of whether the blood flow was through an orifice at the tip or across the tip surface itself for un-bored tips. Such angle-limited, detached droplet volumes were predicted well by applying the same contact angle correction factor as for a flat surface. The maximum droplet volume, formed from a horizontal flat surface, was found to be around 150μL for horse blood. Similar theoretical approaches were then extended to droplets released under centrifugal force appropriate to swing cast-off activity and evaluated using previously published experimental data. Order of magnitude agreement for droplet diameters was fairly good but, more importantly, these were found to be proportional to the inverse of the tangential velocity thereby supporting a model where the blood droplet is released directly from a blood mass itself, such as a ligament, rather than from being pinned to some surface feature or orifice on the weapon. This work provides a sound theoretical and experimental understanding of blood droplet release under these conditions that can underpin both future research and the interpretation of blood evidence in case-work.

Feasibility of a handheld near infrared device for the qualitative analysis of bloodstains

One of the most common tasks in criminal investigation is to determine from which tissue source a biological fluid stain originates. As a result, there are many tests that are frequently used to determine if a stain is blood, semen or saliva by exploiting the properties of certain molecules present within the fluids themselves. These include chemical reagents such as the Kastle-Meyer or Acid Phosphatase tests, as well as other techniques like the use of alternative light sources. However, most of the tests currently available have some major drawbacks. In this study, a handheld near-infrared spectrometer is investigated for the specific identification of deposited bloodstains. First, a calibration was carried out by scanning over 500 positive (blood present) and negative (blood absent) samples to train several predictive models based on machine learning principles. These models were then tested on over 100 new positive and negative samples to evaluate their performance. All models tested were able to correctly classify deposited stains as blood in at least 81% of tested samples, with some models allowing for even higher classification accuracy at over 94%. This suggests that handheld near infrared devices could offer great opportunity for the rapid, low cost and non-destructive screening of body fluids at scenes of crime.

Roughness Influence on Human Blood Drop Spreading and Splashing

Bloodstain pattern analysis (BPA) is used on a daily basis as evidence in crime scene reconstruction. However, though the impact behavior of complex fluid droplets have been extensively studied, important questions still remain. We investigate the influence of surface roughness and wettability on the splashing limit of droplets of blood, a non-Newtonian colloidal fluid. Droplets of blood perpendicularly impacting different surfaces at different velocities were recorded with a high-speed camera. The recordings were analyzed as well as the surfaces characteristics in order to find an empirical solution because we found that roughness plays a major role in the threshold of the splashing/nonsplashing behavior of blood compared to the wettability. Moreover, it appears that roughness alters the deformation of the drip stains. These observations are key to characterizing features of drip stains with the impacting conditions, yielding a more complete forensic analysis in certain cases.

Three physical factors that affect the crown growth of the impact mechanism and its implications for bloodstain pattern analysis

This research uses high-speed video analysis of bloodstain impact events to investigate the influence of impact velocity, fluid depth and free-space on the characteristics of the mechanism. We focus on the changes in the crown growth over time. This work demonstrates qualitative differences in the impact mechanism under a range of impact conditions. These differences are further explained quantitatively as a function of measured crown width and height lengths over time. Fluid dynamic explanations of this growth are featured in the results and discussion. A comparison to water dynamics is reported. Our image analysis demonstrates that droplets are consistently formed at points which are different from the impactor/fluid interface and that this difference is fluid dependent. This fluid dependency demonstrates the importance of accurately modeling fluid dynamics of blood when designing and deploying blood substitutes in forensics applications.

2D Source area prediction based on physical characteristics of a regular, passive blood drip stain

Violent criminal acts are often accompanied by dynamic blood shedding events. Bloodstain pattern analysis particularly deals with estimation of the dynamic blood shedding events from the static bloodstain patterns that have been left at the scene. Of all the stain patterns present at a crime scene, drip stain patterns are common stain patterns one would expect to document at a violent crime scene. The paper documents statistically significant correlations between different physical parameters, such as fall height, total number of spines associated with each stain. Statistical significant correlation between the angle of impact and the total number of spines associated with each stain pattern has been established in this work. The paper propounds that the breadth of a regular drip stain is particularly significant in making predictions empirically as also statistically about the surface area from which blood has dripped leading to the formation of a particular drip stain. A data model has been developed using machine learning techniques to predict the range of surface radius from which blood has dripped and lead to the formation of a particular drip stain (Accuracy: 97.53%, Sensitivity=0.9481, Specificity=1).

Bloodstain Pattern Analysis: implementation of a fluid dynamic model for position determination of victims

Bloodstain Pattern Analysis is a forensic discipline in which, among others, the position of victims can be determined at crime scenes on which blood has been shed. To determine where the blood source was investigators use a straight-line approximation for the trajectory, ignoring effects of gravity and drag and thus overestimating the height of the source. We determined how accurately the location of the origin can be estimated when including gravity and drag into the trajectory reconstruction. We created eight bloodstain patterns at one meter distance from the wall. The origin's location was determined for each pattern with: the straight-line approximation, our method including gravity, and our method including both gravity and drag. The latter two methods require the volume and impact velocity of each bloodstain, which we are able to determine with a 3D scanner and advanced fluid dynamics, respectively. We conclude that by including gravity and drag in the trajectory calculation, the origin's location can be determined roughly four times more accurately than with the straight-line approximation. Our study enables investigators to determine if the victim was sitting or standing, or it might be possible to connect wounds on the body to specific patterns, which is important for crime scene reconstruction.