The effects of the collision of a Near-Earth-Object(NEO) with the Earth could be catastrophic on a local, regional or global scale depending on the size of the NEO. Therefore, there is considerable interest in determining ways to mitigate the threat posed by these objects. This dissertation presents a method utilizing a tethered ballast mass for altering the trajectory of a NEO with an Earth-intersecting orbit so that it avoids hitting the Earth. The method is simulated using four different methods. The first method assumes a rigid massless tether. Using this method, a parametric study was conducted to determine the effectiveness of the technique over a wide parametric space. Specifically, this study provided results in terms of deviation rates over the parametric space. After this, the massless inelastic method was used to study actual miss distances assuming the asteroid was on a collision course with the Earth. After this, a study was conducted, using the massless, inelastic model, in which the mass of the ballast was made constant, in order to determine the minimum tether length required to divert asteroids simulated based on actual asteroids from NASA's potentially hazardous asteroid (PHA) database, again assuming a massless inelastic tether. Finally, it was desired to determine how relaxing the constraints of the massless inelastic model would affect system performance. Therefore, three more models were introduced: massive inelastic, massless elastic, and massive elastic. Using these models, a study was performed to explore the effects of the changed model on system performance and to compare the results in terms of deviation, with those of the massless inelastic model. This was desired because the numerical cost of the new models was much higher than that of the massless inelastic model, so rather than conduct the study over a much larger parametric space, a smaller space was chosen, so that the results could be compared.

A procedure for simulating wall-bounded,separated flows utilizing hybrid large-eddy / Reynolds- Averaged strategies is presented in this work. Following the zonal concept, the proposed hybrid method uses a distance-dependent blending function to shift the turbulence closure from Menter's two-equation model near wall surfaces to a one-equation subgrid model away from walls. The code is parallelized using domain-decomposition / MPI message-passing methods and is optimized for operation on the 720 processor IBM SP-2 at the North Carolina Supercomputing Center. The capabilities of the hybrid method are examined on two benchmark flows: a ramped-cavity flow that is representative of the internal flow field of a high speed propulsion device, and a compression ramp flow that features the classical problem of a shock wave / boundary layer interaction. Results indicate that the hybrid method provides generally good predictions for the ramped-cavity configuration, but less satisfactory predictions for the compression ramp configuration. Nevertheless, the strength of the hybrid method in capturing the recovery of the boundary layer downstream of reattachment is found in both cases, and it is a major improvement over the simulations produced by RANS alone. The weaknesses in simulating the compression ramp flow are also discussed and possible remedies are provided for further investigation in the future.

Soot volume fraction (f_sv) is measured quantitatively in a laminar diffusion flame at elevated pressures up to 25 atmospheres as a function of fuel type in order to gain a better understanding of the effects of pressure on the soot formation process. Methane and ethylene are used as fuels; methane is chosen since it is the simplest hydrocarbon while ethylene represents a larger hydrocarbon with a higher propensity to soot. Soot continues to be of interest because it is a sensitive indicator of the interactions between combustion chemistry and fluid mechanics and a known pollutant. To examine the effects of increased pressure on soot formation, Laser Induced Incandescence (LII) is used to obtain the desired temporally and spatially resolved, instantaneous f_sv measurements as the pressure is incrementally increased up to 25 atmospheres. The effects of pressure on the physical characteristics of the flame are also observed. A laser light extinction method that accounts for signal trapping and laser attenuation is used for calibration that results in quantitative results. The local peak f_sv is found to scale with pressure as <I>p</I>^1.2 for methane and <I>p</I>^1.7 for ethylene.

In support of NASAâs Autonomous Landing and Hazard Avoidance Technology project, an extended Kalman filter (EKF) routine has been developed for estimating the position, velocity, and attitude of a spacecraft during the landing phase of a planetary mission. The EKF is a recursive algorithm for obtaining the minimum variance estimate of a nonlinear dynamic process from a sequence of noisy observations. The proposed filter combines measurements of acceleration and angular velocity from an inertial measurement unit with range and range-rate observations from an onboard light detection and ranging (LIDAR) system. These high-precision LIDAR measurements of distance to the ground and approach velocity will enable both robotic and manned vehicles to land safely and precisely at scientifically interesting sites. The robustness and accuracy of the Kalman filter were first established using a simplified simulation of the final translation and touchdown phase of the Apollo lunar landings. In addition, experimental results from a helicopter flight test performed at NASA Dryden in August 2008 demonstrate the merit in employing LIDAR for pinpoint landing in future space missions.

Testing is performed on the 15 centimeter class pulsejet engine in order to develop, study, and explore the operational characteristics. Valved and valveless operation, hydrogen and propane fuels, various fuel injection methods, and a range of geometric configurations are investigated for operational feasibility. The scaling capabilities of a valveless 15 centimeter class pulsejet of conventional design are studied by methodically varying inlet length, exit length, exit geometry, and inlet area to combustor area ratio (Ai/Ac). Engine performance is defined by measuring chamber pressure, internal gas temperatures, time-resolved thrust, operational frequency, and fuel flow rate. The scaling capability is characterized by the success of self-sustained combustion for each corresponding geometric configuration. Tail pipe length is found to be a function of valveless inlet length and may be further minimized by the addition of a diverging exit nozzle. Chemical kinetic times and Ai/Ac prove to be the two prominent controlling parameters in determining scaling behavior.

