Experimental and numerical investigation of air blast mitigation of single and multi-scale structures

In order to protect civilians and infrastructure against the damaging effects of explosions, understanding blast wave interactions with structures in complex environments is fundamental for effective blast mitigation development. Obstructing the direct propagation of blast waves towards targets using a small, porous structure formed as an array/matrix of smaller structural elements is usually a preferred protective strategy in urban environments due to issues related to sustainability including material use and public perceptions of hazard.

The blast wave mitigation induced by interaction with obstacles in downstream region is primarily dominated by configuration- shapes and lengths of individual elements- of the obstacle itself. Whilst the blast wave mitigation behaviour of obstacles arranged as an array/matrix comprising single-length scale and regular shape is well known, the mitigation behaviour of obstacles arranged as fractals involving multi-scale length and self-similarity features is not clear yet.

The primary aim of this thesis was to extensively understand the blast mitigation behaviour of fractal obstacles of increasing complexity for developing protection strategies and predictive methods. To achieve this, an experiment on blast wave interaction with fractal obstacles was conducted to measure the blast attenuation and observe the mitigation behaviour. CFD numerical models simulating air blast loading in free-field and complex interaction scenarios were validated against experimental data. Utilising the validated numerical models, the mitigation behaviour of blast loading through blast-obstacle interactions process was studied for single obstacle case, and fractal obstacles with increasing complexity scenarios.

The experimental findings exhibited that obstacles with shapes closely resembling true fractals can induce local significant blast attenuation up to 26\% in pressure and 16\% in peak specific impulse attributed to a mechanism known as trapping. This indicated that the mechanism of blast mitigation of fractal obstacles of increasing complexity is fundamentally different from singular or arrays of regular obstacles. Furthermore, blast wave parameters behind different arrangements of fractal obstacles were found to be inherently determinable.

The numerical simulations revealed that the development of a complex flow-field downstream is comprised of two zones: wave shadowing and wave interference in which mitigation patterns, specifically behind the single obstacle, can be predictable despite the complex interactions. It was also shown that increasing the fractal obstacles complexity can produce a better impedance and substantially alter the direction of the incident wave. This has demonstrated that the effectiveness of obstructing a blast wave, measured in terms of blast intensity mitigation, is classified according to the fractal complexity configuration from the lowest level to the highest level. The peak reduction in pressure and specific impulse in the downstream region of fractal obstacle with highest level of complexity was numerically found to be 60\% and 40\% respectively. Therefore, the finding of this can be utilised in structural design and optimization of protective structures with improved blast mitigation.