Thesis (M.S. in Aeronautical Engineering)--Naval Postgraduate School, September 2002. / Thesis advisor(s): Kevin D. Jones, Max F. Platzer. Includes bibliographical references (p. 61). Also available online.

abstract: While much effort in Stirling engine development is placed on making the high-temperature region of the Stirling engine warmer, this research explores methods to lower the temperature of the cold region by improving heat transfer in the cooler. This paper presents heat transfer coefficients obtained for a Stirling engine heat exchanger with oscillatory flow. The effects of oscillating frequency and input heat rate on the heat transfer coefficients are evaluated and details on the design and development of the heat exchanger test apparatus are also explained. Featured results include the relationship between overall heat transfer coefficients and oscillation frequency which increase from 21.5 to 46.1 Wm-2K-1 as the oscillation frequency increases from 6.0 to 19.3 Hz. A correlation for the Nusselt number on the inside of the heat exchange tubes in oscillatory flow is presented in a concise, dimensionless form in terms of the kinetic Reynolds number as a result of a statistical analysis. The test apparatus design is proven to be successful throughout its implementation due to the usefulness of data and clear trends observed. The author is not aware of any other publicly-available research on a Stirling engine cooler to the extent presented in this paper. Therefore, the present results are analyzed on a part-by-part basis and compared to segments of other research; however, strong correlations with data from other studies are not expected. The data presented in this paper are part of a continuing effort to better understand heat transfer properties in Stirling engines as well as other oscillating flow applications. / Dissertation/Thesis / M.S. Mechanical Engineering 2011

This thesis contains an experimental campaign on the practical implementation of oscillating-foil technologies. It explores two possible engineering applications of oscillating-wings: thrust-generation, and energy-extraction. The history of, benefits of, and difficulties involved in the use of oscillating-foils is discussed throughout. Many existing technologies used for thrust generation and hydrokinetic energy extraction are based on rotating blades or foils, which have evolved over decades of use. In recent years, designs that use oscillating-foils, with motions analogous to the flapping of a fish’s tail or a bird’s wing, have shown increased hydrodynamic performance compared to the traditional rotary technologies. However, these systems are complex, both in terms of the governing unsteady fluid dynamics, and the methods by which kinematics are prescribed. Simply put, system complexity and cost need to be reduced before these devices see wide-spread use. For this reason, the work contained within this thesis explores possible methods of reducing the complexity of oscillating-foil systems in an effort to contribute to their development. For thrust-generation applications, this entailed using flexible foils to create passive pitching kinematics. This was parametrically studied by testing foils of different structural properties under a range of kinematics. The results suggested that properly tuning the flexibility of the foil could enhance both the thrust generation, and the efficiency of the propulsive system. With respect to energy-harvesting applications, the reliability of a novel fully passive turbine was assessed. The prototype tested had no active control strategy, and the degreesof-freedom were not mechanically linked, greatly simplifying the design. The prototype was subjected to real-world conditions, including high turbulence levels and the wake of an upstream turbine, and displayed robust performance in most conditions. In both applications, the hydrodynamic performance of the oscillating-wings was directly measured, and particle image velocimetry was used to observe the flow topology in the wakes and boundary layers of the foils. The vortex and stall dynamics were highlighted as key flow features, and are studied in detail. / Graduate

