Towards quantifying effects of refractive index mismatch on PIV results

Refractive index matched particle image velocimetry (RIM-PIV) is an elegant method to measure velocity fields around complex geometries and in difficultly accessible areas. The accuracy of RIM-PIV is strongly affected by the quality of refractive index matching. Measurement errors can be induced by refractive index mismatch at optical interfaces (transparent objects or density variations) in or at the boundaries of the flow. In this paper a method to quantify the influence of refractive index mismatch on PIV results is presented. A generic experiment is designed to ensure known flow conditions for varying refractive indices. This setup allows for evaluation of the refractive index effects on PIV results and the systematical analyses of refractive index mismatch. The variation of the magnification factor caused by the reflection at the wall of the cuvette is determined. Furthermore particle mismapping and particle deformations induced by refractions at non-perpendicular optical interfaces within the fluid are determined. The optical interfaces are formed by transparent objects or large density variations. The magnification factor increases for larger differences between the refractive index of the fluid inside the tank and that of the ambient air outside. Particle mismapping and particle deformation become more important for a larger refractive index mismatch with respect to optical interfaces within the flow.