Free-field schlieren measurements of laser-induced breakdown shock propagation

Abstract

Measurements of free-field shock propagation are fundamental to understanding source characteristics, and for transfer functions through complex environments. Probe interference and undesirable high-frequency response plague typical approaches with acoustic microphones, which are also limited to resolving the pressure field at a single position. Measurements made with optical methods do not have such drawbacks, and schlieren measurements are particularly well suited to measuring both the spatial and temporal evolution of nonlinear pulse propagation. A laser induced breakdown source is used to generate spherically symmetric weak shockwaves. A z-type schlieren instrument and high-speed framing camera are used to measure schlieren images of passing shock waves. Quantitative measurements relate schlieren image intensity values to light refraction angles, and by the inverse Abel transform the density (and pressure) fields induced by the shockwave are recovered. Comparisons of analytical predictions, assuming spherically symmetric and inviscid propagation, with experimentally measured shockwaves are presented.