As propriedades da turbulência induzida em um fluido viscoso através da oscilação de uma grade foram investigadas experimentalmente utilizando a técnica DPIV (Digital Particle Image Velocimetry). Perfis verticais da energia cinética turbulenta, taxa de dissipação de energia, viscosidade turbulenta e escala de comprimento foram obtidos e comparados com soluções analíticas existentes. Os dados experimentais mostram uma boa concordância com as predições teóricas. Adicionalmente, foi observado que há uma variação significativa da intensidade turbulenta para distâncias próximas da grade. Com o aumento da distância, a turbulência se torna mais homogênea no plano horizontal, e estas variações tendem a desaparecer. Os resultados mostram também que o escoamento somente pode ser considerado livre de cisalhamento para grandes distâncias da grade, onde as tensões de Reynolds se aproximam de zero. Além disso, descobriu-se que são necessários no mínimo 450 campos de vetores para assegurar a precisão das medidas. Também foram realizadas medidas de turbulência em uma configuração de duas grades. Os resultados mostram que a região central entre as duas grades possui certas propriedades similares à turbulência isotrópica. Adicionalmente, foram realizadas comparações com o modelo k-&#949, mostrando que o mesmo pode ser aplicado a uma configuração de duas grades. / The properties of turbulence induced in a viscous fluid by oscillating a grid were investigated experimentally using the digital particle image velocimetry (DPIV). Vertical profiles of the turbulent energy, the dissipation rate, the eddy viscosity and a lengthscale have been obtained and compared with available analytical solutions. The experimental data agree well with the theoretical predictions. In addition, it could be observed that there are significant variations of the turbulent intensities at a small distance from the grid. With increasing distance from the grid, the turbulence becomes more homogeneous in the horizontal plane, and the variations in the turbulent intensities are reduced. The results also show that the turbulence can be considered to be shear-free only at a distance sufficiently far away from the grid, where the Reynolds shear stress aproaches zero. It became evident, furthermore, that at least 450 vector maps should be taken to ensure the accuracy of the measured velocity fluctuations. Turbulence was also measured for a two-grid configuration. It was found that the central region between the grids has certain properties similar to that of isotropic turbulence. The results show good agreement with the k-&#949 model, validating the application of the k-&#949 model to a pair of oscillating-grids.

Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Highly-loaded turbomachine blades can stall under off-design conditions. In this regime, the flow can separate close to the leading edge of the blade in a periodic manner that can lead to blade vibrations, commonly referred to as stall flutter. Prior experimental studies on stall flutter have been at large Re (Re ~ 106). In the present work, motivated by applications in Unmanned Air Vehicles (UAV) and Micro Air Vehicles (MAV), we study experimentally the forces and flow fields around an oscillating blade at low Re (Re ~ 3 x 104). At these low Re, the flow even over the stationary blade can be quite different. We experimentally study the propensity of an isolated symmetric and cambered blade (with chord c) to undergo self-excited oscillations at high angles of attack and at low Reynolds numbers (Re ~ 30, 000). We force the blade, placed at large mean angle of attack, to undergo small amplitude pitch oscillations and measure the unsteady loads on the blade. From the measured loads, the direction and magnitude of energy transfer to/from the blade is calculated. Systematic measurements have been made for varying mean blade incidence angles and for different excitation amplitudes and frequencies (f). These measurements indicate that post stall there is a possibility of excitation of the blade over a range of Strouhal Numbers (St = fc/U) with the magnitude of the exciting energy varying with amplitude, frequency and mean incidence angles. In particular, the curves for the magnitude of the exciting energy against Strouhal number (St) are found to shift to higher St values as the mean angle of attack is increased. We perform the same set of experiments on two different blade shapes, namely NACA 0012 and a compressor blade profile, SC10. Both blade profiles show qualitatively similar phenomena. The flow around both the stationary and oscillating blades is studied through Particle Image Velocimetry (PIV). PIV measurements on the stationary blade show the gradual shift of the flow separation point towards the leading edge with increasing angle of attack, which occurs at these low Re. From PIV measurements on an oscillating blade near stall, we present the flow field around the blade at different phases of the blade oscillation. These show that the boundary layer separates from the leading edge forming a shear layer, which flaps with respect to the blade. As the Strouhal number is varied, the phase between the flapping shear layer and the blade appears to change. This is likely to be the reason for the observed change in the sign of the energy transfer between the flow and the blade that is responsible for stall flutter.

