The stability and turbulence characteristics of a vortex filament emanating from a single-bladed rotor in hover are investigated using proper orthogonal decomposition. The rotor is operated at a tip chord Reynolds number and a tip Mach number of 218,000 and 0.22, respectively, and with a blade loading of CT /σ = 0.066. In-plane components of the velocity field (normal to the axis of the vortex filament) are captured by way of 2D particle image velocimetry with corrections for vortex wander being performed using the Γ1 method. Using the classical form of POD, the first POD mode alone is found to encompass nearly 75% of the energy for all vortex ages studied and is determined using a grid of sufficient resolution as to avoid numerical integration errors in the decomposition. The findings reveal an equal balance between the axisymmetric and helical modes during vortex roll-up which immediately transitions to helical mode dominance at all other vortex ages. This helical mode is one of the modes of the elliptic instability. While the snapshot POD is shown to reveal similar features of the first few energetic modes, the classical POD is employed here owing to the easier interpretation of the Fourier-azimuthal modes. The spatial eigenfunctions of the first few Fourier-azimuthal modes associated with the most energetic POD mode are shown to be sensitive to the choice of the wander correction technique used. Higher Fourier-azimuthal modes are observed in the outer portions of the vortex and appeared not to be affected by the choice of the wander correction technique used. / text

This thesis proposes two novel algorithms to analyze whether the power system loses synchronism subsequent to credible contingencies. The two algorithms are based on the concept of Lyapunov exponents (LEs) and the Prony analysis respectively. The concept of LEs is a theoretically sound technique to study the system stability of nonlinear dynamic systems. The LEs measure the exponential rates of divergence or convergence of trajectories in the state space. Considering the higher computational burden associated with the convergence of the true LEs, a modified algorithm is proposed to study the transient stability of the post-fault power system. It is shown that the finite-time LEs calculated by the modified algorithm accurately predicts the said stability. If the power system is transient stable, the rotor angle trajectories of the post-fault system exponentially decay with time. The damping ratios of the dominant oscillatory modes present in these power swings provide the indication on the oscillatory stability. The improved Prony algorithm presented in the thesis can be used to identify the oscillatory stability of the power system subsequent to a contingency. It is shown that that these new algorithms can be used in two applications in power systems, online dynamic security assessment and online oscillations monitoring. The proposed algorithm for rotor angle security assessment first uses the LEs-based algorithm to identify the transient stability. The stable cases are then processed by the improved Prony algorithm. The proposed online oscillations monitoring algorithm uses an event-detection logic and a parallel filter bank before applying the improved Prony algorithm on the measured response to extract the dominant oscillatory modes and to determine their frequencies and damping ratios. The suitability of the two algorithms for the aforementioned applications is investigated using different case studies. It is shown that the computational burdens of the two algorithms are acceptable for the online applications. Furthermore, the oscillations monitoring algorithm, extracts only the dominant modes present in the input signal, extracts both low-frequency inter-area modes and sub-synchronous modes, and performs well under noisy conditions. These features make it more appropriate for wide-area monitoring of power system oscillations using synchronized measurements. / February 2016

This thesis presents numerical predictions for the leakage and direct stiffness coefficients of pocket damper seals. Modifications made to earlier flow-prediction models are discussed. Leakage and static pressure measurements on straight-through and diverging configurations of eight-bladed and twelve-bladed seals were used for code validation and for calculation of seal discharge coefficients. Higher than expected leakage rates were measured in the case of the twelve-bladed seal, while the leakage rates for the eight-bladed seals were predicted reasonably accurately. Results are presented for shake tests conducted on the seals at pressures of up to 1000 Psi (6.90 MPa). Test variables included pressure drop across the seals and rotor speed. The experimentally obtained stiffness coefficients are compared to results of a rotordynamic damper seal code, which uses the corrected mass flow-rate calculation method. Results show that the code under-predicts the magnitude of the seal's stiffness for most test cases. However, general trends in the frequency dependency of the direct stiffness are more accurately predicted. The expectation of high values of negative stiffness in diverging seals is confirmed by the results, but the frequency at which the sign of the stiffness becomes positive is considerably lower than is predicted. In addition to presenting high-pressure test data, this thesis also attempts to provide some insight into how seal parameters can be modified to obtain desired changes in seal stiffness.

