No description available.

The current study investigates a range of issues relating to the use of a vane-recessed tubular-passage casing treatment as a passive stall control technique in a multistage axial-flow compressor. The focus of the research was to determine whether such a treatment could delay the initiation of stall at lower mass flow rates as well as providing the most beneficial improvement in flow characteristics without sacrificing compressor efficiencies. Specific objectives of this study were to examine possible improvements or deterioration in the flow characteristics including stall margin, peak pressure rise coefficients and maximum efficiency in a multistage axial flow compressor. A casing treatment in addition to several spacer rings was developed from two initial designs and tested on the first stage of a low speed three-stage axial-flow compressor with a (0.7) hub to tip diameter ratio. The treatment configuration consisted of three parts: an outer casing ring, with a tubular shaped passage on the inside diameter, a set of 120 evenly spaced curved vanes, and a shroud or inner ring. The casing treatment was positioned following the inlet guide vanes upstream and partly covering the tip of the rotor blades. The main parts of the casing treatment including the recessed vanes in addition to some of the spacer rings were manufactured from high quality acrylic. Eight additional spacer rings of various shapes and geometry were added. The first ring held and partly covered the IGVs, in front of the casing treatment. The rotor tip exposure ratio was thought to have a significant impact on the effectiveness of the casing treatment. Therefore the other seven rings were used to provide the desired uncovered region of the rotor tip axial chord of about 10% in order to provide a range of exposures of (23.2%, 33.3%, 43.4%, 53.5%, 63.6%, 73.7%, and 83.8%). The results showed significant improvements in stall margin in all treated casing configurations along with insignificant efficiency sacrifices in some compressor builds. About (28.56%) of stall margin improvement in terms of corrected mass flow rate was achieved using a casing treatment with a (33.3%) rotor tip exposure. The compressor build with (0.535) rotor exposure ratios was the best configuration in terms of efficiency gain and loss characteristics. This build was able to provide the highest values of the maximum efficiencies in comparison with the performance achieved from the solid casing. An improvement of (1.81%) in the maximum efficiency in terms of the overall total-total pressure ratio, in association with a (22.54%) stall margin improvement in terms of the corrected mass flow rates were achieved by the application of this treatment configuration. The improvement in the peak pressure rise coefficients in terms of the overall total-total pressure ratio, obtained from this build was (2.33%). The compressor configuration using a casing treatment with a (0.636) rotor exposure ratio was the best build in terms of the pressure rise coefficients. This configuration was able to provide highest value of the peak pressure rise in comparison with the characteristics achieved from the datum build. An improvement of (2.65%) in the peak pressure rise coefficient in terms of the overall total-total pressure ratio, in association with a (22.49%) improvement in stall margin in terms of the corrected mass flow rates was achieved from this casing treatment build. The improvement in maximum efficiency in terms of the overall total-total pressure ratio, obtained from this build was (1.03%). The results suggest that the vane-recessed tubular-passage casing treatment designed as part of this investigation achieved the objectives, which were established for the research. In the majority of instances it not only produced gains in flow range, pressure rise coefficients and efficiencies, but also enabled the rotating stall, which developed at much lower mass flow rates in the compressor, to become progressive rather than abrupt.

