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The explosion of massive stars involves the formation of a shock wave. In stars that develop iron cores, this shock wave stalls on its way out due to neutrino emission and the breakup of heavy nuclei flowing through the shock. For the explosion to succeed, a fraction of the gravitational binding energy of the collapsed core that is radiated in neutrinos needs to be absorbed by the material below the shock. How much energy is needed depends on the interplay between non-spherical hydrodynamic instabilities, neutrino heating, and nuclear dissociation. This thesis seeks to understand this interplay through numerical experiments that model the key physical components of the system and separate them out to examine their individual effects. Specifically, one- and two-dimensional time-dependent hydrodynamic simulations are performed to study the effects of non-spherical shock oscillations, neutrino-driven convection, and alpha particle recombination on the dynamics of the system and the critical heating rate for explosion. We find that nuclear dissociation has a significant effect on the linear stability and saturation amplitude of shock oscillations. At the critical neutrino heating rate for an explosion, convection due to a negative entropy gradient plays a major role in driving dipolar shock motions. One dimensional explosions are due to a global instability involving the advection of entropy perturbations from the shock to the region where the accretion flow cools due to neutrino emission. Large scale shock expansions in two-dimensions are due to a finite amplitude instability involving the balance between buoyancy forces and the ram pressure of the flow upstream of the shock. During these expansions, a significant amount of energy is released when nucleons recombine into alpha particles, constituting a significant last step in the transition to explosion. The critical neutrino heating rate for an explosion depends sensitively on the starting radius of the shock relative to the radius at which the binding energy of an alpha particle is comparable to the gravitational binding energy.

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One objective of this project is to set up a two-dimensional model for exploring the flushing process of trapped saltwater subject to upstream freshwater turbulent flow. The multiphase Eulerian model, a part of commercial code FLUENT6.2, has been applied for the first time to study this complex mixing interaction in estuary. The distinguishing characteristic of this model is to treat saltwater and freshwater as two single miscible phases instead of a mixture phase with density variation, and the advantage of using a multiphase approach over a single-phase model is that it can efficiently and accurately treat both the free water surface and relatively high density excess between two fluids simultaneously. The other objective of this project is to develop a three-dimensional model based on the FVCOM open source code, with the aim to better understand the estuarine hydrodynamics with or without the presence of typhoon. It is found that the original FVCOM code can not reproduce an accurate tidal hydrodynamics in estuary. An improved simulation of the bed friction has been incorporated into the existing code for estuarine tide. This model has also been developed by including air-pressure gradient term to study the hydrodynamic response to cyclonic typhoon. To include the effect of typhoon (wind stress and pressure deficit), a symmetrical cyclone model is adopted. However, the typhoon-induced wind field has been predicted poorly when the typhoon enters the near-shore region. This is because the typhoon quickly loses its symmetrical property in the near-shore region. To overcome this difficulty, an asymmetrical cyclone model is derived on the basis of characteristic isobar. The accuracy of open sea boundary for storm surge model has also been improved by using large scale model. Comparison results show a good agreement with numerical simulations and physical measurements.

The Bomb Strike Experiment for Mine Countermeasure Operations, currently sponsored through the Office of Naval Research mine impact burial prediction project, is part of a multi-year, comprehensive effort aimed at enhancing the Navyâ s fleet naval mine clearance capability and success. The investigation discussed in this paper examines the experimental and theoretical characteristics of a rigid body falling through the air, water, and sediment column at high speed. Several experiments were conducted to launch bomb-like rigid bodies with the density ratio similar to operational munitions, namely the MK-84 general purpose bomb, into a hydrodynamic test tank. Careful observations of the bomb-like rigid bodyâ s position and orientation were collected and analyzed to produce a series of three-dimensional coordinate time-space data tables and plots. The resulting data set reveals a strong correlation between shape type and trajectory and dispersion patterns for rigid bodies moving through the water column at high velocity. This data will be used for numerical verification of the initial threedimensional model (STRIKE35) aimed at predicting the overall trajectory, maneuvering, burial depth and orientation of a falling high-velocity rigid body in the air-water-sediment column. The long-term goal of this project is to improve warhead lethality for use in quick, precise and accurate strikes on known enemy naval minefields in the littoral combat environment. / FUNDED BY: N0001406WR20076