In this thesis, a novel approach is presented for blade loss simulation of an aircraft gas turbine rotor mounted on rolling element bearings with squeeze film dampers, seal rub and enclosed in a flexible housing. The modal truncation augmentation (MTA) method provides an efficient tool for modeling this large order system with localized nonlinearities in the ball bearings. The gas turbine engine, which is composed of the power turbine and gas generator rotors, is modeled with 38 lumped masses. A nonlinear angular contact bearing model is employed, which has ball and race degrees of freedom and uses a modified Hertzian contact force between the races and balls and for the seal rub. This combines a dry contact force and viscous damping force. A flexible housing with seal rub is also included whose modal description is imported from ANSYS. Prediction of the maximum contact load and the corresponding stress on an elliptical contact area between the races and balls is made during the blade loss simulations. A finite-element based squeeze film damper (SFD), which determines the pressure profile of the oil film and calculates damper forces for any type of whirl orbit is utilized in the simulation. The new approach is shown to provide efficient and accurate predictions of whirl amplitudes, maximum contact load and stress in the bearings, transmissibility, thermal growths, maximum and minimum damper pressures and the amount of unbalanced force for incipient oil film cavitation. It requires about 4 times less computational time than the traditional approaches and has an error of less than 5 %.

L'objet de cette étude est de mieux formaliser et modéliser de façon générique le processus de structuration d'un produit par le procédé de foisonnement, en reliant les paramètres opératoires aux propriétés des mousses formées et de contribuer ainsi à un meilleur pilotage de l'opération. Une ligne de foisonnement par battage en continu a été instrumentée et l'évolution du diamètre des bulles en fonction des paramètres du produit et du procédé a été suivie pour des milieux modèles newtoniens et rhéofluidifiants. L'analyse dimensionnelle à l'échelle du procédé a permis d'aboutir à un modèle physique de l'opération, et donc d'avoir une compréhension des phénomènes en présence. Elle a aussi permis d'intégrer les paramètres du produit et du procédé et de simplifier la représentation des résultats expérimentaux. Enfin, la cohérence de ce modèle avec d'autres issus de la littérature et une première approche de validation avec un produit réel, semble justifier son caractère générique.

The Estimation of Thevenin Equivalent Parameters is useful for System Monitoring and Protection. We studied a method for estimating the Thevenin equivalent circuits. We then studied two applications including voltage stability and fault location. A study of the concepts of Voltage Stability is done in the initial part of this thesis. A Six Bus Power System Model was simulated using MATLAB SIMULINK®. Subsequently, the Thevenin Parameters were calculated. The results were then used for two purposes, to calculate the Maximum Power that can be delivered and for Fault Location.

Modélisation du comportement dynamique des rotors d'hélicoptèresL'objectif de la thèse est l'étude et la modélisation du comportement dynamique et aérodynamique du rotor de l'hélicoptère en considérant à la fois les forces d'inertie et les forces aérodynamiques et en tenant compte des déformations élastiques des pales. L'algorithme de couplage proposé permet d'effectuer le calcul transitoire avec échange de données entre les solveurs fluide et structure à chaque pas de temps.La particularité de cette étude est l'utilisation du modèle aérodynamique de la ligne active, qui représente les forces de pale appliquées au fluide par des termes sources. Ces termes sources sont répartis dans les cellules de maillage à l'emplacement de la pale. Ainsi, la rotation, la torsion et le battement de la pale peuvent être représentés sans aucune déformation du maillage. Un avantage de la ligne active est que la simulation utilise un nombre réduit de nœuds, car des conditions aux limites "lois des parois" ne doivent pas être modélisées.Le cas d'un petit rotor expérimental d'hélicoptère est étudié en vol d'avancement. Les solveurs de fluide et de structure sont couplés pour calculer le comportement aérodynamique et dynamique du rotor. Pour ce faire, un algorithme de couplage faible en série décalé est appliqué. Les calculs itératifs sont contrôlés par un code spécialement conçu. Au début de chaque itération, le code calcule et répartit les termes sources dans le domaine fluide. A la fin du pas de temps, le code exécute le solveur de calcul de structure pour calculer un seul pas de temps. Ce solveur calcule le déplacement de la pale sous l'effet des forces aérodynamiques, élastiques et d'inertie et renvoi les résultats au solveur fluide. Les déplacements de la pale calculés servent de référence pour le solveur fluide au pas de temps suivant, pour distribuer les termes sources. Le calcul s'arrête lorsque le critère de convergence est vérifié.Afin de valider le cas simulé, des expérimentations sont réalisées en soufflerie. La puissance et la poussée aérodynamique du rotor sont mesurées. La Vélocimétrie par images de particules (PIV) est utilisée pour obtenir le champ de vitesse autour du rotor. Les mesures PIV à phase bloqué dans des plans azimutaux ont permis de reconstituer le champ d'écoulement 3D. La comparaison entre les résultats numériques et les expériences montre un bon accord et permet de valider la méthode de couplage proposée.