Pattern formation, i.e., the outcome of self-organization, has fascinated scientists for centuries. A large effort was devoted to understand the formation of regular patterns in dissipative structures. More recently, it appears that self-organized structures could also be achieved near equilibrium. There is a great variety of physical and chemical systems that, near equilibrium, exhibit periodic patterns. For instance, stripes or bubbles could be observed in thin films of magnetic garnet, superconducting materials, block copolymers, liquid crystals, phospholipids, and ferrofluids. Wrinkling instability of compressed rigid membranes on soft elastic substrates leads also to the formation of periodic patterns near equilibrium. Since the seminal paper of Bowden et al. (Nature 1998), various systems were proposed to generate nano- and micrometric wrinkles via the application of compressive stresses to multilayers. In addition to its purely fundamental interest, these instabilities also offer a new route to build in a simple, cost-effective, and well-defined way nano- and microstructured surfaces without the use of the traditional, robust techniques developed in the microelectronics industry. In this thesis, we develop a new system, metal-polymer-substrate trilayers, that exhibit wrinkling when heated above the glass transition temperature of the polymer. We explain in detail the mechanism at the origin of wrinkling and expand existing models to obtain a complete description of the relevant parameters that govern both the amplitude and the wavelength of the obtained pattern. In light of this, we show that by playing with the rheological properties of the polymer we are able to control precisely the geometry of the wrinkles. Furthermore, to generate surfaces with a tailor-made buckling pattern, we develop an original variant of the experiments. We tune the boundary conditions at the polymer-substrate interface by chemically patterning the substrates with regions of high and low adhesion. In this way, we obtain patterns with wrinkles being oriented differently above the sticky and the slippery regions. This last result is very surprising since it seems, at first sight, unrealistic to imagine that the chemical nature of the substrate could affect the elastic instability of the skin through a micron-thick polymer film. To explore wrinkled patterns with complex morphologies, we couple the wrinkling instability with solvent diffusion. Molecular diffusion in the polymer layer triggers the transition from an unwrinkled to a wrinkled state, provided that stimuloresponsive mutlilayers are used. The wrinkled pattern obtained is determined by the geometry of the diffusion process. To understand this surprising observation, we explain in detail how the scalar field related to the solvent concentration affects so strongly the elastic instabilities usually determined by the tensorial stress field. This mechanism allows us thus to grant exotic stress distributions which lead to very intriguing patterns (e.g. parallel or radial folds, herringbones). Interestingly, we find that under specific conditions, a hierarchical wrinkled morphology, i.e. pattern of wrinkles branching into generations of ever-higher folds, develops. We study other manifestations of hierarchical structures existing around us. In this frame, we derive a general concept that a plate constrained at one edge (with a fixed wavelength) but free at the opposite one evolves naturally to larger wavelengths to minimize its bending energy. We show theoretically that the evolution results from a compromise between the gain in bending energy and the energetic penalties related to the change of wavelength. We demonstrate the universality of these concepts by showing that our commonplace suspended curtain behaves like nanometer-thick polystyrene films deposited on water and further compressed. We close this thesis by making a short review of the main applications related to wrinkling that are already described in literature and develop in detail one of them, the use of wrinkling to investigate cell contact guidance.

A novel gas channel experiment was constructed to study the development of high Atwood number Rayleigh-Taylor mixing. Two gas streams, one containing air and the other containing helium-air mixture, flow parallel to each other separated by a thin splitter plate. The streams meet at the end of a splitter plate leading to the formation of an unstable interface and of buoyancy driven mixing. This buoyancy driven mixing experiment allows for long data collection times, short transients and was statistically steady. The facility was designed to be capable of large Atwood number studies of ABtB ~ 0.75. We describe work to measure the self similar evolution of mixing at density differences corresponding to 0.035 < ABtB < 0.25. Diagnostics include a constant temperature hot-wire anemometer, and high resolution digital image analysis. The hot-wire probe gives velocity, density and velocity-density statistics of the mixing layer. Two different multi-position single-wire techniques were used to measure the velocity fluctuations in three mutually perpendicular directions. Analysis of the measured data was used to explain the mixing as it develops to a self-similar regime in this flow. These measurements are to our knowledge, the first use of hot-wire anemometry in the Rayleigh-Taylor community. Since the measurement involved extensive calibration of the probes in a binary gas mixture of air and helium, a new convective heat transfer correlation was formulated to account for variable-density low Reynolds number flows past a heated cylinder. In addition to the hot-wire measurements, a digital image analysis procedure was used to characterize various properties of the flow and also to validate the hot-wire measurements. A test of statistical convergence was performed and the study revealed that the statistical convergence was a direct consequence of the number of different large three-dimensional structures that were averaged over the duration of the run.

Thesis (Ph. D.)--West Virginia University, 1999. / Title from document title page. Document formatted into pages; contains iv, 131 p. : ill. (some col.) Includes abstract. Includes bibliographical references (p. 127-131).

Planetesimals may form from the gravitational collapse of dense particle clumps initiated by the streaming instability. We use simulations of aerodynamically coupled gas-particle mixtures to investigate whether the properties of planetesimals formed in this way depend upon the sizes of the particles that participate in the instability. Based on three high-resolution simulations that span a range of dimensionless stopping times 6 X 10(-3) <= tau <= 2, no statistically significant differences in the initial planetesimal mass function are found. The mass functions are fit by a power law, dN/dM(p) proportional to M-p(-p), with p = 1.5-1.7 and errors of Delta p approximate to 0.1. Comparing the particle density fields prior to collapse, we find that the high-wavenumber power spectra are similarly indistinguishable, though the large-scale geometry of structures induced via the streaming instability is significantly different between all three cases. We interpret the results as evidence for a near-universal slope to the mass function, arising from the small-scale structure of streaming-induced turbulence.

No description available.

