Importance of Refractive Index Matching of Fluids for PIV and PLIF Measurements in Buoyant Jets

Abstract

From violent volcanic eruptions to the rising clouds, buoyant jets and plumes are omnipresent in nature at wide range of scales (Woods in Annu Rev Fluid Mech 42:391–412, 2010 [15]). The mixing and growth dynamics of a plume is important in fundamental understanding of turbulence as well as in modeling this complex fluid mechanics phenomenon. This necessitates simultaneous measurements of velocity and density of the mixing fluids. Particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) are the common methods employed for velocity and density measurement. The usual laboratory fluids, salt solution and pure water, however, introduce optical distortions caused by refractive index mismatch in the two fluids. Although widely known, issues caused by refractive index mismatch are rarely measured and quantified in the literature. Here, we present the results of simultaneous time-resolved measurement of velocity and density in an axisymmetric turbulent plume using 2D-2C PIV and PLIF, respectively. The experiments are performed with and without refractive index matched fluids to compare the uncertainties due to optical inhomogeneities caused by the mixing of two different density fluids. Both mean and turbulence quantities are compared. It is observed that refractive index mismatch adds noise to the measurement, resulting in over prediction of the turbulence quantities. As the distance from jet exit increases the effect of the mismatch decreases.