Vertical axis wind turbines (VAWTs) have with time been outrivaled by the today common and economically feasible horizontal axis wind turbines (HAWTs). However, VAWTs have several advantages such as the possibility to put the drive train at ground level, lower noise emissions and better scaling behavior which still make them interesting for research. The work within this thesis is made in collaboration between the Department of Construction and Energy Engineering at Halmstad University and the Division for Electricity at Uppsala University. A 200 kW VAWT owned by the latter and situated close to Falkenberg in the southwest of Sweden has been the main subject of the research even if most learnings has been generalized to fit a typical vertical turbine. This particular turbine has a wooden tower which is semi-guy-wired, i.e. the tower is both firmly attached to the ground and supported by guy-wires. This thesis has two main topics both regarding VAWTs: eigenfrequency of the tower and the noise generated from the turbine. The eigenfrequency of a semi-guy-wired tower is studied and an analytical expression describing this is produced and verified by experiments and simulations. The eigenfrequency of the wire itself and how it is affected by wind load are also studied.  The noise characteristics of VAWTs have been investigated, both theoretically and by noise measurement campaigns. Both noise emission and frequency distribution of VAWTs has been studied. The work has resulted in analytical expressions for tower and wire eigenfrequency of a semi-guy-wired tower as well as recommendations for designing future towers for VAWTs. The noise emission of VAWTs has been studied and proven low compared to HAWTs. The noise frequency distribution of the 200 kW VAWT differs significantly from that of a similar size HAWTs with for example lower levels for frequencies below 3000 Hz.

Malgré les progrès faits dans les dernières décennies en CFD, les techniques RANS instationnaires pour les turbomachines multi-étages sont toujours très couteuses en temps de calcul, réduisant leur intérêt en conception industrielle. Grâce à une analyse de Fourier, les équations instationnaires de Navier-Stokes peuvent être considérées comme 2N+1 équations stationnaires couplées par un terme source. Cette approche calcule efficacement les écoulements instationnaires périodiques et montre de forts gains en terme de temps de calcul. Cependant, l'expression du terme source est algébrique, ce qui provoque des difficultés dans le calcul de l'inverse de la transformée de Fourier directe. Afin d'améliorer la robustesse et la précision de la méthode, une approche basée sur un échantillonnage temporel non-uniforme est adoptée. Pour réduire le domaine de calcul à un unique passage inter-aube des conditions aux limites de chrochronicité sont développées. Dans un premier temps, un rotor et un stator sont simules avec en une entrée du domaine de calcul une injection qui modélise le sillage de la roue directrice d'entrée. Ainsi, deux fréquences fondamentales sont vues par le rotor. L'influence du contenu fréquentiel dans le rotor (i.e. le nombre d'harmoniques de la fréquence de passage des roues et leurs combinaisons) est analysée. Les résultats sont valides contre ceux obtenus avec des simulations instationnaires classiques. Ensuite, la méthode est appliquée a deux compresseurs industriels, le compresseur transsonique ECL4 et le compresseur d'étude CREATE.

iv In this thesis, numerical solutions of a 2D stator compressor cascade at a given inlet Mach number (0.7) and four values of incidence (49&deg / , 51&deg / , 53&deg / and 55&deg / ) are obtained. Reynolds averaged, thin layer, compressible Navier Stokes equations are solved. Different grid types have been generated. Finite differencing approach and LU - ADI splitting technique are used. Three block parallel Euler and Navier Stokes solutions are compared with the experimental results. Baldwin-Lomax turbulence model is used in the turbulent predictions and boundary layer comparisons and numerical results are in good agreement with the experiment. On the last part of the study, a rotor wake in the inlet flow has been introduced in the steady and unsteady analyses. The influence of this wake and the wake location in the inlet flow, to the total force and pressure is presented. The results have been showed that there is a relationship between the wake position and the incidence value of the case.