The understanding and predictive modeling of explosive blasts require advanced experimental diagnostics that can provide information on local state variables with high spatiotemporal resolution. Current datasets are predominantly based on idealized spherically symmetric explosive charges and point-probe measurements, although practical charges typically involve multidimensional spatial structures and complex shock-flow interactions. This work introduces megahertz-rate background-oriented schlieren tomography to resolve transient, three-dimensional density fields, as found in an explosive blast, without symmetry assumptions. A numerical evaluation is used to quantify the sources of error and optimize the reconstruction parameters for shock fields. Average errors are ∼ 3 % in the synthetic environment, where the accuracy is limited by the deflection sensing algorithm. The approach was experimentally demonstrated on two different commercial blast charges (Mach ∼ 1.2 and ∼ 1.7 ) with both spherical and multi-shock structures. Overpressure measurements were conducted using shock-front tracking to provide a baseline for assessing the reconstructed densities. The experimental reconstructions of the primary blast fronts were within 9% of the expected peak values. The megahertz time resolution and quantitative reconstruction without symmetry assumptions were accomplished using a single high-speed camera and light source, enabling the visualization of multi-shock structures with a relatively simple arrangement. Future developments in illumination, imaging, and analysis to improve the accuracy in extreme environments are discussed.

Falling particle curtains are important in many engineering applications, including receivers for concentrating solar power facilities. During the formation of such a curtain, we observe a multiphase analog of Rayleigh-Taylor instability (RTI). It was originally described in 2011 for a situation when air sparsely seeded with glycol droplets was placed above a volume of unseeded air, producing an unstably stratified average density distribution that was characterized by an effective Atwood number 0.03. In that case, the evolution of the instability was indistinguishable from single-phase RTI with the same Atwood number, as the presence of the droplets largely acted as an additional contribution to the mean density of the gaseous medium. Here, we present experiments where the volume (and mass) fraction of the seeding particles in gas is considerably higher, and the gravity-driven flow is dominated by the particle movement. In this case, the evolution of the observed instability appears significantly different.

The formation of a gravity-driven falling particle curtain is important for many problems, including solar tower particle receivers and setting the correct initial conditions for modeling shock interaction with multiphase media. One important characteristic of the curtain is the time history of its fractal dimension that characterizes the evolutionary growth of perturbations along the curtain's extent. For multiphase flows, fractal dimension can be used to help predict the types of instabilities that will occur within the flow. Our experiment aimed to establish the transient and steady-state fractal dimension of a dense particle curtain containing particles with a density of 1.4416 gm/cm3 and nominal diameter ranging from 30 to 50 microns. High-speed video of the curtain was captured and analyzed. This data from this experiment, besides providing insights into the relevant physical processes, will be used to validate numerical models for multiphase flows.