Spotlight on shock waves

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Shock waves' impacts on industry and medicine.

shock wave

An abrupt change in the density, pressure and temperature of a flow as opposed to a gradual increase is known as a shock wave – a phenomenon that could prove promising for a wide range of applications in industry and medicine. However, this would first require significantly greater understanding of the occurrence and impact of these shocks. Professor Nikolaus Adams intends to devote the next five years to intensive research into their potential for biomedicine and nanotechnology – supported by an ERC Advanced Grant of €2.4 million.

Molecules in a gas or liquid seek equilibrium. If the density, pressure and temperature of a fluid changes, the molecules usually have enough time to reach equilibrium – but not in the event of a shock wave. “In that case, the molecules cannot spontaneously achieve equilibrium. Instead, they are in a state of thermodynamic non-equilibrium,” explains Prof. Adams, Chair of the Institute of Aerodynamics and Fluid Mechanics at TUM.

Shock waves are sudden changes in flow states across very short distances. A typical example is the sonic boom we hear when an aircraft flies overhead faster than the speed of sound. The air molecules are forced out of the path of the aircraft so quickly that maintaining thermodynamic equilibrium is no longer possible. A shock wave forms that moves with the aircraft and is perceived as a boom by people on the ground when the sharp rise in pressure passes over them. The formation, impact and potential for targeting shock waves is the focus of Adams’ research efforts, which he will now pursue intensively in a five-year project. This is facilitated by an ERC Advanced Grant – one of Europe’s most prestigious sources of research funding. The European Research Council awards these grants, endowed with €2.4 million, to scientists who already have an outstanding track record and intend to pursue ambitious, pioneering and unconventional research.

From industry to medicine – of potential interest to a wide range of applications

Both the spatial localization and strength of shock waves means they hold potential for a wide range of applications. Direct fuel injection for diesel engines is a case in point. The automotive industry is keen to increase injection pressure as this would enable engineers to also reduce harmful emissions. However, extreme injection pressures result in extreme tensions in liquid fuel, which then evaporates without heating. Vapor bubbles, known as cavitation bubbles, form in the fuel and suddenly collapse (implode). This creates shock waves that are so strong, they can even damage hardened metals in fuel injector components on impact. This phenomenon is called cavitation erosion and is also a familiar problem in the operation of marine propellers and water turbines. Yet what is a downside in engine technology could prove a benefit in medicine. “A fundamental problem in cancer treatment is that diffusion is a relatively slow process, so it takes time for drugs to reach the cancer cells, which have a higher internal pressure than healthy cells,” describes Adams. In contrast to diffusion, shock waves are rapid processes and could be used to accelerate drug uptake significantly. Adams envisages the following scenario: “Tiny vapor bubbles are produced – for instance by ultrasound – in the vicinity of a diseased cell and then collapse. The shock wave this generates perforates the cell walls, with the subsequent flow allowing a rapid influx of drugs into the cell. As long as the perforation is small enough, the cell wall can close again afterwards.” This would be a therapy that could be precisely targeted, substantially reducing the amount of medication required and thus also the side effects.

Mastering complexity through simulation

But before this type of scenario can become reality, scientists first need to improve their understanding of the physical processes involved. Adams is particularly interested in ways of generating tailored shock waves and how shocks interact with phase boundaries and nanoparticles: “These phenomena are so tiny and fast that they are still largely unresearched and obtaining experimental data is a major challenge. Quantitative and detailed investigation is only possible by computer, using numerical simulation.”

In numerical simulation, scientists first formulate the basic properties of shock waves by means of physical and mathematical models, then implement these in dedicated programs on high-performance supercomputers. The decisive factor here is the number of degrees of freedom – that is, the number of variable data describing the flow. “The more degrees of freedom, the more accurate the simulation,” Adams emphasizes. So this entails processing huge volumes of data. Thanks to improved processing power, the possibilities of numerical flow simulation have grown enormously over the last few years. However, for large-scale simulations, the processes call for a lot more computational power – another area Adams is focusing on. As part of a team of researchers, he received the Gordon Bell Prize in 2013 for a flow simulation of a cavitation bubble cloud with 13 trillion degrees of freedom – the largest and most efficient ever performed at that time. The researchers simulated the simultaneous collapse of 15,000 gas bubbles within a liquid. To accomplish this, they used one of the world’s fastest supercomputers, reaching a processing speed of 14 petaflops. That equates to 14 quadrillion (14,000,000,000,000,000) computer operations per second.

The burning question: can they be technically controlled?

The ERC Advanced Grant is crucial to Adams’ ambitious research endeavors. Thanks to this funding, he is now able to extend an excellent research group by four doctoral students and one postdoc. Adams is particularly pleased to receive such “stable support over an unusually long period” and explains that, as a rule, grants tend to be smaller, shorter and thus more suited to incremental project proposals. In particular, the ERC Advanced Grant allows the scientists a fairly high degree of research freedom. “Disruptive research also becomes possible – we can redirect our efforts to pursue a new avenue if unexpected results come up,” confirms Adams. The scientists will now start by using their simulations to address the burning question, namely can tailored shock waves be generated in complex environments such as living organisms? “We are already well able to simulate individual phenomena – the challenge lies in managing the complex interactions between them,” clarifies Adams. Or, as he frames the question that drives him: “Is the level of complexity so great that the formation and impact of shock waves simply cannot be predicted, which means they cannot be controlled technically?” Only if he and his team succeed in managing this complexity can they then turn to investigating the mechanisms and properties that enable controlled formation of shock waves and their possible impacts – to the benefit of applications in industry and medicine.

Article by Gitta Rohling, Faszination forschung 17/15

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