Veronica Eliasson

Shock wave focusing in water inside convergent structures

Experiments on shock focusing in water-filled convergent structures have been performed. A shock wave in water is generated by means of a projectile, launched from a gas gun, which impacts a water-filled convergent structure. Two types of structures have been tested; a bulk material and a thin shell structure. The geometric shape of the convergent structures is given by a logarithmic spiral, and this particular shape is chosen because it maximizes the amount of energy reaching the focal region. High-speed schlieren photography is used to visualize the shock dynamics during the focusing event. Results show that the fluid-structure interaction between the thin shell structure and the shock wave in the water is different from that of a bulk structure; multiple reflections of the shock wave inside the thin shell are reflected back into the water, thus creating a wave train, which is not observed for shock focusing in a bulk material.

Light emission during shock wave focusing in air and argon

The light emission from a converging shock wave was investigated experimentally. Results show that the shape of the shock wave close to the center of convergence has a large influence on the amou ...

Shock Wave Response of Iron-based In Situ Metallic Glass Matrix Composites

The response of amorphous steels to shock wave compression has been explored for the first time. Further, the effect of partial devitrification on the shock response of bulk metallic glasses is examined by conducting experiments on two iron-based in situ metallic glass matrix composites, containing varying amounts of crystalline precipitates, both with initial composition Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4. The samples, designated SAM2X5-600 and SAM2X5-630, are X-ray amorphous and partially crystalline, respectively, due to differences in sintering parameters during sample preparation. Shock response is determined by making velocity measurements using interferometry techniques at the rear free surface of the samples, which have been subjected to impact from a high-velocity projectile launched from a powder gun. Experiments have yielded results indicating a Hugoniot Elastic Limit (HEL) to be 8.58 ± 0.53 GPa for SAM2X5-600 and 11.76 ± 1.26 GPa for SAM2X5-630. The latter HEL result is higher than elastic limits for any BMG reported in the literature thus far. SAM2X5-600 catastrophically loses post-yield strength whereas SAM2X5-630, while showing some strain-softening, retains strength beyond the HEL. The presence of crystallinity within the amorphous matrix is thus seen to significantly aid in strengthening the material as well as preserving material strength beyond yielding.

Quantitative Pressure Measurement of Shock Waves in Water Using a Schlieren-Based Visualization Technique

Many fluid flow applications, for example shock wave propagation and jet flows, involve variation of thermodynamic state variables such as pressure or density of the fluid. In order to fully understand the dynamics of the flow, quantitative information about the variables is desirable during experimental measurements of these applications. The background oriented schlieren (BOS) method is a relatively new quantitative visualization method that uses a digital image correlation algorithm and a numerical solver. It has been applied for measuring the density variation in gaseous fluids. In this paper, the main focus is to apply BOS to the study of shock dynamics in water and provide a detailed protocol for such a method. The experimental results indicate that this technique is a reliable and robust way of probing pressure variations in water for shock impact events.

Shaping Converging Shock Waves by Means of Obstacles

The schlieren photographs show a cylindrical converging shock wave at different time instants. Eight cylindrical obstacles, with diameters of 15 mm, are placed in an octagonal pattern to create octagonally shaped shock waves. At first, eight concave forward sides are created, Fig. 1 and 2. The concave sides will first get plane and then the shock wave will transform into a double octagon, Fig. 3. After some time it will transform back into an octagon again, with opposite orientation relative to the first one, Fig. 4. This behavior will repeat during the whole focusing process and depends on the nonlinear coupling between the local form of the shock front and the local propagation velocity. Fig. 1. A converging shock wave with concave sides in an octagonal pattern, t = t0. Fig. 2. Converging shock wave, t = t0 + 5 μs.

Experimental Investigation of Shock Wave Amplification Using Multiple Munitions

A laboratory setup has been constructed to generate expanding shock waves in air using exploding wires. High-speed schlieren video is captured to characterize the shock fronts. Current work aims to validate the system with single exploding wires, after which multiple exploding wires will be used to study the interaction of the shock waves. Preliminary results confirm the system is able to generate expanding shocks. Wires of diameter 0.15 mm have generated shock waves with Mach numbers of 1.3. Following further testing of the system, multiple exploding wires will be used to study the interaction of the shock waves.

