Global ETD Search
Search theses and dissertations gathered from participating repositories worldwide. Every result links back to the library that holds it. No account is needed.
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Showing 1 to 17 of 17 for “"parametric instability"”.
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Parametric Excitation of a DWSC
Parametric excitation of the DWSC (Deep Water Stable Craneship) is studied in this thesis. It occurs for a system without any external forcing, when one of the coefficients in the equation of motion (EOM) modeling the system varies with time. Parametric instability might be triggered for certain …
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On subharmonic instability in boundary layers
The subharmonic instability in two-dimensional boundary layer on a flat plate is analyzed using the parametric instability model and the resonant triad model. The problems arising from both models are solved numerically using the shooting technique and results are presented. It is found that in the …
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Nonlinear hydrodynamic instability and turbulence in eccentric astrophysical discs
The nonlinear evolution of the parametric instability of inertial waves inherent to eccentric discs is studied by way of a new local numerical model. Mode coupling of tidal deformation with the disc eccentricity is known to produce exponentially growing eccentricities at mean-motion resonances, …
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The instability of time-dependent jets
The central focus of my thesis is to study the instability jets of various complexity by analyzing the linear and nonlinear dynamics. We applied this methodology to four different situations in order to learn the following. First, what asymmetries develop between cyclones and anticyclones because …
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Numerical simulation of wave-plasma interactions in the ionosphere
… motion allowed or suppressed indicated that a parametric instability was responsible for irregularity production. Simulation of EM wave fields confirmed that X-mode waves are capable of exceeding the threshold for parametric instability excitation under certain conditions.
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Semi-Analytical Model to Study Vibrations of High-Speed, Rotating Axisymmetric Bodies Coupled to Other Rotating/ Stationary Structures
… to critical speeds, divergence and flutter instability phenomena, and eigenvalue veering are investigated for spinning systems with asymmetric features. The developed semi-analytical model is used for rotationally periodic systems, for example, planetary gears. Rotationally periodic systems …
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Harmonic generation and parametric excitation in a beam-plasma system
… by wave-wave and wave-particle interactions and parametric excitation of plasma and ion-acoustic waves by a beam-plasma instability are studied, theoretically and experimentally, in a cold beam-plasma system. The dispersion relations for plasma and ion-acoustic waves are calculated. Coupling …
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Simulation Studies of Parametric Processes Associated with Ionospheric Electromagnetic Radiation
Parametric instability processes are thought to produce Stimulated Electromagnetic Emissions (SEE) during ionospheric heating experiments. The phenomenon is primarily attributed to plasma turbulence excited by the high frequency HF heater in the altitude region where the pump frequency <i>ω</i>₀ is …
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Radiation pressure effects in a suspended Fabry-Perot cavity
… x 105 N/m. We also measure and characterize the parametric instability, caused by the coupling of the cavity field to the acoustic modes of the mirror, and find an instability strength R - 3.
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Passive damping of a LIGO mirror
… mirrors and excite mechanical motion. If such a parametric instability occurs, it will disrupt the operation of the interferometer. In this thesis I test a method to passively damp the acoustic modes of the mirror, using a mass-spring damper. The damper can be modeled as a piezoelectric "spring" …
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Temporal and spatial growth of subharmonic disturbances in Falkner-Skan flows
… modeling of the subharmonic disturbance as a parametric instability arising from the presence of a finite amplitude TS wave has given results in quantitative agreement with experiments conducted in a Blasius boundary-layer. The present work extends the analysis to the Falkner-Skan family of …
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Numerical simulation of stimulated electromagnetic emissions in the ionosphere
… that the DM and UM are generated by the same parametric instability involving lower hybrid waves while the BUM is produced by other different mechanisms. Boundary effects are found important on the generation of all three features in the simulation. Moreover, detailed investigation of the …
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Hydromagnetic Oscillations and Instabilities in Astrophysical Discs
… from the excitation of inertial waves by a local parametric instability provides one mechanism for curbing this growth. However, eccentric discs also provide an environment for the excitation of additional, inherently global oscillations, and I present a framework facilitating a semi-analytical …
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The structure and stability of vortices in astrophysical discs
… shearing flow inside the vortex. A ‘saddle point instability’ is seen for elliptical–streamline vortices with small aspect ratios and we also find that this is true in general. However, only higher aspect ratio vortices act as dust traps. For constant–density vortices with a concentrated vorticity …
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Theoretical and Numerical Studies of Frequency Up-shifted Ionospheric Stimulated Radiation
… upshifted maximum (BUM). In this study, the instability processes thought to be responsible to the BUM spectra in the SEE experiments are discussed and analyzed using theoretical and electrostatic particle-in-cell (PIC) models. From characteristics of this feature, a four-wave parametric …
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The effect of AC magnetic fields on liquid-metal free surfaces in industrialMHD
… modes are caused by a Mathieu-type subharmonic instability. We next consider the effect of a medium-frequency magnetic field on an initially circular pool of liquid metal. Experimentally we study the effect of a vertical alternating medium-frequency magnetic field on an initially circular pool. …
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Nonlinear Dynamics of Warped Astrophysical Discs
… the growth, saturation and feedback of a parametric instability, which feeds off the free-energy associated with the warped geometry. Using a Lagrangian-Godunov code, we perform simulations of our tilting ring and identify several locally growing modes, as predicted by a three-mode …