Analysis of blast mitigation strategies exploiting fluid-structure interaction

Blast attacks have become the most pervasive threat in both civil and military contexts. However, there is currently a limited understanding of the mechanisms of loading, damage and failure of structures, and injury to humans produced by blast. This thesis seeks to advance our current understanding of the mechanisms of blast loading on structures. Towards this end, a comprehensive analytical and numerical study of basic problems in the interaction of blast waves with structures is conducted. The analysis is of interest in the conception of blast mitigation strategies and in the design and optimization of protection systems with improved performance against blast. The approach builds on a classic solution by G. I. Taylor on the interaction of acoustic blast waves with free-standing plates (In G. K. Batchelor, editor, The Scientific Papers of Sir Geoffrey Ingram Taylor, vol. III, p.287-303, Cambridge University Press, 1963). Taylor’s analysis demonstrates that the coupled fluid-structure interaction effect can be exploited for the purpose of reducing the impulse transmitted from the blast to the structure. This basic result is not applicable to the case of air blasts due to non-linear compressibility effects. In this thesis, a number of extensions of Taylor’s theory is proposed. The case of air blast waves interacting with free-standing plates of variable mass is given special attention. The limiting cases of extremely heavy and extremely light plates are explored analytically for arbitrary blast intensities, from where it is concluded that a modified non-dimensional parameter representing the mass of compressed fluid relative to the mass of the plate governs the fluid-structure interaction. The intermediate asymptotic regimes are studied using a numerical method based on a Lagrangian formulation of the Euler equations of compressible flow and conventional shock-capturing techniques. Based on the analytical and numerical results, approximate formulae for the transmitted impulse describing the entire range of relevant conditions are proposed. The main conclusion of the theory is that non-linear fluid compressibility further enhances the beneficial effects of fluid-structure interaction in reducing the impulse transmitted to the structure. More specifically, it is found that impulse re3 ductions due to fluid-structure interaction are more significant than in the acoustic limit when compared to those obtained ignoring fluid-structure interaction effects. In addition, a number of acoustic results for uniform shocks, viscoelastic supports, two fluid media, impulsively deployed and pressure actuated plates are proposed which provide the basis for evaluation of the benefits of the fluid-structure interaction in a wide variety of settings. The governing non-dimensional parameters in each specific context are determined and exact solutions to the fluid-structure interaction problem are provided. The results for the actively deployed plates reveal that significant cancellation of the blast impulse can be achieved thus suggesting a plausible blast mitigation strategy. Thesis Supervisor and Committee Chair: Raúl Radovitzky Associate Professor of Aeronautics and Astronautics Thesis Supervisor Committee Member: Mary Boyce Gail E. Kendall Professor of Mechanical Engineering Committee Member: John Hutchinson Professor of Engineering & Applied Sciences Harvard University Committee Member: Gareth McKinley Professor of Mechanical Engineering