Thesis (Ph.D.)--University of Iowa, 2009. / Thesis supervisor: Craig A. Kletzing. Includes bibliographical references (leaves 129-133).

No description available.

This thesis describes the analysis, fabrication and testing of a novel magnetohydrodynamic plasma actuator for aerodynamic flow control, specifically, retreating blade stall. A magnetohydrodynamic plasma actuator is comprised of two parallel rail electrodes embedded chord-wise on the upper surface of an airfoil. A pulse forming network generates a low-voltage, high-current repetitive pulsed arc. Self-induced electromagnetic fields force the pulsed arc along the length of the rail electrodes at high velocities, transferring momentum to the surrounding air, creating a high-velocity pulsed air wall jet. A systematic experimental investigation of the effect of plasma actuators on the surrounding air is conducted in stagnant air conditions to gain an understanding of the physical characteristics. These characteristics include voltage and current measurements, pulsed arc velocity measurements, and high speed video imaging. The results show typical pulsed arc velocities of about 100 m/s can be induced with discharge energies of about 300 J per pulse. Additional experimental studies are conducted to quantify the performance of the pulsed arc for potential use in subsonic flow control applications. To gain an estimate of the momentum transferred from the pulsed arc to the surrounding air the plasma actuator is placed in a subsonic open-circuit wind tunnel at a Reynolds number of 4.5 x 105. The induced velocity of the pulsed wall jet is measured using a Laser Doppler Anemometer. The measurements show that the pulsed arc creates a high-velocity pulsed wall jet that extends 40 mm above the airfoils surface and has an induced velocity of 15 m/s greater than the unaltered air flow over the airfoil, with peak velocities of 32 m/s. The magnetohydrodynamic plasma actuator proved to induce velocities an order of magnitude greater than the velocities attained by current state-of-the-art plasma actuators. Moreover, the RailPAc is found to posses the potential for alleviation of retreating blade stall. Future work will include experiments to gain a detailed understanding of the improvements to the static stall angle, the optimal actuator geometry, excitation duty cycle, magnetic field augmentation, and behavior of the plasma armature at high Mach/Reynolds numbers. Particle Image Velocimetry (PIV) will be utilized to improve the induced flow velocity measurements acquired with the LDA. / text

No description available.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1984. / Typescript. Vita. Includes bibliographical references.

Thesis (M.S.)--West Virginia University, 2006. / Title from document title page. Document formatted into pages; contains iv, 22 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 22).

This thesis is concerned with the effects of magnetic atmospheres on solar oscillations. The behaviour of magnetohydrodynamic surface waves propagating on a single magnetic interface is discussed ignoring the effects of gravity. The effects of non-parallel propagation (where the wave vector is at an angle to the magnetic field direction) are considered. The effects of chromospheric magnetic fields on solar p- and f-modes in a stratified atmosphere are examined for three different models. In the first of these models, the chromosphere is assumed to be isothermal and permeated by a uniform and horizontal magnetic field. A dispersion relation for the p-modes trapped below such an atmosphere is derived. Asymptotic and numerical solutions for the p-modes are discussed in detail. An increase in chromospheric magnetic field strength leads to an increase in the frequency of the p-modes, whereas an increase in the chromospheric temperature leads to a decrease in the frequencies of these modes. Comparison with observational data suggests that both these effects may indeed take place. The second model is set up for magnetic fields which decrease with height in such a way that the Alfven speed remains constant. In addition to magnetic effects, the effects of non-parallel propagation on and f-modes are considered in the presence of such a non-uniform magnetic field. After deriving a very general dispersion relation, various asymptotic and numerical solutions have been obtained and the possible effects of magnetic fields and non-parallel propagation on these modes are examined. The presence of a horizontal non-uniform chromospheric field produces changes in the frequencies of the p- and f-modes, reducing the frequencies of p-modes and increasing the frequency of the f-mode. Besides depending upon magnetic field strength, frequencies also depend on both the mode's order n and its degree l. The effects of non-parallel propagation are found to be most significant for the f-mode and the low order p-modes. The magnetic structure of the chromosphere has been further generalised by combining the two models described above. In this three layer model, a dispersion relation is derived in a general manner and discussed in detail for the p-modes. The role of magnetoacoustic cut-off frequency is studied. Again, the results are qualitatively similar to those found from observation.

Thesis (Ph.D.) - University of St Andrews, March 2007.

Thesis (Ph.D.)--University of California, Berkeley, 1961. / Errata sheet at end. Includes bibliographical references (p. 102-104).

Self-organising processes occurring in isotropic turbulence and homogeneous magnetohydrodynamic (MHD) turbulence are investigated in relation to the stability of helical flow structures. A stability analysis of helical triad interactions shows that compared to hydrodynamics, equilibria of the triadic evolution equations have more instabilities with respect to perturbations on scales larger than the characteristic scale of the system. Some of these instabilities can be mapped to Stretch-Twist-Fold dynamo action and others to the inverse cascade of magnetic helicity. High levels of cross-helicity are found to constrain small-scale instabilities more than large scale instabilities and are thus expected to have an asymmetric damping effect on forward and inverse energy transfer. Results from a numerical investigation into the influence of helicity on energy transfer and dissipation are consistent with this observation. The numerical work also confirms the predictions of an approximate method describing the Reynolds number dependence of the dimensionless dissipation coefficient for MHD turbulence. These predictions are complemented by the derivation of mathematically rigorous upper bounds on the dissipation rates of total energy and cross-helicity in terms of applied external forces. Large-scale helical flows are also found to emerge in relaminarisation events in direct numerical simulations of isotropic hydrodynamic turbulence at low Reynolds number, where the turbulent fluctuations suddenly collapse in favour of a large-scale helical flow, which was identified as a phase-shifted ABC-flow. A statistical investigation shows similarities to relaminarisation of localised turbulence in wall-bounded parallel shear flows. The turbulent states have an exponential survival probability indicating a memoryless process with a characteristic lifetime, which is found to depend super-exponentially on Reynolds number akin to well-established results for pipe and plane Couette flow. These and further similarites suggest that the phase space dynamics of isotropic turbulence and wall-bounded shear flows are qualitatively similar and that the relaminarisation of isotropic turbulence can also be explained by the escape from a chaotic saddle.