Refractive Index Matching for Optical Flow Investigation With High Density Stratification

Turbulent mixing in density stratified environments represents a challenging task in experimental turbulence research. When optical measurement techniques like Particle Image Velocimetry (PIV) are applied to stratified liquids, it is common practice to combine two aqueous solutions with different densities but equal refractive indices. As a result, light deflections/distortions due to the mixing of the fluids can be suppressed. While refractive image matching (RIM) was developed in the late ’70s, the limit of a 4% density ratio had yet to be reported before this work. In the present work, a methodology based on the behavior of changes in a multi component system while mixing is presented. This methodology allows RIM for solutions with higher density differences. The applicability of this methodology is experimentally demonstrated with a turbulent buoyant jet using a ternary combination of water, isopropanol and glycerol, for which an index matched density ratio of 8.6% has been achieved (Krohn et al. 2018). Measurements were conducted with a high fidelity synchronized PIV/PLIF system and the results are qualitatively compared in terms of turbulent statistics.