Robust optical tracking of individual ejecta particles in hypervelocity impact experiments

Abstract

New insights into the kinematics of ejecta clouds and the dynamics of crater formation are gained from the introduction of an approach to track individual particles ejected from a horizontal hypervelocity impact of a 2 mm aluminum sphere at 6.3km s−1 into vertically aligned Carrara marble. Particle trajectories are determined with 500 ns temporal resolution inside a 1–2 mm thick laser light sheet illuminating a single plane within the ejecta plume. In contrast to optical flow analysis, the methodology presented here enables us to track individual particles instead of relying on field-averaged information. This is realized by correlating particles not via their geometric shape but through their trajectory directly. It robustly identifies even partially obscured or strongly tumbling particles, allowing for a comprehensive physical description of the highly dynamical excavation process based on the precise determination of position, time, and ejection velocity of each individual particle. Specifically, we find ejecta particles launched in a short window of about 0–25 μs after impact and up to a radial distance of 10 mm from the impact location. During this time interval, the transient crater radius grows from 2 ±1 mm to 6 ± 2 mm. Velocities between 70 m s−1 and 1 km s−1 are observed and reveal a substantial steepening of the ejecta curtain within 15 μs after the impact. We additionally determine the particle size and find a μ-parameter of 0.6 for Carrara marble which is consistent with theoretical predictions for nonporous materials

Experimental impact cratering: A summary of the major results of the MEMIN research unit

“…By investigating the impact of a 2 mm aluminum sphere at 6.3 km s −1 into Carrara marble, Gulde et al. () found that ejecta particles were launched in a short time window of about 0–25 μs and up to a radial distance of 10 mm from the impact location. During this time interval, the transient crater radius grew from 2 ± 1 to 6 ± 2 mm.…”

“…Based on the experimental laser light sheet (LLS) technique as employed by Cintala et al (1999), we developed and implemented a new methodology to robustly track single ejecta particles with very high accuracy (Gulde et al 2018). In particular, instead of relying on the challenging correlation of particles via their geometric shape, the novel methodology identifies particles by means of their trajectory.…”

“…In the present work, a single plane within the ejecta plume was illuminated using a time resolution of 500 ns as well as a spatial resolution of 150 lm. By investigating the impact of a 2 mm aluminum sphere at 6.3 km s À1 into Carrara marble, Gulde et al (2018) found that ejecta particles were launched in a short time window of about 0-25 ls and up to a radial distance of 10 mm from the impact location. During this time interval, the transient crater radius grew from 2 AE 1 to 6 AE 2 mm.…”

Effect of target properties and impact velocity on ejection dynamics and ejecta deposition

“…; Gulde et al. ). Some studies are able to track individual particles or give at least a particle size distribution for their total detected ejected material (Buhl et al.…”

“…; Gulde et al. ). These experiments provide empirical relationships between ejection speed and angle and launch position within the crater for a number of different target materials (Housen et al.…”