A Refractive Index Matched Experimental Investigation Of The Dynamics Of Inertial Spheres Across Round Pipes And Buoyancy-Driven Flows

Abstract

A series of refractive index-matched time-resolved tomographic PIV experiments are performed to study the dynamics of motion of solid spheres in flow. The refractive index matching of acrylic spheres with the working fluid facilitates unobstructed optical access around the spheres, enabling the simultaneous estimation of both the 3D flow field and the fluid-structure interactions governing the kinematics of the spheres. Additionally, using normal tracers as well as fluorescent tracers for PIV helps identify the most suitable approach for multiphase index-matched PIV studies. Two different test cases are presented: (1) solid spheres moving in a round channel with an abrupt area expansion and, (2) solid spheres rising in a quiescent flow at terminal velocity. Despite both falling under the category of solid-liquid flows, these cases exhibit entirely different mechanisms and flow characteristics. Nonetheless, refractive index matching provides a valuable tool that can be used to characterize the flow as well as the motion dynamics of the free spherical particles in a liquid. These time-resolved 3D experiments offer a comprehensive insight into the transient evolution of the flow fields and the pressure forces on the spheres, providing a complete picture of the dynamic process. The emphasis of the present paper lies in the experimental methodologies, challenges and data analysis techniques used for refractive index-matched multiphase flow studies.