The wake of an axisymmetric bluff body was investigated using water tunnel experiments. The parameters common to all investigations were a Reynolds number of 1000 or 1500 based on the body diameter, and a boundary layer thickness entering the body base of 30% of the base diameter. Harmonic forcing was accomplished using eight individual piston pump actuators providing blowing and suction disturbances into the boundary layer close to the body base, or into the wake at the base of the body. This setup allowed the excitation of azimuthal mode numbers up to four. The resulting flow field was evaluated using flow visualization, single wire hot film anemometry, and direct drag force measurements. Four different helical mode combinations were used to force the wake, ±1, ±2, ±3, and ±4. The ±1 modes are dominant in the natural wake. When forcing the ±1 modes it was possible to lock their frequency and phase to the forcing over a relatively large frequency range. Within the lock-in range, the wake drag increased by up to 40%. The mean flow of the wake was axisymmetric. Forcing the ±2 modes, the lock-in frequency range was significantly smaller and was centered at somewhat higher frequencies. The mean flow in this case was distorted to a four-lobed polygon, and the drag increased by more than 60%. The ±3 forcing yielded a flow response that involved neighboring modes with significant amplitudes, which was most likely caused by the decreased quality of the spatial representation of the forcing input due to the limited number of pistons. The combination of the different modes resulted in a mean flow distortion and amplitude distribution with five lobes. The frequency range for which lock-in could be observed was further reduced when compared to the ±2 case. For forcing modes ±4, the flow responded only locally to the forcing, and the decay of the forced modes in downstream direction was very rapid, for example, at three diameters downstream the forced modes were no longer detectable.

Aspects of the self consistent acceleration and transport of cosmic rays in astrophysical fluid flows and associated numerical methods are studied. Problems investigated are: (i) magnetohydrodynamic (MHD) wave interactions and instabilities in two-fluid models of cosmic ray modified shocks and flows; (ii) two dimensional, self consistent models of cosmic ray acceleration by the first order Fermi mechanism in supernova remnant shocks; (iii) new Riemann solver for the two-dimensional Euler equations and adaptive mesh refinement scheme for the coupled MHD and cosmic ray transport equations. The interaction of short wavelength MHD waves and instabilities in cosmic ray modified flows are investigated using asymptotic analysis and numerical simulations, with application to cosmic ray driven squeezing instabilities in supernova remnant shocks. In the linear wave regime, the waves are coupled by wave mixing due to gradients in the background flow; cosmic-ray squeezing instability effects, and damping due to the diffusing cosmic-rays. Numerical solutions of the fully nonlinear two-fluid cosmic ray MHD equations are compared with solutions of the wave mixing equations for oblique, cosmic ray modified shocks. A two-dimensional, self-consistent, adaptive mesh refinement numerical algorithm is developed for the solution of the ideal magnetohydrodynamic equations coupled to the kinetic transport equation for energetic charged particles. The method is used to simulate the evolution of the momentum distribution function of the cosmic rays accelerated at supernova remnant shocks. The numerical methods were tested on a variety of fluid dynamics and MHD problems, and previous models of cosmic ray modified supernova remnant shocks. A Riemann solver based on two-dimensional multi-state Riemann problems was developed. The scheme generalizes the traditional one-dimensional flux calculation to include contributions to the flux through the cell edges of the waves originating at cell corners. The multidimensional flux corrections increase the accuracy and stability of the scheme. An adaptive mesh refinement technique was used to study the Von Neumann paradox associated with the formation of three shocks, when a low Mach number, supersonic flow impinges on a thin wedge. For the first time, the region near the triple point has been resolved in a numerical solution of the Euler equations.

Richtmyer-Meshkov (R-M) instability occurs when two different density fluids are impulsively accelerated in the direction normal to their nearly planar interface. The instability causes perturbations on the interface to grow and to possibly become turbulent. R-M instability is a fundamental fluid instability that is important to fields ranging from astrophysics to high-speed combustion. For example, R-M instability is currently one of the limiting factors in achieving a positive net yield in laser driven inertial confinement fusion experiments. This experimental study investigates the instability of an interface between incompressible, miscible liquids with an initial sinusoidal perturbation. After undergoing a nearly impulsive acceleration, the initial perturbation quickly inverts and then grows in amplitude. The vorticity on the interface eventually coalesces into a series of alternating signed vortices. Disturbance amplitudes are measured and compared to theoretical predictions. Linear stability theory gives excellent agreement with the measured initial perturbation growth rates, while the predicted amplitudes differ by less than 10% from experimental measurements up to a nondimensional time kȧ₀t = 0.7. Fourth order, single-mode perturbation theory extends the 1.0% amplitude agreement up to a nondimensional time kȧ₀t = 1.3. A discrete vortex model and a combined model equation are within 10% of the experimental amplitude measurements up to the maximum experimental nondimensional time kȧ₀t = 30. The effects of Reynolds number (based on circulation) on the vortex core evolution and overall growth rate of the interface are also investigated. In addition, an instability in the vortex cores is observed for the first time and criteria established for its occurrence.

