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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.

In this thesis I study a variety of two-dimensional turbulent systems using a mixed analytical, phenomenological and numerical approach. The systems under consideration are governed by the two-dimensional Navier-Stokes (2DNS), surface quasigeostrophic (SQG), alpha-turbulence and magnetohydrodynamic (MHD) equations. The main analytical focus is on the number of degrees of freedom of a given system, defined as the least value $N$ such that all $n$-dimensional ($n$ ≥ $N$) volume elements along a given trajectory contract during the course of evolution. By equating $N$ with the number of active Fourier-space modes, that is the number of modes in the inertial range, and assuming power-law spectra in the inertial range, the scaling of $N$ with the Reynolds number $Re$ allows bounds to be put on the exponent of the spectrum. This allows the recovery of analytic results that have until now only been derived phenomenologically, such as the $k$[superscript(-5/3)] energy spectrum in the energy inertial range in SQG turbulence. Phenomenologically I study the modal interactions that control the transfer of various conserved quantities. Among other results I show that in MHD dynamo triads (those converting kinetic into magnetic energy) are associated with a direct magnetic energy flux while anti-dynamo triads (those converting magnetic into kinetic energy) are associated with an inverse magnetic energy flux. As both dynamo and anti-dynamo interacting triads are integral parts of the direct energy transfer, the anti-dynamo inverse flux partially neutralises the dynamo direct flux, arguably resulting in relatively weak direct energy transfer and giving rise to dynamo saturation. These theoretical results are backed up by high resolution numerical simulations, out of which have emerged some new results such as the suggestion that for alpha turbulence the generalised enstrophy spectra are not closely approximated by those that have been derived phenomenologically, and new theories may be needed in order to explain them.

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.

Numerical simulations of magnetohydrodynamic (MHD) hypersonic flow are presented for both laminar and turbulent flow over a cylinder and flow entering a scramjet inlet. ANSYS CFX is used to carry out calculations for steady flow at hypersonic speeds (Mach number > 5). The low magnetic Reynolds number (<

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.

We explore, via analytical and numerical methods, the Kelvin-Helmholtz instability in relativistic magnetized plasmas, with applications to astrophysical jets. We solve the single-fluid relativistic magnetohydrodynamic (RMHD) equations in conservative form using a scheme which is fourth-order in space and time. To invert the RMHD equations, we use a highly accurate, rapidly-convergent algorithm which improves upon such schemes as the Newton-Raphson method. Though the exact RMHD equations are marginally stable, numerical discretization renders them unstable. Artificial viscosity restores numerical stability, but also models physical processes (momentum transfer, conductivity). We use a two-dimensional slab geometry with periodic boundary conditions in both directions. The background velocity peaks along the central axis and vanishes asymptotically at the transverse boundaries. Remaining background quantities are uniform, with a flow-aligned background magnetic field. In both the linear and nonlinear analysis, we have successfully unified the HD, MHD, RHD, and RMHD regimes.The early nonlinear regime corresponds to the formation of counter-rotating vortices, connected by filaments, which persist in the absence of a magnetic field. A magnetic field inhibits the vortices through a series of stages, namely, field amplification, vortex disruption, turbulent breakdown, and an approach to a flow-aligned equilibrium configuration. Similar stages have been discussed in MHD literature. We examine how and to what extent these stages manifest in RMHD for a set of representative field strengths. To characterize field strength, we define a relativistic extension of the Alfvenic Mach number MA. We observe close complementarity between flow and magnetic field behavior. Weaker fields exhibit more vortex rotation, magnetic reconnection, jet-broadening, and intermediate turbulence. Sufficiently strong fields (MA < 6) completely suppress vortex formation. Maximum jet deceleration, and viscous dissipation, occurs for intermediate vortex-disruptive fields, while maximum electromagnetic energy extraction occurs for the strongest fields which allow vortex formation. Our results are qualitatively similar to observations of numerous jets, including M87, whose knots may exhibit vortex-like behavior.Highly relativistic flows destabilize the system, supporting modes with near-maximum growth at smaller wavelengths than the shear width of the velocity. This helps to explain early numerical breakdown of highly relativistic artificial viscosity simulations, a long-standing problem.

The present work addresses performance optimization of a small-scale, electric discharge excited, gasdynamic oxygen iodine laser (DOIL). For this, (i) nitric oxide has been added to the laser mixture, and (ii) iodine vapor was dissociated in an auxiliary electric discharge prior to its injection into the laser flow. The addition of NO has a significant effect on the laser performance, increasing small signal gain in the supersonic laser cavity from 0.05 %/cm to 0.08 %/cm. On the other hand, although large iodine dissociation fractions in the laser cavity have been achieved using an auxiliary discharge (up to 50%), only modest increase in gain was detected. The DOIL laser apparatus has been scaled up, with the main electric discharge volume increased by a factor of four and the flow rate through the laser doubled. The scaled-up laser has been tested using a nanosecond pulser / DC sustainer discharge or a capacitively coupled radio frequency discharge (CCRF) sustained at powers up to 2.7 kW and 4.5 kW, respectively. In both these cases, single-delta oxygen yield of up to 3-4% has been measured. Small signal gain up to 0.116 %/cm has been measured in the laser cavity while using the CCRF discharge to generate singlet delta oxygen. Numerical modeling of magnetohydrodynamic deceleration of a low-temperature M=4 flow was conducted using a three-dimensional compressible Navier-Stokes flow code. The results are in good agreement with recent experiments conducted at Ohio State, where flow deceleration by up to 2% has been demonstrated.