Investigating Inertial Spheres In a Flow Using Refractive Index-Matched Tomographic PIV*

A series of refractive index-matched, time-resolved tomographic PIV experiments are performed to study the dynamics of inertial spheres in a flow. Using a 62% aqueous sodium iodide solution matching the refractive index of acrylic spheres, we achieve unobstructed optical access, enabling simultaneous recording of 3D flow fields and fluid-structure interactions. Two cases are presented: (1) spheres in a round channel with abrupt area changes and (2) spheres rising in quiescent flow at terminal velocity.

Four high-speed cameras record the flow field around the sphere. 3D particle tracking is used to reconstruct the transient flow field, turbulence, and vortical structures. Data from refractive index matching allows measurements of 3D time-resolved velocity and sphere trajectory. For the first case, we used 1/4” to 1/2” spheres allowed to rise freely in a tank, and for the pipe case freely, we used 1/8” and 1/16” spheres embedded in a flow of up to 5m/s.   

Despite having different mechanisms and flow characteristics, both scenarios benefit from refractive index matching in order to characterize flow and motion dynamics. These experiments offer valuable insights into the transient flow evolution and forces acting on the spheres, thereby elucidating their behavior. The talk will focuses on the experimental methodologies, challenges, and data analysis techniques for refractive index-matched multi-phase flow studies, with a focus on the complex fluid-structure interaction that governs sphere motion.