Scalar Velocimetry using Cross-Scanning LIF

Volumetric measurements of three-dimensional velocity fields are most often accomplished by seeding the liquid with small solid particles and then follow them around using multiple cameras. The resolution of this measurement is limited by the size and number of these particles. A second approach is to stir fluorescent dye into the liquid and infer the velocity field from the changes in the volumetric fluorescent field with time, by inverting the advection-diffusion equation. To track changes in the volumetric dye-field, one images its concentration plane-by-plane by illuminating it with a rapidly scanning laser-sheet, each video-frame measuring the intensity in one plane I(x,y). In practice, one can only image 10-100 adjacent planes, to capture the time-variations of the field. The drawback in this approach is the reduced resolution in the scanning z-direction. While you have 1000x1000 pixels in each (x,y)-plane, there are only of the order of 50 planes in the depth. Our approach overcomes this by using two perpendicular scanning directions and two high-speed cameras to record the fluorescent intensity field in (x,y) and (x,z) directions respectively. This will allow much denser velocity measurements in a larger volume. However, the data acquisition and analysis are more complicated than for the single scan direction. Below we describe how this is being implemented, along with preliminary results for a turbulent jet.