The interaction between cylindrically converging shock waves (SWs) in a water–gelatine solution and a coaxial cylindrical air bubble is studied experimentally and numerically. Two configurations are considered: (i) an azimuthally symmetric, cylindrically converging SW of Mach 1.35 impinging on a coaxial cylindrical bubble, and (ii) a semicylindrical converging SW of Mach 1.45 (corresponding to half of the cylindrical front), interacting with the same target. Shock waves are generated by exploding wire arrays driven by a high-voltage pulsed power system at beamline ID19 of the European Synchrotron Radiation Facility, delivering currents up to 130kA with rise times of 0.35 and 0.55 µs to the cylindrical and semicylindrical wire loads, respectively. X-ray radiography is conducted at a pulse repetition rate of 5.68 MHz using two synchronised high-speed cameras. Numerical hydrodynamic simulations are performed using a compressible multiphase Navier–Stokes solver. A Gilmore-type model for compressible cylindrical bubble pulsation provides an independent analytical estimate of the interface evolution. In the cylindrical SW configuration, the bubble collapse in experiments exhibits Richtmyer–Meshkov instability spikes. The cylindrically converging shock is analysed with Guderley’s solution and Whitham’s approximation using a real-gas equation of state, predicting Mach 14.1 near the focus. In the semicylindrical configuration, momentum focuses into a single supersonic jet with a speed of 885 ± 30 m s−1, producing localised high-pressure regions, coherent vortices and complex internal Mach reflections. Experiments, simulations and theory are consistent in collapse time, interface motion and overall flow dynamics.

Convergence of cylindrical shock in argon is studied both experimentally and numerically. Shock tube experiments are conducted, where a planar shock is first transformed to a cylindrical shape and then converged to its focal axis. Numerical simulations of the converging shock using equations of state for an ideal gas and a real gas (SESAME 5173 model) are conducted and compared. High temporal resolution data of cylindrical shock convergence is presented. When comparing the trajectories of the converging shock of initial shock Mach number (M-S) of 4.63, the convergence exponent (alpha) in experiments is found to be 0.833. This alpha value in experiments is higher than the value obtained from computations with argon treated as an ideal gas but agrees well with the real gas computations. It is revealed that the form of convergence varies with different M-S. An asymptotic approach of alpha toward the self-similar solution for high M-S is attributed to an earlier transition of shock motion to self-similarity, while a significantly higher alpha observed at lower M-S is attributed to the negative influence of upstream nonuniformities and weaker initiation of the shock. It is found that even before the shock reflection, real gas effects are significant enough to affect the convergence of the shock and limit the extreme conditions predicted by the ideal gas computations. For an M-S of 4.63, the maximum temperature reached is 9250 K before reflection, leading to 0.12% of the argon gas undergoing the first stage of ionization.

The ability to control the structure of the wood-pulp fiber cell wall is an attractive means to obtain increased accessibility to the fiber interior, providing routes for functionalization of the fibers that support further processing and novel material concepts, e.g. improved degree of polymerization, nanofiltration as demonstrated in previous studies. It has been proposed that dynamic compression and decompression of the cellulose pulp fibers in the wet state make it possible to modify the cell wall significantly. We hypothesize that hydrostatic pressure exerted on fibers fully submerged in water will increase the accessibility of the fiber wall by penetrating the fiber through weak spots in the cell wall. To pursue this, we have developed an experimental facility that can subject wet cellulose pulp samples to a pressure pulse -10 ms long and with a peak pressure of -300 MPa. The experiment is thus specifically designed to elucidate the effect of a rapid high-pressure pulse passing through the cellulose sample and enables studies of changes in structural properties over different size ranges. Different characterization techniques, including Scanning electron microscopy, X-ray diffraction, and wide- and small-angle X-ray scattering, have been used to evaluate the material exposed to pulsed pressure. The mechanism of pressure build-up is estimated computationally to complement the results. Key findings from the experiments consider a decrease in crystallinity and changes in the surface morphology of the cellulose sample.

A supersonic jet of Mach number M = 4.5 in air is produced experimentally at the apex of a miniature 150 x 50 x 5 mm converging section with a 2 x 5 mm opening by the principle of blast wave amplification through focusing. An initial plane blast wave of M = 2.4 in the convergence section is generated by the exploding wire technique. The profile of the convergence section is specially tailored to smoothly transform a plane blast wave into a perfectly cylindrical arc, imploding at the apex of the section. The cylindrical form of the imploding shock delivers maximum shock amplification in the two-dimensional test section and maximum subsequent jet flow velocity behind the shock front. Blast wave propagation in the convergence chamber as well as jet generation through a 2 mm opening at the apex into the adjacent exhaust chamber is optically captured by a high-speed camera using the shadowgraph method. Visualizing the flow provided a distinct advantage not only for obtaining detailed information on the flow characteristics but also for validating the numerical scheme which further enhanced the analysis. Experimental images together with the numerical analysis deliver detailed information on the blast wave propagation and focusing as well as subsequent jet initiation and development. One of the main advantages of the described method apart from being simple and robust is the effective focusing of low initial input energy levels of just around 500 Joules, resulting in production of supersonic jets in a small confined chamber.