The dropping of a water-filled latex balloon onto a flat, rigid surface is an experiment that is known and has been performed by many, but on which there is no existing published work. High-speed images taken of the process revealed a range of phenomena, many of which had not been previously observed. After release, a water-filled balloon accelerates down and impacts with the tank floor. Upon impact, the balloon deforms through the propagation of waves up the balloon from the impact point. If the balloon does not rupture during this deformation, it then bounces up off the surface, the whole process similar to that when a water droplet bounces on a hydrophobic surface. Often, however, the balloon ruptures. This occurs through the propagation of one or more cracks through the balloon, leading to the rapid retraction of the membrane over the water’s surface, and consequent ejection of a fine spray of water droplets behind it. If there are any waves on the balloon at the moment of rupture, a larger-scale growth of the interfacial amplitude occurs, of the same wavelength as the preburst waves. Eventually, gravity dominates, as the water slumps down and spreads over the flat surface. In this thesis, the process described above is examined in detail, both experimentally and theoretically. To gain some insight into the behaviour of the latex balloons, their static and quasi-static behaviour is examined. A experimental method better than simply dropping the balloons was derived, permitting the accurate quantitative measurement of the process. In this new method, the balloons were held stationary, forced at a set frequency, then ruptured with a pin. The pre-burst waves are then shown to be accurately modelled by linear theory, with tension in the membrane acting much like a fluid surface tension. The behaviour of the rubber in retraction from large initial stretches is shown to be disperse, in contrast with that observed for retractions from small initial stretches, due to both non-linearity in the rubber and drag from the water on the strip. The spray ejected behind the rubber is explained as consequence of the inherent instability of the Gaussian velocity field in the wake. Finally, the late-time growth of the interfacial amplitude is examined, and argued to be closely-related to the Richtmyer-Meshkov instability. A model is then derived for the case of a balloon oscillated and burst with water both inside and outside, and is shown to be in approximate agreement with experiments.

Pressure pulsations, or thermoacoustic instabilities, as they are called in the research community, can cause extensive damage in gas turbine combustion chambers. To understand the phenomena related to thermoacoustics, a simple Rijke-type tube combustor was built and studied. Extensive experimental results, as well as theoretical analyses related to the Rijke tube are presented in this thesis. The results, attributable to both the analyses and the experiments, help explain all the phenomena affecting the acoustic pressure in the combustor. The conclusion is that there are three separate yet related physical processes affecting the acoustic pressure in the tube. The three mechanisms are as follows: a main thermoacoustic instability in accordance to the Rayleigh Criterion; a vibrating flame instability where the flame sheet exhibits mode shapes; and a pulsating flame instability driven by heat losses to the flame stabilizer. All these instabilities affect the heat released to the gas in the combustor. The energy from the oscillating heat couples with the acoustics of the volume bounded by the tube structure. The experimental results in the study are important in order to obtain model parameters for prediction as well as for achieving control of the instabilities. / Master of Science

Plasmas involving strong electron drift in crossed electric and magnetic fields are of great interest for a number of applications such as space propulsion and material processing plasma sources. Specific applications include Hall thrusters, which are high efficiency, low thrust propulsion systems used on many missions for satellite orbit corrections and for future planned interplanetary missions, as well as magnetrons of various configurations used in plasma deposition devices. Similar conditions also exist in the E-layer of the ionosphere and on the Sun. Despite many successful applications of Hall thrusters and other Hall plasma sources, some aspects of their operation are still poorly understood. A particularly important problem is the anomalous electron transport, which greatly exceeds classical collisional values. Hall plasma devices exhibit numerous turbulent fluctuations in a wide frequency range and it is believed that fluctuations resulting from plasma instabilities are likely one of the main causes of the observed anomalous transport. Plasma turbulence also affects many other important processes such as electron injection, location of the ionization region and wall erosion among others that influence the operation and efficiency of Hall thrusters. In Hall thrusters, the E0xB0 flow is made unstable due to gradients in the plasma density, temperature and magnetic field. The gradient drift instabilities are long wavelength instabilities that propagate in the azimuthal direction. A fluid theory of these unstable modes is proposed. It is shown that a full account of the compressibility of the electron flow in inhomogeneous magnetic field leads to quantitative modifications of the previously obtained instability criteria and characteristics of the unstable modes. The ExB drift also drives ion sound type instabilities in Hall thrusters. The reactive/dissipative response of the closure current to the thruster walls drives these negative energy modes. A model for this type of instabilities is proposed and analyzed for typical Hall thruster conditions. It is shown how wall parameters modify the characteristic growth rate and frequency of the unstable modes and the related anomalous transport. Nonlinear phenomena are important to understand different aspects of the Hall thruster plasma dynamics. A nonlinear fluid model for the typical Hall thruster plasma is proposed. The model takes into account electron inertia, electron collisions with neutrals, density gradients as well as various nonlinear terms that arise from the electron drift and nonlinear polarization that were included via the gyroviscous cancellation. The proposed model includes the long wavelength and the low hybrid modes destabilized by density gradients and collisions. This system of fluid equations was implemented using the computational framework BOUT++ from which a set of nonlinear simulations of plasma turbulence was performed. It is shown from these first principles nonlinear simulations that small scale low hybrid oscillations result in an anomalous electron current significantly exceeding the classical collisional current.