Publications

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Experiments, modeling and simulation were used to study the nonlinear dynamics of a jointed-structure in a shock tube. The structure was a full-span square cylinder with internal bolted connections excited by fluid loading. The width-based Reynolds number was ≈10⁵. The cylinder was exposed to an impulsive force associated with the incident shock followed by transverse loading imposed by vortex shedding. In the experiment, aerodynamic loading was characterized with high-speed pressure sensitive paint (PSP). Digital image correlation (DIC) concurrently measured the structural response. The maximum displacement occurred when the vortex shedding frequency most closely matched the structural mode of the beam associated with a rocking motion at the joint. A finite element model was developed using Abaqus, where the nonlinear contact dynamics of the joint were simulated using Coulomb friction. The PSP data loaded the model and the interaction was treated as one-way coupled. The simulations well-matched the trends observed in the experiment. Overall, the root-mean-square values of the transverse displacement agreed to within 24% of the experiment. The modeling showed rocking about the joint during vortex shedding was critical to the nonlinear damping and energy dissipation in the structure. We conclude this campaign highlights the importance of jointed-connections to energy dissipation in structures under aerodynamic loading.

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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.

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Conventional particle image velocimetry (PIV) configurations require a minimum of two optical access ports, inherently restricting the technique to a limited class of flows. Here, the development and application of a novel method of backscattered time-gated PIV requiring a single-optical-access port is described along with preliminary results. The light backscattered from a seeded flow is imaged over a narrow optical depth selected by an optical Kerr effect (OKE) time gate. The picosecond duration of the OKE time gate essentially replicates the width of the laser sheet of conventional PIV by limiting detected photons to a narrow time-of-flight within the flow. Thus, scattering noise from outside the measurement volume is eliminated. This PIV via the optical time-of-flight sectioning technique can be useful in systems with limited optical access and in flows near walls or other scattering surfaces.

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.

Backscatter Particle Image Velocimetry via Optical Time-of-flight Sectioning (PIVOTS) is a novel method of performing PIV in situations where conventional PIV presents difficulties. The PIVOTS technique is introduced along with recent applications and results.

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.

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.

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