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51 
Deviation of Earth Threatening Asteroids Using Tether and BallastFrench, David Bruce 28 July 2009 (has links)
The effects of the collision of a NearEarthObject(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 Earthintersecting 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.

52 
Hybrid ReynoldsAveraged / LargeEddy Simulations of RampedCavity and Compression Ramp FlowfieldsFan, Thomas ChenChuan 24 July 2002 (has links)
A procedure for simulating wallbounded,separated flows utilizing hybrid largeeddy / Reynolds Averaged strategies is presented in this work. Following the zonal concept, the proposed hybrid method uses a distancedependent blending function to shift the turbulence closure from Menter's twoequation model near wall surfaces to a oneequation subgrid model away from walls. The code is parallelized using domaindecomposition / MPI messagepassing methods and is optimized for operation on the 720 processor IBM SP2 at the North Carolina Supercomputing Center. The capabilities of the hybrid method are examined on two benchmark flows: a rampedcavity 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 rampedcavity 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.

53 
The Effect of Elevated Pressure on Soot Formation in a Laminar Jet Diffusion FlameMcCrain, Laura L. 18 July 2003 (has links)
Soot volume fraction (f<sub>sv</sub>) 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<sub>sv</sub> 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<sub>sv</sub> is found to scale with pressure as <I>p</I><sup>1.2</sup> for methane and <I>p</I><sup>1.7</sup> for ethylene.

54 
LidarAided Inertial Navigation with Extended Kalman Filtering for Pinpoint LandingAitken, Matthew Lawrence 23 July 2009 (has links)
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 rangerate observations from an onboard light detection and ranging (LIDAR) system. These highprecision 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.

55 
Experimental Investigations in 15 Centimeter Class Pulsejet EnginesSchoen, Michael Alexander 08 August 2005 (has links)
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, timeresolved thrust, operational frequency, and fuel flow rate. The scaling capability is characterized by the success of selfsustained 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.

56 
Experimental Investigations of MiniPulsejet EnginesKiker, Adam Paul 11 August 2005 (has links)
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.

57 
Hybrid LargeEddy Simulation/ReynoldsAveraged NavierStokes Methods and Predictions for Various HighSpeed FlowsBoles, John Arthur 02 October 2009 (has links)
Hybrid Large Eddy Simulation/ReynoldsAveraged NavierStokes (LES/RANS) simulations of several highspeed flows are presented in this work. The solver blends a Menter BSL twoequation model for the RANS part of the closure with a Smagorisnky subgrid model for the LES component. The solver uses a flowdependent blending function based on wall distance and a modeled form of the Taylor microscale to transition from RANS to LES. Turbulent fluctuations are initiated and are sustained in the inflow region using a recycling/rescaling technique. A new multiwall recycling/rescaling technique is described and tested. A spanwiseshifting method is introduced that is intended to alleviate unphysical streamwise streaks of high and lowmomentum fluid that appear in the timeaveraged solution due to the recycling procedure. Simulations of sonic injection of air, helium and ethylene into a Mach 2 crossflow of air are performed. Also, simulations of Mach 5 flow in a subscale inlet/isolator configuration with and without backpressuring are performed. Finally, a Mach 3.9 flow through a square duct is used as an initial test case for the new multiwall recycling and rescaling method as well as a multiwall shifting procedure. A discussion of the methods, implementation and results of these simulations is included.

58 
ReynoldsAveraged NavierStokes Analysis of the Flow through a Model RocketBased Combined Cycle Engine with an IndependentlyFueled Ramjet StreamBond, Ryan Bomar 18 August 2003 (has links)
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 NavierStokes 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 coldflow 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.

59 
Algorithmic Enhancements to the VULCAN NavierStokes SolverLitton, Daniel 15 August 2003 (has links)
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 NavierStokes 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 highlystretched meshes. The second algorithm addresses the shortcomings associated with densitybased schemes by the addition of a timederivative 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 multiblock, highlystretched meshes and also can accurately solve the NavierStokes equations for very low Mach numbers.

60 
Numerical Simulation of the Internal TwoPhase Flow within an AeratedLiquid Injector and its Injection into the Corresopnding Highspeed Crossflows.Tian, Ming 17 September 2002 (has links)
Aeratedliquid atomization, which is produced by the introduction of gas directly into a liquid flow immediately upstream of the injector exit orifice to generate a twophase flow, has been shown to produce wellatomized sprays in a quiescent environment with only a small amount of aerating gas at relatively low injection pressures. A timederivative preconditioning method using the LowDiffusion FluxSplitting Scheme (LDFSS) has been extended to a ?mixture? model of twophase flow and applied to simulate the structure of internal twophase flow for aeratedliquid 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 subiterative time integration method based on a planar GaussSeidel partitioning of the system matrix is used with implicit source terms as a means of solving the threedimensional, timedependent form of the governing equations. The calculations are parallelized using domaindecomposition and MessagePassing Interface (MPI) methods, and are optimized for operation on the 720 processor IBM SP2 at the North Carolina Supercomputing Center (NCSC). Simulation results for 2D aeratedliquid injector flowfields at gastoliquid (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 coreannular twophase flow in the injector. Results at GLR = 2.45% indicate that a coreannular 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.

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