Simple distinction of similar-looking inertial particles and fluid tracers on camera images

Dispersed two phase flows cover a wide range of natural phenomenon and technological applications. When studying such complex systems, having access to the velocities of both phases is necessary to fully understand their dynamics. In experiments, inertial particles have to be distinguished from tracers which image the movement of the carrier fluid. For this, most studies rely on differences in size or brightness. This article presents a method for separating particles and tracers if those are of similar size and brightness. It employs fluorescent tracers and a two camera acquisition system. One camera records both particles and tracers, while the second camera only records the tracers by using an optical filter for the fluorescent light. A straightforward image processing algorithm then identifies and removes the tracers from the first image. This allows for the velocities and positions of the particles to be measured in conjunction with the velocity field of the carrying phase. A series of tests are performed on the method to demonstrate its reliability. In addition to ensuring that the method functions in a satisfactory manner, these tests give guidelines on how to use the method correctly. To illustrate an application of this method, measurement results of ceramic particles settling in still water are presented.