The unstable evolution of an elongated elliptically shaped inhomogeneity that is embedded in ambient air and aligned both normal and at an angle to an incident plane blast wave of impact Mach number 2.15 is investigated both experimentally and numerically. The elliptic inhomogeneities and the blast waves are generated using gas heating and exploding wire technique and their interaction is captured optically using shadowgraph method. While two symmetric counter-rotating vortices due to Richtmyer-Meshkov instability are observed for the straight interaction, the formation of a train of vortices similar to Kelvin-Helmholtz instability, introducing asymmetry into the flow field, are observed for an inclined interaction. During the early phase of the interaction process in the straight case, the growth of the counter-rotating vortices (based on the sequence of images obtained from the high-speed camera) and circulation (calculated with the aid of numerical data) are found to be linear in both space and time. Moreover, the normalized circulation is independent of the inhomogeneity density and the ellipse thickness, enabling the formulation of a unique linear fit equation. Conversely, the circulation for an inclined case follows a quadratic function, with each vortex in the train estimated to move with a different velocity directly related to its size at that instant. Two factors influencing the quadratic nature are identified: the reduction in strength of the transmitted shock thereby generating vortices with reduced vorticity, along with the gradual loss of vorticity of the earlier-generated vortices.

An alternate mechanism explaining the shock broadening and splitting effects observed during its propagation through an elongated region with transverse thermal inhomogeneity is described. The shock wave is generated by exploding wire technique and its propagation is captured optically using shadowgraph method. Visualizing the flow provided distinct advantage not only for obtaining detailed information on the propagation characteristics but also for validating the numerical scheme used in the analysis. Three physical features namely shock jump, precursor region and vorticity induced flow, are identified to contribute to the shock structure with the latter two being responsible for the pressure profile ‘broadening’. The physical behavior of the incident shock is also analyzed along with other factors like temperature and curvature effects.

The mitigation of externally generated strong blast waves by an aqueous foam barrier of varying configurations within fixed distance between the explosion origin and the object to be protected is investigated and quantified both experimentally and numerically. The blast waves of shock Mach number 4.8 at 190 mm from the explosion plane are generated using exploding wire technique. The initially cylindrical blast waves are transformed into a plane blast wave in a specially constructed test unit in which the experiments are performed. The shock waves emanating from the foam barrier are captured using shadowgraph technique. A simple numerical model treating the foam by a pseudo-gas approach is used in interpreting and reconstructing the experimental results. The additional contribution of the impedance mismatch factor is analysed with the aid of numerical simulation and exploited for achieving greater blast wave pressure reduction.

Spherical shock wave implosion in argon is studied both theoretically and experimentally. It is shown that as the strength of the converging shock increases the nonideal gas effects become dominant and govern the evolution of thermal and transport gas properties limiting the shock acceleration, lowering the gas adiabatic index and the achievable energy density at the focus. Accounting for multiple-level ionization, excitation, Coulomb interaction and radiation effects, the limiting equilibrium temperatures to be achieved during the shock implosion are estimated. Focal temperatures of the order of 30 000 K are measured in experiments where converging spherical shock waves are created using a conventional gas-dynamic shock tube facility.

A semi-conservative, stable, interphase-capturing numerical scheme for shock propagation in heterogeneous systems is applied to the problem of shock propagation in liquid-gas systems. The scheme is based on the volume-fraction formulation of the equations of motion for liquid and gas phases with separate equations of state. The semi-conservative formulation of the governing equations ensures the absence of spurious pressure oscillations at the material interphases between liquid and gas. Interaction of a planar shock in water with a single spherical bubble as well as twin adjacent bubbles is investigated. Several stages of the interaction process are considered, including focusing of the transmitted shock within the deformed bubble, creation of a water-hammer shock as well as generation of high-speed liquid jet in the later stages of the process.

A complex system of waves propagating inside a water column due to the impact of plane shock wave is investigated both experimentally and numerically. Flow features, such as, focusing of expansion waves generating large negative pressure, nucleation of cavitation bubbles, and a re-circulation zone are observed and discussed qualitatively and quantitatively. Experiments are conducted on a 22 mm diametrical water column hit by shock waves with Mach numbers 1.75 and 2.4 in a newly constructed exploding wire facility. A new technique to create a properly shaped, repeatable, large diameter water column with straight walls is presented. Qualitative features of the flow are captured using the shadowgraph technique. With the aid of numerical simulations the wave motions inside the column are analyzed; the spatial location of the expansion wave focusing point and the corresponding negative peak pressures is estimated.