An experimental 8 cm pulsejet was developed using scaling laws from research on both 50 and 15 cm pulsejets. The 8 cm jet operates in three different inlet configurations?conventional, perpendicular, and rearward. The rearward configuration features inlets facing in the opposite direction of the flight path and develops the maximum net thrust. Using a high frequency pressure transducer, the operational frequency of the pulsejet was obtained by monitoring the combustion chamber pressure. It was found that in the rearward configuration, the operational frequency of the jet decreases with increasing inlet length. In addition, the combustion chamber peak pressure rise per cycle increases significantly if the exhaust diameter is reduced. Using information from the 8 cm pulsejet, a 4.5 cm pulsejet was developed and is operational.

Hybrid Large Eddy Simulation/Reynolds-Averaged Navier-Stokes (LES/RANS) simulations of several high-speed flows are presented in this work. The solver blends a Menter BSL two-equation model for the RANS part of the closure with a Smagorisnky sub-grid model for the LES component. The solver uses a flow-dependent blending function based on wall distance and a modeled form of the Taylor micro-scale to transition from RANS to LES. Turbulent fluctuations are initiated and are sustained in the inflow region using a recycling/rescaling technique. A new multi-wall recycling/rescaling technique is described and tested. A spanwise-shifting method is introduced that is intended to alleviate unphysical streamwise streaks of high- and low-momentum fluid that appear in the time-averaged solution due to the recycling procedure. Simulations of sonic injection of air, helium and ethylene into a Mach 2 cross-flow of air are performed. Also, simulations of Mach 5 flow in a subscale inlet/isolator configuration with and without back-pressuring are performed. Finally, a Mach 3.9 flow through a square duct is used as an initial test case for the new multi-wall recycling and rescaling method as well as a multi-wall shifting procedure. A discussion of the methods, implementation and results of these simulations is included.

A new concept for the low speed propulsion mode in rocket based combined cycle (RBCC) engines has been developed as part of the NASA GTX program. This concept, called the independent ramjet stream (IRS) cycle, is a variation of the traditional ejector ramjet (ER) design and involves the injection of hydrogen fuel directly into the air stream, where it is ignited by the rocket plume. Experiments and computational fluid dynamics (CFD) are currently being used to evaluate the feasibility of the new design. In this work, a Navier-Stokes code valid for general reactive flows is applied to the model engine under cold flow, ejector ramjet, and IRS cycle operation. Pressure distributions corresponding to cold-flow and ejector ramjet operation are compared with experimental data. The engine response under independent ramjet stream cycle operation is examined for different reaction models and grid sizes. The engine response to variations in fuel injection is also examined. Mode transition simulations are also analyzed both with and without a nitrogen purge of the rocket. The solutions exhibit a high sensitivity to both grid resolution and reaction mechanism, but they do indicate that thermal throat ramjet operation is possible through the injection and burning of additional fuel into the air stream. The solutions also indicate that variations in fuel injection location can affect the position of the thermal throat. The numerical simulations predicted successful mode transition both with and without a nitrogen purge of the rocket; however, the reliability of the mode transition results cannot be established without experimental data to validate the reaction mechanism.

VULCAN (Viscous Upwind aLgorithm for Complex flow ANalysis) is a cell centered, finite volume code used to solve high speed flows related to hypersonic vehicles. Two algorithms are presented for expanding the range of applications of the current Navier-Stokes solver implemented in VULCAN. The first addition is a highly implicit approach that uses subiterations to enhance block to block connectivity between adjacent subdomains. The addition of this scheme allows more efficient solution of viscous flows on highly-stretched meshes. The second algorithm addresses the shortcomings associated with density-based schemes by the addition of a time-derivative preconditioning strategy. High speed, compressible flows are typically solved with density based schemes, which show a high level of degradation in accuracy and convergence at low Mach numbers (M < 0.1). With the addition of preconditioning and associated modifications to the numerical discretization scheme, the eigenvalues will scale with the local velocity, and the above problems will be eliminated. With these additions, VULCAN now has improved convergence behavior for multi-block, highly-stretched meshes and also can accurately solve the Navier-Stokes equations for very low Mach numbers.

Aerated-liquid atomization, which is produced by the introduction of gas directly into a liquid flow immediately upstream of the injector exit orifice to generate a two-phase flow, has been shown to produce well-atomized sprays in a quiescent environment with only a small amount of aerating gas at relatively low injection pressures. A time-derivative preconditioning method using the Low-Diffusion Flux-Splitting Scheme (LDFSS) has been extended to a ?mixture? model of two-phase flow and applied to simulate the structure of internal two-phase flow for aerated-liquid injectors, with each phase governed by its own equation of state. The Continuum Surface Force (CSF) model of Brackbill, et al. is adapted to model compressible fluid flow influenced by interfacial surface tension. A sub-iterative time integration method based on a planar Gauss-Seidel partitioning of the system matrix is used with implicit source terms as a means of solving the three-dimensional, time-dependent form of the governing equations. The calculations are parallelized using domain-decomposition and Message-Passing Interface (MPI) methods, and are optimized for operation on the 720 processor IBM SP-2 at the North Carolina Supercomputing Center (NCSC). Simulation results for 2-D aerated-liquid injector flowfields at gas-to-liquid (GLR) mass ratios of 0.08% and 2.45% are discussed. In accord with experimental visualization data, the results for GLR = 0.08% indicate a combination of slugging and core-annular two-phase flow in the injector. Results at GLR = 2.45% indicate that a core-annular flow mode dominates, again in agreement with experimental results. The effects of the choice of reference velocity and the level of surface tension on the injector flowfield solutions are also examined.