Dynamic Compression and Energy Concentration in Fluids Using Shock Waves

QC261005

Abstract

Shock waves provide an effective means of dynamically compressing fluids and generating intense, localized thermodynamic and mechanical states involving high temperatures and pressures. Such fluid states are relevant to applications including inertial confinement fusion, shock-wave lithotripsy, targeted drug delivery, materials research, and studies of fuel ignition and detonation. This thesis investigates two related flow scenarios in which shock dynamics and shock-induced interactions govern strong compression, energy concentration, and the resulting flow evolution in fluids.

The first part of the thesis examines shock convergence in gases, where a curved shock strengthens as it propagates toward its focus, producing high temperatures upon focusing and subsequent reflection. The experiments are performed in a shock tube, which enables controlled generation and study of the converging shocks. High-speed shadowgraphy, optical emission spectroscopy, and photometry are used to measure the shock trajectory and focal temperature, while numerical simulations employing ideal- and real-gas equations of state complement the experiments. The results show that the convergence process is influenced by the initial shock strength, upstream nonuniformities, and real-gas effects, which together govern the focal temperatures that can realistically be achieved. 

A generalized scaling for the focal temperature is developed, incorporating the effects of shock symmetry, convergence ratio, and initial gas pressure. Demonstrated for argon, the scaling enables prediction of the focal temperature from accessible initial parameters and provides a basis for inverse design, allowing the conditions required to reach a target temperature to be estimated. Subsequent to focusing, the shock reflects and propagates upstream, where it interacts with the incoming boundary layer. These interactions are found to depend strongly on the specific heat ratio of the gas and can lead to bifurcation of the reflected shock. Such non-ideal flow effects alter the thermodynamic state behind the reflected shock, which is important to characterize in studies of chemical kinetics and detonation employing shock-focusing geometries.

The second part of the thesis investigates shock--bubble interactions in a water--gelatine solution used as a water surrogate. These studies address how shocks interacting with light inhomogeneities in liquids can generate localized pressure peaks, high-speed jets, and coherent vortical structures. Experiments in the water--air limit remain relatively scarce because generating strong shocks in water is challenging, the interactions involve gigapascal-scale pressures, and strong density gradients limit the use of conventional optical visualization techniques. Planar Mach 1.4 and Mach 1.5 blast waves are generated by exploding copper foils using a pulsed-power driver, and their interaction with cylindrical air bubbles is visualized using X-ray phase-contrast imaging at a synchrotron. The measurements resolve the interaction from incident-wave impact through jet formation, water-hammer shock emission, remnant-lobe collapse, and late-time vortex-pair formation. Compared with the corresponding idealized step-like shock interaction, the finite-width blast-wave driver, combined with the cylindrical geometry of the bubble, produces thinner and slower jets, weaker water-hammer shocks, larger remnant lobes, and a sequence of events that is moreclearly separated in space and time. This separation allows the mechanism responsible for a second upstream pressure peak and the subsequent multi-stage vorticity evolution to be identified.

Finally, the effect of shock curvature is investigated through the interaction of cylindrical and semi-cylindrical converging shocks with a coaxial cylindrical air bubble. In the cylindrical configuration, the bubble undergoes inward collapse accompanied by Richtmyer--Meshkov instability spikes, while the transmitted shock focuses within the bubble and reaches high Mach numbers, leading to elevated temperatures in the bubble gas. In the semi-cylindrical configuration, the asymmetric interaction concentrates momentum into a single supersonic jet and gives rise to localized high-pressure regions, internal Mach reflection, and coherent vortical structures.

Overall, the thesis shows how shock waves can generate intense thermodynamic and mechanical states through geometric convergence and interactions with gas--liquid interfaces. It further advances understanding of how the interactions and flow features that develop during these phenomena influence the resulting pressure and temperature states and the subsequent evolution of the flow. This understanding is important for interpreting these phenomena, both to better harness shock-driven compression and energy concentration and to anticipate and limit their potentially damaging effects in natural and engineered systems.

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