Measuring velocities in a suspension

An optical method for determining the velocity fields of both phases in dilute particle - laden flows is presented. The method was developed to study the sedimentation of particles through a rapidly evolving mixing layer. A laser light sheet in a vertical plane through the flow was used to illuminate both the sediment, and small, neutrally buoyant fluorescent spheres which served as markers for the fluid motion. The fluorescent light was separated from the reflected laser light by an optical low-pass filter. Both images were viewed through a glass wall of the tank and recorded on separate CCD cameras. To obtain a high correlation between subsequent images of the rapidly evolving flow field, a second pair of cameras was used, and triggered a sufficiently short time after the first pair. The velocity fields for both phases were then determined by least square matching of corresponding particle groups on subsequent frames.