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Modeling Gas Bubble Behaviour and Loading on a Rigid Target due to Close-Proximity Underwater Explosions: Comparison to Tests Conducted at DRDC Suffield

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Technical memorandum

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This study describes recent simulation results for underwater explosions in close-proximity to a rigid target. Simulations were performed using Chinook, an Eulerian computational fluid dynamics CFD code. Predicted fluid pressures, impulse loading on the target, gas bubble size and bubble collapse times are compared with measurements taken from a series of experiments and compared to empirical models. The experiments, which were conducted at DRDC Suffield as part of the Force Protection TDP in 2008, involved 1.1 g detonator charges and 40 g C4 charges detonated at standoff distances ranging from 0.2 up to 2.0 times the free-field bubble radius from a rigid target. At these ranges of standoff distance, both the shock wave and gas bubble make significant contributions to the loading on the target surface. The simulations of the rigid target tests primarily focused on the modelling of gas bubble collapse and water jetting behaviour. Both two-dimensional and three-dimensional simulations were performed. Chinook was found to qualitatively predict the general trends correctly, however quantitative gaps still exist. From the high speed experimental videos and the animation of the numerical simulations it was found that the maximum impulse due to a bubble collapse occurs for the largest standoff in which the bubble contacts the target before collapse. The results were found to be very mesh dependent, and refining the mesh did not always produce better agreement with experiments. This was observed most often in the bubble collapse impulse loading. The two-dimensional approach provided a good initial understanding of the physical behaviour for a limited computational effort, and produced better bubble periods when compared to experimental data. The three-dimensional simulations were found to produce improved impulse predictions for the rigid targets.

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  • Underwater Ordnance
  • Explosions

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