Application of multiple exposure digital in-line holography to particle tracking in a Bénard–von Kármán vortex flow

Digital in-line holography is applied to studying the trajectories of individual water droplets in airflow. In order to track the particles, multiple exposure holography is performed using a modulated laser diode emitting at the wavelength of 635 nm and a lens-less CCD camera. This method leads to an accuracy better than 100 µm on the axial location. A study of the signal-to-noise ratio of such holograms shows that the number of exposures must be limited. Preliminary tests of this method are carried out in a Bénard–von Kármán street first characterized by laser Doppler velocimetry and hot wire anemometry. An example of a trajectory of a water droplet obtained in this flow at Reynolds number Re = 63 and Strouhal number St = 0.13 shows that digital holography is a promising method to extract the trajectories of droplets in laminar or turbulent flows.