Publications

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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.

The development of the unsteady pressure field on the floor of a rectangular cavity was studied at Mach 0.9 using high-frequency pressure-sensitive paint. Power spectral amplitudes at each cavity resonance exhibit a spatial distribution with a streamwise-oscillatory pattern; additional maxima and minima appear as the mode number is increased. This spatial distribution also appears in the propagation velocity of modal pressure disturbances. This behaviour was tied to the superposition of a downstream-propagating shear-layer disturbance and an upstream-propagating acoustic wave of different amplitudes and convection velocities, consistent with the classical Rossiter model. The summation of these waves generates a net downstream-travelling wave whose amplitude and phase velocity are modulated by a fixed envelope within the cavity. This travelling-wave interpretation of the Rossiter model correctly predicts the instantaneous modal pressure behaviour in the cavity. Subtle spanwise variations in the modal pressure behaviour were also observed, which could be attributed to a shift in the resonance pattern as a result of spillage effects at the edges of the finite-width cavity.

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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.

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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.

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