Low pressure plasma discharges have been an important process in the manufacturing of microelectronics devices since the late seventies. Therefore, the knowledge and control of the physical and chemical phenomena in plasmas are important for reactor design and process development. In order to understand the process, it is necessary to be able to make accurate measurements of plasma parameters, such as charged particle density, electron temperature, and ion energy. There are three objectives in this research; the first objective is to develop a new automatic electrostatic probe system in order to make accurate measurements of plasma parameters such as plasma potential φ₀, electron temperature Tₑ, electron density nₑ, and electron energy distribution function (EEDF). The second objective is to investigate the forces acting on contaminant particle which can be generate during process. The final objective is to understand the physical nature of the plasma which is very sensitive to changes of processing variables such as rf power and pressure. It was discovered that both ion drag and electrostatic forces induce particle trapping. Additionally, over the range of processing variables explored, the mode of heating transited from ohmic to stochastic, resulting in a variation of the plasma parameters.

We have developed a two-dimensional heliospheric model that includes galactic and anomalous cosmic rays as well as pickup ions. Cosmic rays are described via their number density in phase space, rather than pressure, as every preceding 2-D model has done. Cosmic-ray pressure is included in the total energy budget, allowing us to compute dynamical effects of the energetic particles on the solar wind. We include the magnetic field as well in order to consistently compute cosmic-ray diffusion coefficients. To accommodate' lower-energy cosmic rays with their short diffusion length, we implemented an adaptive mesh refinement code featuring improved spatial resolution near the termination shock. Our simulations show that galactic cosmic rays could substantially change the solar wind flow in the outer heliosphere. In particular, the solar wind is deflected towards the ecliptic plane during the positive solar cycle, resulting in faster wind near the current sheet. This is a result of large latitudinal gradients in the cosmic-ray pressure, caused by the difference in cosmic-ray drift patterns over latitude. We also found that anomalous cosmic rays have a minor effect on the solar wind. Their pressure is not sufficient to modify the termination shock significantly, a conclusion based on comparing model cosmic-ray spectra with observations. However, anomalous cosmic-ray acceleration occurs somewhat differently than thought before, and shock drift effects are not prominent. The spectra of these particles have an enhancement near the cutoff, that is not caused by shock drifts.

Particle contamination in plasma processing is a serious and challenging issue for the semiconductor industry. In this work, Laser Doppler Velocimetry, Laser Light Scattering, and Optical Emission Spectroscopy are used to elucidate the physical behavior of particles trapped in a plasma. Coulomb theory is used to describe the motion of particles. The theory agreed very well with experimental data and was explored to evaluate conditions for which particle agglomeration is likely. Finally, it was observed that particles may fall out of the particle trap during plasma ignition and subsequently contaminate the substrate.

In this work, a physical model describing the electric arc attachment on electron emitting non-refractory (cold) cathodes is developed and applied to Cu, Fe and Ti cathodes. The model considers the possibility of a pressure build up in the cathode region due to the strong vaporization of the cathode, the formation of a cathode sheath according to the Bohm's model, and the ion-enhanced thermo-field emission of electrons by the cathode surface. The self-sustaining operating conditions of the discharge are defined by two simple criteria based on particle and energy balance considerations. Results clearly show the necessity of having high local metallic vapor pressures in the cathode region of non-refractory cathodes in order to have a self-sustaining arc attachment. A minimum pressure of at least 19 atm is needed for a Cu cathode. This minimum pressure is shown to decrease as the cathode material boiling temperature increases according to an exponential decay law. Current densities of the order of 1010 A m--2 are maintained at the surface of a Cu cathode mainly by the emitted electrons. A comparison of the three different models for the electron emission current found in the literature allowed to define the limits of validity of each model for two typical arc-cathode interaction systems, and to evaluate the underestimation made on the emission current density when a less appropriate model is used. This underestimation is shown to cause an overestimation of important parameters such as the cathode surface temperature and metallic vapor pressure in the cathode region. An analysis of the mechanisms of heat transfer to the cathode surface allowed to show that the confinement of the cathode spot plasma forming the arc attachment could favor the production of vapors to the detriment of liquids. Such a phenomenon is of importance in Arc Ion Plating for instance. Heat losses by conduction in the cathode bulk larger than 1010 W m--2 are shown to favor the formation of liquid