Parallel implementation of geometrical shock dynamics for two dimensional converging shock waves

Abstract Geometrical shock dynamics (GSD) theory is an appealing method to predict the shock motion in the sense that it is more computationally efficient than solving the traditional Euler equations, especially for converging shock waves. However, to solve and optimize large scale configurations, the main bottleneck is the computational cost. Among the existing numerical GSD schemes, there is only one that has been implemented on parallel computers, with the purpose to analyze detonation waves. To extend the computational advantage of the GSD theory to more general applications such as converging shock waves, a numerical implementation using a spatial decomposition method has been coupled with a front tracking approach on parallel computers. In addition, an efficient tridiagonal system solver for massively parallel computers has been applied to resolve the most expensive function in this implementation, resulting in an efficiency of 0.93 while using 32 HPCC cores. Moreover, symmetric boundary conditions have been developed to further reduce the computational cost, achieving a speedup of 19.26 for a 12-sided polygonal converging shock.

Non-Penetrative Blast-Induced Traumatic Brain Injury: Visualization of Representative Human Skull and Brain Response to Shock and Blast Loading

Traumatic brain injury (TBI) is a significant biological concern that has been known to result from exposure of the brain to shock and blast events. Experiments have been carried out at The Aerospace Corporation’s shock tube facility aimed at studying the interaction between a shock (M1.23 corresponding to an overpressure of 0.57 atm) and a realistic human skull and brain system. Visualization was accomplished via high-speed Schlieren, dynamic photoelasticity, and high speed video. Results showed that the shock propagated into the cranial cavity via the eye sockets (i.e. soft tissues) and focused near the posterior region of the brain. The brain material had a delayed response to the shock, and underwent violent oscillations in the forward/backwards directions. Cavitation was observed in the anterior region of the brain. Weak shock waves were observed up to 800 μs after the shock incident, while mild oscillations of the brain material were observed for several seconds after the shock incident. Visible macroscopic injury was observed in the brain material after the shock tube tests in the anterior region of the brain due to the cavitation and forwards/backwards motions of the brain material. Little or no visible damage was observed in the remainder of the brain. Preliminary studies with a faceshield showed promise for diminishing the effects of the shock and overpressure wave by blocking the main path of shock transmission through the eye sockets. Future studies will focus on mitigation of the shock and overpressure wave via novel materials and helmet/faceshield configurations.

The Production of Converging Polygonal Shock Waves by Means of Reflectors and Cylindrical Obstacles

Converging and reflecting strong shock waves are investigated experimentally in a horizontal co‐axial shock tube. The shock tube has a test section mounted at the end of the tube. Two different methods to produce various geometrical shapes of shock waves are tested. In the first method the reflector boundary of the test section is exchangeable and four different reflectors are used: a circle, a smooth pentagon, a heptagon and an octagon. It is shown that the form of the converging shock wave is influenced both by the shape of the reflector boundary and by the nonlinear dynamics between the shape of the shock and the velocity of the shock front. Further, the reflected outgoing shock wave is affected by the shape of the reflector through the flow ahead of the shock front. In the second method we use cylindrical obstacles, placed in the test section at various positions and patterns, to create disturbances in the flow that will shape the shock wave. It is shown that it is possible to shape the shock wave in ...

Shock Focusing in Nature and Medicine

We will here get acquainted with some spectacular examples of shock focusing occurring in nature, from a tiny bubble that emits light during its periodic compression and expansion to a supernova that rebounds after a gravitational collapse producing the most powerful energy burst known to us with light intensity comparable to that of the whole galaxy. Interestingly, some of the small sea creatures such as the so-called snapping shrimp, just some 20 mm in length, use cavitation to create a powerful outgoing blasts to hunt their pray. Despite the very small scale compared to astronomical events, the tiny bubble functions as a focusing lens and is able to generate extreme accelerations, forces, and temperatures during nano- and picosecond time intervals. Shock wave lithotripsy is one of the most known medical applications of shock wave focusing. The method, developed three decades ago, uses repeated focused pressure pulses and is now the primary method for treatment of kidney stones.