Ultrafast Optical Kerr Effect Gated Imaging of Dynamic Bubbly Flows.

Despite the pervasiveness of bubbly flows, taking reliable measurements in these multiphase conditions remains onerous, especially with high void fractions and turbulence. Bubbles are excellent photon scatterers, and when present in high void fractions with small sizes, they increase the flow's optical thickness. This increase leads to greater attenuation, blurring, occlusion, and loss of information, limiting the use of available measurement techniques.

Ultrafast optical measurements offer new methods for probing these optically thick, bubbly flows. Through Optical Kerr Effect (OKE) Gating, images have been acquired with exposure times on the order of 1 picosecond. With such short exposures, the destructive high-order scattering modes are negated, preserving image quality and information, increasing confidence in measurements. Furthermore, by gating images in femtosecond intervals, the lowest-order scattering modes can be imaged in isolation, temporally decomposing the bubbly scenes. Quantifying the time between scattering modes from this decomposition provides a new method of determining bubble size, even when bubbles are partially occluded.

By applying this ultrafast method to a newly constructed vertical bubble column, dynamic bubble plumes have been temporally decomposed for the first time. Designed for controlled conditions, with optical thicknesses ranging from 1 to 10, and in both quiescent and transitional conditions, we demonstrate the vast potential of OKE imaging to enhance collective understanding of multiphase flows.