<p> The ever-present need for reducing greenhouse gas emissions associated with transportation motivates this investigation of a novel ignition technology for internal combustion engine applications. Advanced engines can achieve higher efficiencies and reduced emissions by operating in regimes with diluted fuel-air mixtures and higher compression ratios, but the range of stable engine operation is constrained by combustion initiation and flame propagation when dilution levels are high. An advanced ignition technology that reliably extends the operating range of internal combustion engines will aid practical implementation of the next generation of high-efficiency engines. This dissertation contributes to next-generation ignition technology advancement by experimentally analyzing a prototype technology as well as developing a numerical model for the chemical processes governing microwave-assisted ignition. </p><p> The microwave-assisted spark plug under development by Imagineering, Inc. of Japan has previously been shown to expand the stable operating range of gasoline-fueled engines through plasma-assisted combustion, but the factors limiting its operation were not well characterized. The present experimental study has two main goals. The first goal is to investigate the capability of the microwave-assisted spark plug towards expanding the stable operating range of wet-ethanol-fueled engines. The stability range is investigated by examining the coefficient of variation of indicated mean effective pressure as a metric for instability, and indicated specific ethanol consumption as a metric for efficiency. The second goal is to examine the factors affecting the extent to which microwaves enhance ignition processes. The factors impacting microwave enhancement of ignition processes are individually examined, using flame development behavior as a key metric in determining microwave effectiveness. </p><p> Further development of practical combustion applications implementing microwave-assisted spark technology will benefit from predictive models which include the plasma processes governing the observed combustion enhancement. This dissertation documents the development of a chemical kinetic mechanism for the plasma-assisted combustion processes relevant to microwave-assisted spark ignition. The mechanism includes an existing mechanism for gas-phase methane oxidation, supplemented with electron impact reactions, cation and anion chemical reactions, and reactions involving vibrationally-excited and electronically-excited species. Calculations using the presently-developed numerical model explain experimentally-observed trends, highlighting the relative importance of pressure, temperature, and mixture composition in determining the effectiveness of microwave-assisted ignition enhancement.</p>

Dielectric losses and dielectric constants at radio frequencies for nitrobenzene, water and xylene. The method of resistance variation was used to measure the phase difference psi and dielectric constant K for frequencies between 2 x 105 and 14 x 105 cycles/sec. Special cells were required. (1) Variation with frequency. The results agree approximately with the equations: For carefully dried nitrobenzene at 30&deg;C, psi = .028&deg; + 6.03 x 104/f; for distilled water at 23.5&deg;, psi = 0.8&deg; + 2.09 x 106/ f. These indicate that in addition to the true dielectric loss there is a leakage through the liquid proportional to 1/f. For xylene, psi was too small to measure, less than .01&deg; at 3 x 10 5 cycles. K was found to be practically independent of the frequency, being 2.24 for xylene and of the order of 100 for water. (2) Variation with temperature, for nitrobenzene. K decreased from 42 at 20&deg; to 24 at 14.2&deg;, while psi increased in the same range in the ratio of 7 to 1. These values were obtained, however, for a sample of nitrobenzene for which psi was 12 times as great as for a carefully dried sample.

Interchange instability is an important dynamic mechanism in plasma physics and has been advanced as an explanation of a variety of phenomena in the magnetospheric physics. This work derives a new instability criterion for interchange motion in a plasma that connects to a finite-conductivity wall. The new criterion is for a arbitrary magnetic &lt;beta> (ratio between thermal pressure and magnetic pressure averaged within flux tube) system, which contains background shear flow, whereas most classical criteria did not consider all of these conditions. Thus this new result is more appropriate to be applied in a real plasma system like the Earth's plasma sheet, which exhibits a wide range of &lt;beta> values and background shear flow. Based on magnetosphere-ionosphere coupling theory and ideal MHD adiabatic theory in the inner plasma sheet, a theoretical model was constructed in the ionosphere region. A finite boundary layer was set up between two regions of uniform-content flux tubes, and a perturbation on the boundary layer was investigated. Both analytical and numerical approaches are used to study the stability of the plasma configuration. The flux tubes are interchange unstable when the angle between the gradient of flux tube volume, defined as V = dsB , and the gradient of adiabatic specific entropy PV 5/3 is larger than arccos&lt;b&gt; 1lnPV5/3 1ln V/ 21+5&lt;b&gt;/6 . Combining this new criterion with the statistical calculation of the plasma sheet characteristics by using the Tsyganenko magnetic field model (the 1996 version) and the Tsyganenko-Mukai plasma model, it is found that, in the Earth's inner plasma sheet, the angle between the two gradients is typically of the order of 15&deg;, which indicates that the statistical-average Earth's plasma sheet is interchange stable. This result is applicable to the study of interchange instability and plasma transport in the global-MHD and other large-scale magnetosphere simulations, and provides a theoretical base for the study of analogous dynamic processes in the magnetospheres of other planets like Jupiter.