The forward and reverse flow characteristics have been measured for Zobel type vortex diode with 19 mm throat diameter using superheated stea with inlet pressures up to 12 bar absolute, and exhausting into a subatmospheric condenser to achieve inlet to outlet pressure ratios up to 30 A discharge factor, Cf, which relates the measured mass flow rate to the theoretical mass flow rate of a critical flow through a comparable isentropic nozzle, has been used to describe the resistance of the diode. In the choked region of the forward and reverse flow, Cf has a constant value of 0.95 and 0.38 respectively. The critical pressure ratios in the forward and reverse flow are approximately 2 and 4. Repeating the tests using wet steam with known dryness fractions, ha shown separately the effects of wetness on the diode performance. The forward flow discharge factor in the choked condition is seen to be independent of dryness fraction, and found to lie between 0.9 and 1.0, which is similar to that found with superheated steam. The reverse flow discharge factor in the choked condition varied from about 0.4 for a dryness fraction of, 0.98, to 0.48 for a dryness fraction of 0.92. Excessive steam wetness (quality less than 0.93) in the reverse flow direction led to a build-up of water and when this was eventually swept through to the diode, the resistance was seen to fall substantially as th strong internal vortex was destroyed. This problem can be overcome in practice, however, by installing a water separator before the diode. Reverse flow characteristics of four 10 mm throat diameter Zobel typ vortex diodes have been measured using air with inlet pressure up to 31 bar absolute, and exhausting into atmosphere. The characteristics are seen to be similar to those found with superheated steam, with a value of Cf of 0.38. The effect of installing the four diodes in series was investigated. It was shown that by sharing the pressure drop between the diodes and moving the operating point into the incompressible regime, som of the high resistance performance could be recovered. A detailed study of vortex flow was carried out using a large vortex throttle with superheated steam as the working fluid. The static pressur distribution has been determined experimentally both across the vortex an along the axis of the chamber exit duct. The chamber internal wall temperatures have been obtained using insulated, flush-mounted thermocouples. The measurements enabled the velocity field to be calculated. The bulk of the internal vortex was found to have an exponer. of 0.69. It was found that the vortex throttle choked at an upstream to downstream pressure ratio of about 6 with corresponding Cf value of 0.28. The resistance of vortex chambers is known to be strongly influenced by the presence of reversed flow in the exit, due to vortex breakdown. Schlieren photography of the swirling exhaust flow was used to show that whilst vortex breakdown does occur, it can only do so after the flow has become subsonic downstream of the exit and cannot therefore influence the vortex chamber resistance.

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Reducing the deleterious effect of Vortex Induced Vibrations (VIV) in marine risers is an important task for ocean engineers; and many competing factors exist in the design of VIV suppression devices. This thesis explores the experimental minimization of the drag force and the disruption of the vortex formation by utilizing VIV suppression devices. Two series of tests are conducted- both utilizing separate testing designs. The first tests are the flexible cylinder experiments, detailed in Chapter 2, which determine the drag force and vibration amplitude of numerous, original testing configurations. The second series of tests are the rigid cylinder, PIV experiments, detailed in Chapter 3. These rests measure both the drag force on the cylinder and the oscillating component of the lift force, the latter of which is a good indication of vortex formation. The Chapter 3 tests also image the test section wake- providing helpful insight into the physical process of vortex formation. / Contract number: N62271-97-G-0026.

A new method for dynamic chirality control of a magnetic vortex is demonstrated with micromagnetic simulations. Spin transfer torque and giant magnetoresistance in an asymmetric spin valve nanopillar provide fast, reliable, and compact single-bit manipulation and readout. Magnetization relaxation following chirality switching proceeds via formation and dissipation of spin wave eigenmodes. Combined time- and frequency-domain analysis reveals a novel radial eigenmode spectrum with large edge amplitudes and nonuniform phase in the fundamental mode, in contrast with existing analytical models and experimental precedents. With the aim to determine the sources of this departure, we implement signal processing methods to identify and characterize the effects of interlayer coupling and nanoscale spatial confinement on the magnetization dynamics. Variation of the interlayer coupling and relative chirality is found to modify the eigenfrequencies but not the eigenfunctions. Examination of the interlayer phase and dynamic stray field provides quantitative and qualitative explanation of frequency splitting with relative chirality. / Graduate / 0611, 0607

