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 20 of 20 for “"Perturbation equations"”.
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A source-extraction based coupling method for computational aeroacoustics
… are separated artificially. A set of acoustic perturbation equations is derived by decomposing all flow variables into their dominant part and their fluctuating part, and neglecting some small-magnitude terms, and further simplified into a set of isentropic perturbation equations. Accompanying …
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Covariant perturbations in f (R) - gravity of multi-component fluid cosmologies
We study the evolution of scalar cosmological perturbations in the 1+3 Covariant Gauge-Invariant formalism for generic f(R) theories of gravity. Working in the energy frame of the total matter, we give a complete set equations describing the evolution of matter and curvature fluctuations for a …
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Estudo analítico da convecção natural junto a uma placa inclinada para números de Prandtl baixos
The present work performs a perturbation analysis of laminar free convection about an inclined isothermal plate, using the classical boundary layer solution as the zeroth-order perturbation. The analysis is extended to second-order perturbations and the effects of the additional terms considered in …
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Cosmological Models and Observation
… of cosmology. We introduce gauge-invariant perturbation theory to derive the linear perturbation equations and use a matrix formalism to find suitable initial conditions for numerical integration. With the cosmological software package Cmbeasy we compute model predictions and compare these …
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Rotordynamic analysis of circumferentially grooved annular pump seals with turbulent flow and inlet swirl
… and land regions of the seal. The governing equations are derived using Hirs’ turbulent lubrication theory. The equations are expanded to yield zeroth and first order perturbation equations for small rotor displacements about a centered position. The leakage and velocity distribution is …
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Analysis of various forced oscillation techniques for obtaining dynamic stability derivatives
… angle of attack are described. From the general equations of motion of an aircraft in space, the perturbation equations of motion are derived for applying to a wind tunnel aircraft model. Data Reduction and test procedures for important dynamic tests are evolved systematically from the restrained …
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A characteristic mode perturbation approach for antenna loading design
… In this thesis, a novel characteristic mode perturbation approach is developed that provides a quantitative description of how mode eigenvalues and eigencurrents transform under reactive loading and frequency variation. Analytical equations are derived using a novel application of eigenvalue …
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Relativistic neutron stars in general relativity and fourth order gravity
… of which involves a stiff system of differential equations. In each case, we deal with stiffness by employing an IMEX Runge-Kutta scheme as opposed to the more computationally intensive fully implicit schemes or other adaptive Runge Kutta methods that may be impractical for partial differential …
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Radio and optical studies of natural and artificially stimulated geospace plasmas
… modeled using electron temperature and density perturbation equations. It is concluded that a maximum electron temperature enhancement of up to ∼5700 K was achieved on average. Possible mechanisms and improvements to the model are discussed. The artificial optical rings were produced by …
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Adaptive mesh refinement for computational aeroacoustics
… intake duct is modelled by the linearised Euler equations, while the radiation from an exhaust duct is modelled by the extended acoustic perturbation equations to suppress hydrodynamic instabilities generated in a sheared mean flow. After solving the near-field sound solution, the associated …
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Gravitational Waves From Perturbed Kerr Black Holes: Two Investigations
… the response of the black hole to external perturbations. Relying on the formalism developed by Mano, Suzuki and Takasugi, we improve and extend previous calculations of the contribution to the Green's function coming from quasinormal mode residues in the complex frequency plane (“excitation …
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Leveraging numerical relativity formulations for perturbative analysis: applications to quasinormal modes in general relativity
… and other nonlinear phenomena. At the same time, perturbation theory provides a powerful analytical framework for understanding small deviations from equilibrium configurations, offering insights into gravitational wave emission, stability, and fundamental mode structures of relativistic systems. …
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Position and vibration control of flexible space robots
… elastic vibration of the arms and the rigid-body perturbations. The equations of motion governing the robot dynamics are derived by means of Lagrangian mechanics and they include actuator dynamics. Based on the assumption that the elastic deformations and the rigid-body perturbations are small …
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Hydrodynamic stability of liquid films adjacent to incompressible gas streams including effects of interface mass transfer
… Orr-Sommerfeld, temperature and concentration perturbation equations amenable to closed form analytical solutions and therefore enables one to isolate the effects of mass transfer. The present analysis considers instabilities in both gas and liquid motions and thus departs from the customary …
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PROBING THE NATURE OF COMPACT OBJECTS: SCATTERING, TIDES AND QUASINORMAL MODES
… effective field theories (WEFTs) and BH perturbation theory (BHPT), with emphasis on selected scattering processes in General Relativity (GR). Scattering of two BHs in the post-Minkowskian (PM) regime has proven valuable for extracting binary dynamics from gauge-invariant scattering …
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Quasinormal Modes of Nearly Extremal Black Holes
… black holes emit radiation as a response to perturbations. After a dynamical event, for example a black hole merger, the system is expected to relax to a stationary black hole solution. After sufficient time, the system can be treated as a perturbation to this stationary solution in what is …
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An evaluation of component mode synthesis for modal analysis of finite element models
… formulations in the form of matrix eigen value equations are derived from these mode sets and describe approximately the low-frequency free vibration modes of the structure. They are improvements over previous formulations in that they can be systematically and efficiently applied to linear, …
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Numerical investigation of slat noise attenuation using acoustic liners
… impedance values by solving the linearized Euler equations (LEE) for a modeled acoustic source located at the trailing edge of the slat to find a optimized one. Using the optimized impedance value, URANS computations for the wing with liner treatment are conducted. The results show that acoustic …
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Numerical investigation of air vehicle noise propagation effects
… numerically generated database, ii) correlation equations and iii) the phase assignment method. Near field propagation of air vehicle noise requires different treatment than far field. The effects which are mainly affecting the propagation are geometrical spreading, convection and refraction …
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A theoretical study of stellart pulsations in young brown dwarfs
This thesis reports the results of a twofold study on the recently proposed phenomenon of ‘stellar pulsations’ in young brown dwarfs by the seminal study of Palla and Baraffe (2005) (PB05, thereafter). The PB05 study presents results of a non-adiabatic linear stability analysis showing that young …