Tracking blood drops ejected from the nasal cavity during violent assault

Expirated blood is blood released from the mouth or nose which leaves characteristic stains at crime scenes which may convey valuable information about the disposition of assailant and victim. Little work has been done to determine the typical velocities and trajectories of the droplets which make such patterns. Stereoscopic Particle Image Velocimetry (SPIV) and a model of the human upper airway were used to measure the velocity field near the nostrils during exhalation. The trajectories of blood drops emerging from the nose were computed with a numerical model. For an expiration flow rate of 32 l/min in vivo there is a maximum exit velocity from the nostril of approximately 4 m/s. From a height of 1.7 m blood drops between 0.5 mm and 2 mm reach terminal velocity. Their exit velocity at the nostril does not affect impact velocity, but it does influence impact location. Drop size affects both impact location and impact velocity. Compared to the 2 mm drop, the 0.5 mm drop had a reduced impact velocity, but its impact location in the ground plane was further from the nostril exit. Understanding the physics of expirated blood flight allows better interpretation of expirated stains on different surfaces at crime scenes.