In Tribology, the well-known Stribeck curve is often used to relate friction behaviour to the properties of a system. Richard Stribeck, (Jacobson, 2003), (Stribeck, 1902) developed these curves while researching various bearings and found that the Stribeck curve can be generated for all tribological contacts of the Hertzian type. These curves give a relation between the coefficient of friction and the Sommerfeld number for a lubricant and given surfaces. All his test work was done on contacts that move in a single direction, unidirectional motion. This leaves the question, could Stribeck curves be applied to reciprocating contacts and how does linear velocity affect wear in these contacts? This research project is aimed to further the knowledge on how linear velocity affects reciprocating contacts with a focus on Stribeck curves and lubricity. Tests were conducted on two reciprocating instruments using ball and disk configurations. Two parameters were varied to change the linear velocity, namely oscillating frequency, and stroke length. To shift focus away from viscosity, n-Hexadecane was used as the base fluid due to its lack of lubrication properties. To improve the base fluid lubricity 3 carboxylic acids with 3 different chain lengths were used as additives. / Dissertation (MEng (Chemical Engineering)) Univercity of Pretoria, 2021. / Chemical Engineering / MEng (Chemical Engineering) / Restricted

The collector of an oscillating water column system (OWC) for wave energy utilization requires a bidirectional turbine that copes with pneumatic power while providing specified impedance or, in terms of an OWC designer, “damping”. Damping is realized by keeping to a specific flow rate through the turbine at a given pressure head due to the individual performance characteristic of the turbine. With the number of turbine designs increasing designers of OWC systems are facing more options to select and dimension a bidirectional turbine. Energy yield, size and hence cost of the turbine and electric generator, operational behaviour, envisaged control strategy and noise emitted by the turbine are possible criteria for selection. The primary objective of this paper is to describe a simple procedure for making a first choice of a turbine for a particular OWC application. Here we confine ourselves to a family of reaction type of turbines (axial-flow Wells and mixed-flow turbines by Moisel) with their approximately linear pressure head/volume flow rate characteristics. Starting point is the set of non-dimensional steady-state characteristics of each turbine in the family. Utilizing standard scaling laws and a very simple time domain model for the cyclic turbine operation (i.e. based one single sea state and turbine operation assumed to be fixed rotational speed), first estimates of turbine size and rotor speed, number for stages or flows, and performance curves can be determined. The resulting turbine may also serve as a starting configuration for a refined analysis, e.g. the optimization of the turbine and the complete OWC system, utilizing more complex stochastic models. Three case studies illustrate the application of the method: selection and scaling of turbines, effect of collector parameters, turbines in series and parallel.

In this paper the popup height of rising buoyant spheres is studied over a range of distinct release depths along with the accompanying velocities and accelerations near the free surface. In the past, regimes of motion due to vortex induced vibrations have been classified based on trajectories below the free surface. This study focuses on the popup height, velocity and acceleration at free surface exit, and vortex shedding in order to further define regimes of motion experienced by a rising buoyant sphere. Varying the release depth below the free surface reveals varying exit angles, velocities, accelerations, and popup heights at surface exit. Vortex shedding prior to free surface exit causes decelerations contributing to the variation in exit velocities and resulting popup heights. Using high-speed imaging and particle image velocimetry, we examine the trajectories, accelerations, velocities and vortex shedding events for spheres of different mass ratios over a range of Reynolds number (2e4 >Re> 6e5). At lower Re, spheres released from shallow release depths result in greater accelerations and velocities at free surface exit along with greater popup heights compared to releases from deeper depths. After reaching a depth which results in a minimum popup height, further increasing the release depth reveals an increase in popup height demonstrating an oscillatory pattern due to the sphere being released from vortex forces after shedding. This pattern is repeated as the popup height again decreases with greater release depths. For spheres of greater Re, popup height increases linearly with release depth, demonstrating continued accelerations at free surface exit.

The theory which relates the natural frequency of oscillation of a drop and its surface tension was investigated. Based on this, a new experimental technique of measuring surface tension was developed. The surface tension of pure iron and pure nickel were measured over the temperature ranges, 1550-1650°C and 1475-1625°C, respectively. / Thesis / Master of Science (MSc)