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Laboratory and numerical studies of gas escape through early time casing fractures

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Title: Laboratory and numerical studies of gas escape through early time casing fractures
Authors: Hoing, Craig Andrew
Item Type: Thesis or dissertation
Abstract: Current accepted theory for the calculation of gas flow through narrow openings, such as fractures in weapon casings during expansion, relies on the use of ideal nozzle flow theory being applicable throughout the fracture process. The thesis initially looked to determine the acceleration phase of fragments within expanding detonation gases; however, the scope narrowed to determine the applicability of the current gas flow model. The revised scope combines numerical and experimental research to investigate the flow Mach number within narrow openings, along with any influence of internal pressure, opening width or opening length. Numerical modelling conducted investigated internal pressures of 10 bar, 18 bar, 50 bar and 200 bar for opening diameters of 1 mm to 6 mm in 1 mm steps. Opening lengths of 5 mm, 10 mm and 20 mm were considered to determine any casing thickness effects. The modelling showed the internal pressure to have minimal effect on the flow, with the main driver being the opening diameter. In general, the flow stabilised at Mach numbers greater than unity for each case, returning Mach 1.2 for a diameter of 1 mm which increased to Mach 1.35 for diameters of 3 mm and above. The experimental investigation measured static and stagnation pressures within the flow through a 6 mm diameter opening requiring the design, construction, and calibration of a novel miniature pressure probe. Experimental pressures were limited by the probe response with tests conducted at 8 bar and 10 bar to study flow stability. Aerodynamic blockage effects were considered and detailed analysis of the experimental data is presented, using compressible flow theory and Rayleigh supersonic pitot theory. Supplementary numerical modelling could not definitively determine detached bow shock presence within the experimental set up used, so uncertainties in the final analyses were calculated for inappropriate use of either theory. The most conservative approach was to use compressible flow theory as this underestimated the flow Mach number should a bow shock exist. Experimentally, the flow was approximately Mach 1.15 which gives reasonable agreement with the numerical modelling; both returning low supersonic flow. Finally, two alternative model hypotheses are presented; however, neither was developed into a working model due to experimental limitations.
Content Version: Open Access
Issue Date: Mar-2020
Date Awarded: Aug-2020
URI: http://hdl.handle.net/10044/1/83167
DOI: https://doi.org/10.25560/83167
Copyright Statement: Creative Commons Attribution NonCommercial NoDerivatives Licence
Supervisor: Proud, William
Sponsor/Funder: Great Britain. Ministry of Defence
Department: Physics
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Physics PhD theses



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