FractalBlast: Understanding and predicting the interaction of blast waves with multi-scale obstacles.

Terrorist attacks are becoming more deadly, and terrorist groups are beginning to engage in tactics with the specific aim to cause maximum casualties. Such attacks range from those targeting personnel, e.g. in crowded places such as Istanbul Airport and Manchester Arena, or those targeting infrastructure, e.g. Metrojet Flight 9268. In order to ensure that engineers provide effective and efficient protection systems, we must fully understand how a blast load develops in a complex, crowded environment, and how the presence of obstacles and obstructions alters the propagation of a blast wave.

Whilst it is known that a blast wave will reflect off and diffract around an obstacle, accurate quantification of this effect and a detailed understanding of the mechanisms governing this behaviour have, to date, eluded researchers in the field of blast protection engineering. This project aims to address this knowledge gap through experimental work at a world-leading facility, involving direct measurement of blast wave parameters both at the source and downstream of the obstacle. This experimental work will be supplemented with cutting-edge numerical analysis, using tools specifically designed for simulation of blast wave propagation in complex environments. Both of these approaches will then be combined to develop generalised relationships for the interaction of blast waves with obstacles, enabling a semi-empirical tool to be developed for rapid calculation of the flow field surrounding an obstacle following detonation of a high explosive.

This project aims to prove the concept of porous blast barriers, i.e. barriers comprising a series of smaller obstacles rather than a large, imposing, (typically concrete) monolithic structure. Such designs, it is envisaged, will become the next generation of urban blast protection strategies: engineered systems with tailored properties to achieved maximum blast protection, but compatible with a modern, open, green city. Blast protection systems that do not look like blast protection systems.

FractalBlast is expected to have long-term, wide-ranging impact for society, industry, and academia. The ability to design better protective systems will allow us to provide more blast protection for the same cost, or to implement blast protection strategies in places where it had not been previously possible. Furthermore, the provision of better protective systems can reduce loss of income and, crucially, loss of life. The ability to design optimised, porous, nature-inspired fractal protective structures has clear economic and environmental benefits in reducing material usage, as well as key societal implications. With blast protection and terror activities becoming increasingly prominent in the public domain, a move away from oppressive structures will ease public opinion and reduce the psychological effects of increased terror activity.

Impact will be maximised through several industry and academic links (see Project Partner Letters of Support). Strategic collaborations with small-scale experimentalists (Sochet) and numerical model developers (Schwer) will be used to generate a full suite of multi-scale experimental data for model validation, and the implementation of new, computationally efficient numerical techniques into LS-DYNA, one of the most widely used explicit finite element codes worldwide. The involvement of Dstl as industry mentors (Pope) ensures that the project remains rooted to current requirements for UK MoD. Additionally, FractalBlast will engage with project partners from key industrial partners from UK resilience, security and risk engineers (Arup, BakerRisk), and academic partners with extensive experience in engaging with policymakers (Gebbeken) to maximise and streamline dissemination into UK practice.

The fast running engineering model (FREM) developed as part of this project will be released to partner institutions through the PI's membership to the Energetic Materials Blast Information Group (EMBIG). This tool will enable practicing engineers to rapidly and accurately quantify blast loading on structures, providing first-order improvements on existing calculation methods used in industry. Dr Rigby has a proven track record for FREM development through his PhD work and follow-up collaborations with the Canadian Standards Association. In addition to the standard methods of dissemination at international conferences and journal paper publication, an exhibition day will be hosted at the University of Sheffield Blast and Impact Laboratory and Buxton, bringing together key UK academics and engineers and rapidly sharing findings from FractalBlast.

There are a number of connected research areas where fractal geometries could be exploited. If fractal barriers are demonstrated to attenuate pressure waves, there is a natural extension of this research to focus on attenuation of sound waves for use as sound barriers for inner-city motorways or airport approaches, temporary fencing around construction and demolition sites, or commercial blast research facilities. If FractalBlast can develop validated relationships between fractal geometry and energy losses through vortex shedding, then academics can begin to research the use of fractals to enhance drag, or mixing, for a lower mass/volume of obstacle. This will be achieved through engagement with Project Partners Higham and Bogosian. Potential applications are fractal aerofoils to increase drag and reduce weight of commercial aircraft, and more efficient mixing in the chemical process industry.

It is envisaged that FractalBlast will have lasting impact on setting the longer-term direction of blast research. The establishment of a virtual blast research group will help foster ongoing collaborations and unite currently disparate research themes, ensuring that the blast protection community is better equipped to handle the diverse and ever-changing challenges of accidental and malicious explosions and their effects of people and infrastructure.