Using laser-induced fluorescence to quantify velocity in a turbulent jet

Turbulent jets have been widely studied due to their ubiquitous presence in natural and industrial systems. As such, characteristics of steady round jets, e.g., time-averaged profiles showing Gaussian-like distributions of velocity and passive scalars, along with self-similarity, have been established via laboratory and numerical studies. This study aims to characterize a starting jet using indirect measurements, without a detailed understanding of the source conditions. We present a methodology for determining jet velocity using laser-induced fluorescence - a non-invasive laboratory method typically intended for mass concentration measurements. We estimate the front and centerline velocities using the variation in image intensity (i.e. brightness) at each pixel. Fluorescent-dyed water is pumped through a nozzle into quiescent water to create the jet. A laser light sheet passes through the jet centerline, fluorescing the dye; a camera positioned outside of the tank records as the jet flows from its source. Front velocity is approximated by tracking the movement of the jet, while the centerline velocity is calculated using cross-correlations of the intensity signals. We find that spectra of the time-history of intensity at a point follow a -5/3 slope, indicating information about turbulence can be extracted from image intensities. Simultaneous testing via particle image velocimetry is underway to validate our approach.