A new free-piston driven shock tube is being constructed at Sandia National Laboratories for generating extreme aerodynamic environments relevant for the study of reacting particle dispersal. The high-temperature shock tube (HST) is designed to reach post-incident shock temperatures more than 2000 K, starting from a driven section initially at ambient temperature and pressure. A design study is presented on different driver methods, leading to the selection of a free-piston driver. The tuning and performance of this driver is analyzed using the Hornung one-dimensional model and the L1d quasi-one-dimensional flow solver. The final mechanical design is shown and compared to the X2 free-piston facility. Construction was completed in mid-2018, and an initial analysis of facility performance from the first shots is presented.
Liquid metal breakup processes are important for understanding a variety of physical phenomena including metal powder formation, thermal spray coatings, fragmentation in explosive detonations and metalized propellant combustion. Since the breakup behaviors of liquid metals are not well studied, we experimentally investigate the roles of higher density and fast elastic surface oxide formation on breakup morphology and droplet characteristics. This work compares the column breakup of water with Galinstan, a room-temperature eutectic liquid metal alloy of gallium, indium and tin. A shock tube is used to generate a step change in convective velocity and back-lit imaging is used to classify morphologies for Weber numbers up to 250. Digital in-line holography (DIH) is then used to quantitatively capture droplet size, velocity and three-dimensional position information. Differences in geometry between canonical spherical drops and the liquid columns utilized in this paper are likely responsible for observations of earlier transition Weber numbers and uni-modal droplet volume distributions. Scaling laws indicate that Galinstan and water share similar droplet size-velocity trends and root-normal volume probability distributions. However, measurements indicate that Galinstan breakup occurs earlier in non-dimensional time and produces more non-spherical droplets due to fast oxide formation.
Wagner, Justin W.; Demauro, Edwward P.; Casper, Katya M.; Beresh, Steven J.; Pruett, Brian O.
Here, the impulsive start of a circular cylinder in a shock tube was characterized with time-resolved particle image velocimetry measurements (TR-PIV) using a pulse-burst laser. Three Reynolds numbers Re of 1.07 × 105, 1.63 × 105 and 2.46 × 105 were studied adding insight into the transient process in the vicinity of the drag crisis. One symmetric vortex pair was shed from the cylinder at the lower Reynolds number prior to the wake going asymmetric in a fashion analogous to studies at lower Re. At Re ≥ 1.63 × 105, two or more symmetric vortex pairs occurred prior to asymmetry and the eventual transition to a von Kármán vortex street. The non-dimensional rise time for vortex shedding to begin, as quantified by wavelet analysis, was found to be lower at the two higher Re. Finally, the study indicates a transformation in the impulsive wake to occur at Re near the critical regime and may serve as a benchmark for high Re numerical solutions.
Simultaneous pressure sensitive paint (PSP) and stereo digital image correlation (DIC) measurements on a jointed beam structure are presented. Tests are conducted in a shock tube, providing an impulsive starting condition followed by approximately uniform high-speed flow conditions for 5.0 msec. The unsteady pressure loading generated by shock waves and vortex shedding results in the excitation of various structural modes in the beam. The combined data characterizes the structural loading input (pressure) and the resulting structural behavior output (deformation). Time-series filtering is used to remove external bias errors such as shock tube motion, and proper orthogonal decomposition (POD) is used to extract mode shapes from the deformation data. This demonstrates the utility of using fast-response PSP together with stereo digital image correlation (DIC), which provides a valuable capability for validating structural dynamics simulations.
The spanwise variation of resonance dynamics in the Mach 0.94 flow over a finite-span cavity of variable length-to-width ratio was explored using time-resolved particle image velocimetry (TR-PIV) in a planform plane above the cavity and time-resolved pressure sensitive paint (TR-PSP) on the floor and adjacent exterior surface. The TR-PIV showed a significant variation in resonant fluctuations to occur across the span of the cavity, which appears to arise from spillage vortices stemming from finite width effects. Thus, the spanwise variation was a strong function of the cavity aspect ratio and was only weakly dependent on the cavity mode number. Modal streamwise velocity fluctuations in the spillage vortices showed large peaks at modes one through three, indicating that resonance dynamics, and not just broadband turbulence effects, are prevalent near the sidewalls. Large peaks in modal pressures were also present on the walls just outside of the cavity. Interestingly, prominent peaks at the mode frequencies were observed in the spanwise velocity spectra as well. These peaks were strongest near the cavity sidewalls suggesting a coupling between the resonance mechanism and the spillage vortices.
Time-resolved tomographic particle image velocimetry measurements of the vortex organization in cylinder wakes at Reynolds numbers from 8,200 to 53,000 is presented. Flow is generated in a shock tube, providing an impulsive starting condition followed by approximately uniform flow conditions for 8.0 msec. A pulse-burst laser and four high-speed cameras enable time-resolved measurements at 10 kHz for the entire test duration; approximately 90 volumetric velocity fields are acquired for each shot. The high energy provided by the pulse burst laser allows for a large measurement volume exceeding most other time-resolved experiments in air. The work demonstrates the feasibility of time-resolved tomographic PIV of large volumes in high-speed air flows, and its utility for maximizing data acquisition in a transient facility. The latter is particularly useful for quantifying the behavior of impulsive flows. A single-image self-calibration procedure is demonstrated to accommodate facility vibrations, and an uncertainty analysis of the measurement is performed. The initial wake development and transition to regular Kármán shedding in the cylinder wake is analyzed in terms of the vortex topology and associated spatial scales as a function of time.