17 th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014 Measurement of Ejecta from Hypervelocity Impacts with a Generalized High-Speed Two-Frame 3D Hybrid Particle Tracking Velocimetry Method Brendan Hermalyn 1,2* , James T. Heineck 2 , Edward T. Schairer 2 , and Peter H. Schultz 1 1: Hawaii Space Flight Laboratory, University of Hawaii, Honolulu, HI 2: NASA Ames Research Center, Moffett Field, CA 3: Department of Geological Sciences, Brown University, Providence, RI correspondent author: hermalyn@hawaii.edu Abstract Impacts on planetary surfaces occur almost exclusively at oblique angles. Generally, the shape of the final crater retains circularity for all but the lowest angle impacts, and vertical incidence is often assumed to greatly simplify the dynamics. The ejecta distribution deposited on the surface, however, belies the obliquity of the impact. These effects are especially apparent in oblique impacts because the zone of coupling is extended laterally along the impact trajectory (rather than in the downward direction alone, as in the case for vertical impacts). Particle tracking velocimetry (PTV) and particle imaging velocimetry (PIV) are full-field, non-intrusive imaging techniques that allow determination of the instantaneous velocity of a flow-field. While a significant amount of research has gone into these techniques and their extension to the three dimensions required for many fluidic measurements, past work has set requirements or introduced errors that are not conducive to many high-speed tests (e.g., wind tunnels). Here we describe an implementation of a generalized two-frame hybrid laser particle tracking velocimetry system to measure, with a high degree of accuracy, the three-dimensional location and velocities of particles from simultaneous images taken from multiple arbitrary (e.g., non collinear) view points applied for granular flow driven from a hypervelocity impact. These data provide a time-resolved view of the ejecta evolution for the first time, and allow development of an analytical description of the velocity distribu- tion as a function of time and azimuth. 1 Introduction and Background A hypervelocity impact (e.g., above the speed of sound in the target) sets up a shock-driven flow of material. As the expansion wave passes through the target behind the shock, some of this material (termed “ejecta”) is launched out of the growing crater, and travels ballistically above the target surface (see Fig. 1). It is eventually emplaced some distance away from the impact point (unless the material is traveling at sufficient velocity to encounter the vacuum chamber walls). At laboratory scales, the crater is formed in less than 100msec for impacts into granular particulate targets such as sand. The ejecta velocities are controlled by the initial conditions of the impact; e.g., the size, velocity, and density of the impactor, and the properties of the target. In addition, virtually all impacts occur at some degree of obliquity to the target surface, with a probability distribution that yields a maximum likelihood of 45 ◦ (Gilbert, 1893; Shoemaker, 1962). Measurement of the ejecta velocity distribution for these oblique impacts is considerably more difficult due to the spatially and temporally varying ejecta velocities and launch angle that result from asymmetries in the shock (Dahl and Schultz, 2001), yet is vitally important in understanding the appearance of planetary surfaces, interpreting impact mission data (such as NASA’s LCROSS and Deep Impact missions), and in the constraint of shielding requirements for human habitation of the Moon and beyond. Particle tracking velocimetry (PTV) and particle imaging velocimetry (PIV) are full-field, non-intrusive imaging techniques that allow determination of the instantaneous velocity of a flow-field. In the most general terms, the methodology of the techniques is quite intuitive, dating back to at least Leonardo DaVinci: by tracking the location of particles (including those suspended in a fluid), the flow-field can be visualized and measured. PTV techniques are the purest form of this procedure- literally tracking particles in a Lagrangian fashion over time. The PIV methodology, however, is slightly convoluted in that small interrogation areas (comprised, ideally, of ∼9 particles each) are used to determine the flow field at Eulerian locations in the flow. In practice, common implementations of these techniques include a pulsed illumination source (typically a laser) synchronized to one or more cameras. The light source is optically spread to a sheet or volume of some discrete thickness, and serves as the sole illumination of the particles in the flow field. The camera(s) record images of the particles at multiple instances as they are displaced over time, thereby allowing computation of the velocity after spatial calibration of the images. Both techniques have a rich history and are now routinely used in experimental studies. While fluidic studies comprise the vast majority of implementations, experiments - 1-