Approved for public release; distribution is unlimited / Motivation to expand the understanding of a helicopter rotor descending into the vortex ring state (VRS) stems from the aircraft mishaps that have plagued the helicopter community. The V-22 has become the most recent victim of encounters with VRS. The onset of VRS is associated with the collapse of the helical vortex wake in the plane of the rotor. The resulting wake disturbances develop an irregular and aperiodic flow. Rotor blade interaction with the disturbed vortices causes large variations in the blade spanwise aerodynamic load distribution. Harmonic analysis of the loading indicates that higher harmonic content becomes prevalent in this state. The dynamic flow similarities achieved in a water tunnel are used to explore flow visualization and conduct vibration analysis of a rotor system operating in the VRS. A scaled rotor system was operated in the NPS Aeronautical Engineering Department's water tunnel. Sensors were used to gather thrust and vibration power spectrum data when operating in VRS. Experimental results correlate with full scale flight data and show a significant increase in the vibration levels of the even multiples of the blade passage frequency. The relative strength of these higher harmonics can be used as an indicator of impending VRS encounters. / Major, United States Marine Corps

Approved for public release, distribution unlimited / An investigation of vortex shedding downstream of a cascade of compressor stator blades, at off-design inlet-flow angles of 35, 33 and 31 degrees and Reynolds numbers, based on chord length, of 625,000, 750,000 and 800,000 is reported. The objective of the study was to characterize the flow and vortex shedding through blade surface pressure measurements and hot-wire anemometry. Vortex shedding was determined to be a leading edge phenomenon as periodic shedding was only detected on the pressure side of the wake. The relationship between vortex shedding frequency and Reynolds number was nearly linear. The vortex shedding frequency at three incidence angles was observed to be quite similar at lower Reynolds number (i.e. 450,000 and below) but developed into a larger scatter at higher Reynolds number. Similarly, the Strouhal numbers were observed to be fairly consistent (0.22 to 0.24) at low Reynolds number and more scattered (0.18 to 0.25) with increasing Reynolds number. The result obtained was comparable to the experimental results obtained by Roshko[Ref. 14], for vortex shedding behind a circular cylinder. / Major, Republic of Singapore Air Force

La thèse présentée dans ce document concerne la caractérisation du comportement d’une hydrolienne à membrane ondulante. Ce dispositif novateur utilise les instabilités de flottement d’une membrane semi-rigide précontrainte dans un écoulement pour capter l’énergie des courants marins. À partir d’une certaine vitesse critique de l’écoulement, une onde se propage le long de la structure, ce qui actionne des convertisseurs linéaires fixés sur celle-ci. Un modèle expérimental à échelle réduite 1/20eme est développé et testé en bassin d’essai. Le système de conversion est simulé par des amortisseurs hydrauliques. Les mesures de trajectoire et d’efforts permettent d’analyser la dynamique de la membrane pour un grand nombre de configurations et d’aboutir à une étude paramétrique. Des conditions réalistes d’écoulement sont étudiées, notamment l’influence de la direction du courant et celle de la houle sur le fonctionnement du système. La caractérisation du sillage est effectuée par mesures de vélocimétrie laser en 2D. Les effets d’échelle sont présentés à travers une comparaison des essais en bassin et en mer. Un modèle analytique et un modèle numérique sont développés et comparés avec les résultats d’expériences. Le modèle analytique linéaire est basé sur la théorie des poutres de Euler-Bernouilli et la théorie des profils minces de Lighthill. Il est résolu dans le domaine fréquentiel et donne de bons résultats en termes de fréquence d’ondulation et de vitesse critique. Le modèle numérique est fondé sur le couplage fort entre un code fluide basé sur la méthode vortex et un code structure utilisant les éléments finis en corotationnel. Ce modèle est validé sur un cas expérimental. / The thesis presented in this document deals with the characterization of the behavior of an undulating membrane tidal energy converter. This kind of device uses the flutter instabilities occurring between a semi-rigid pre-strained membrane and a fluid flow in order to convert the sea currents energy. Above a certain critical flow speed, the structure undulates, thus activating the linear converters fixed on it. In order to study this system, an experimental model is developed and tested in a flume tank. The power conversion system is simulated by hydraulic dampers. The membrane’s dynamics is analyzed in many configurations through trajectory and force measurements, and leads to a parametric study. More realistic flow conditions are also studied, such as the impact of current direction influence and the influence of surface waves on the behavior of the system. Wake characterization is carried out with two-dimensional PIV measurements. Scale effects and confinement are also studied through a comparison with a bigger scale prototype tested in tank and at sea.In addition to the experimental study, an analytical model and a numerical model are developed and compared with experiments. The linear analytical model is based on Euler-Bernouilli’s beam theory and on Lighthill’s slender body theory. It is solved in the frequency domain and gives good undulation frequency and critical speed results. The numerical model uses strong interactions between a fluid code using the vortex method and a structure code based on corotationnal finite elements. This model is validated on an experimental case.