Oscillating heat pipes (OHPs) have been actively investigated since their inception due to their ability to manage high heat/heat fluxes. The OHP is a passive, wickless, two-phase heat transfer device that relies on pressure driven fluid oscillations within a hermetically-sealed serpentine channel structure. The cyclic phase-change heat transfer drives additional sensible heat transfer, and this combination causes OHPs to have high effective thermal conductivities. Many strides have been made, through both experimentation and modeling, to refine the design and implementation of OHPs. However, the main objective in OHP research has been to better understand the thermodynamic and fluid mechanic phenomena so as to enhance OHPs' thermal performance. The current work presents methods for using OHP in thermal-to-electric energy harvesting, which would allow for ‘dual-purpose’ OHP applications in which thermal management can be combined with work output. Energy harvesting occurred when a portion of the thermally-driven fluidic motion was used to generate a voltage either by electromagnetic induction or by a piezoelectric transducer imbedded in an OHP tube. For the induction approach, two methods were used to create the time-varying magnetic field required for induction. In the first, a ferrofluid was used as the OHP working fluid. Because the magnetic dipoles of the nanoparticles are randomly aligned naturally, two static, external ‘bias’ magnets were required to create a uniform magnetic field to align the particle dipoles for a non-zero magnetic flux change through a coaxial solenoid. The second method used a small rare-earth magnet confined inside a set length of an OHP channel that had a coaxial solenoid. As the OHP working fluid moved inside the harvesting channel, a portion of the fluid's momentum was transferred to the magnet, causing it to oscillate. For the piezoelectric approach, a narrow piezoelectric transducer was placed in a bow-shaped configuration along the inside of an OHP channel. Passing fluid would deflect the piezo creating a potential difference across its leads, which protruded out of the channel walls. All three of these methods successfully produced a voltage while retaining the excellent thermal performance synonymous with OHPs.

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Ocean wave energy has the potential to play a crucial role in the shift to renewable energy. In order to improve wave energy conversion techniques, a recognition of the sub-optimal nature of traditional sequential design processes due to the interconnectedness of subsystems such as the geometry, power take-off, and controls is necessary. A codesign optimization in this paper seeks to include effects of all subsystems within one optimization loop in order to reach a fully optimal design for an oscillating-surge wave energy converter. A width and height sweep serves as a brute force geometry optimization while optimizing the power take-off components and controls using a pseudo-spectral method for each geometry. An investigation of electrical power and mechanical power maximization also outlines the contrasting nature of the two objectives to illustrate electrical power maximization's importance for identifying optimality. The codesign optimization leads to an optimal design with a width of 12 m and a height of 10 m. The power take-off and controls systems are also examined more in depth to identify important areas for increased focus during detailed design. Ultimately, the codesign optimization leads to a 61.4% increase in the objective function over the optimal design from a sequential design process while also requiring about half the power take-off torque. / Master of Science / Ocean wave energy has the potential to play a crucial role in the shift to renewable energy sources. The Earth's vast oceans have immense energy potentials throughout the world, which often follow the seasonal trends of electricity demand in temperate climates. Wave energy harvesting is a technology which has been studied significantly, but has not yet experienced commercial success, partially due to the lack of convergence on a type of wave energy converter. In order to improve wave energy conversion techniques and support the convergence on a particular type, a recognition of the sub-optimal nature of traditional sequential design processes due to the interconnectedness of subsystems is necessary. A codesign optimization in this paper seeks to include effects of all subsystems within one optimization loop in order to reach a fully optimal design for an oscillating-surge wave energy converter. A width and height sweep serves as a brute force geometry optimization while optimizing the power take-off and control components for each geometry. The codesign optimization leads to an optimal design with a width of 12 m and a height of 10 m. Ultimately, the codesign optimization leads to a 62% increase in performance over the result from a sequential design process.

A power system should always operate in a balanced and stable condition at its designed frequency. Any significant upset of this balance will produce a change in the power system frequency. It is the responsibility of the monitoring and protective devices to detect and restore the system to the equilibrium operating condition at the nominal frequency as soon as it is practical to do so. An accurate measurement of both frequency deviation and rate of change of frequency will greatly facilitate the restoration process. In this thesis, a recursive algorithm for precise frequency and rate of change of frequency measurement is presented. The algorithm consists of three major steps. First, a rough frequency estimation for a data window is computed using a second order least error square approximation on the phasor angles of the input waveform. Then, a resampling based on the rough frequency estimation is carried out, followed by another second order least error square approximation to obtain the final results. The results of simulations using this approach are provided. / Master of Science

Renewable energy is a critical component in combating climate change. Ocean wave energy is a source of renewable energy that can be harvested using Wave Energy Converters (WECs). One such WEC is the floating Oscillating Water Column (OWC), which has been successfully field tested and warrants further exploration. This research implements a publicly accessible code in MatLab and SimuLink to simulate the dynamics of a floating OWC in the time domain. This code, known as the Floating OWC Iterative Time Series Solver (FlOWCITSS), uses the pressure distribution model paired with state space realization to capture the internal water column dynamics of the WEC and estimate pneumatic power generation. Published experimental results of floating moored structures are then used to validate FlOWCITSS. While FlOWCITSS seemed to capture the period and general nature of the heave, surge, and internal water column dynamics, the magnitude of the response sometimes had errors ranging from 1.5% −37%. This error could be caused by the modeling techniques used, or it could be due to uncertainties in the experiments. The presence of smaller error values shows potential for FlOWCITSS to achieve consistently higher fidelity results as the code undergoes further developments. To demonstrate the use of FlOWCITSS, geometry variations of a Backward Bent Duct Buoy (BBDB) are explored for a wave environment and mooring configuration. The reference model from Sandia National Labs, RM6, performed significantly better than a BBDB with an altered stern geometry for a 3 second wave period, indicating that stern geometry can have a significant impact on pneumatic power performance. / Master of Science / Renewable energy is a critical component in combating climate change. Ocean wave energy is a source of renewable energy that can be converted into electricity using Wave Energy Converters (WECs). One such WEC is the floating Oscillating Water Column (OWC), which has been successfully field tested and warrants further exploration. Floating OWCs are partially submerged floating structures that have an internal chamber which water oscillates in. The motions of the water displace air inside this chamber, causing the air to be forced through a high speed turbine, which generates electricity. This research develops a publicly accessible code using MatLab and SimuLink to evaluate the motions and power generation capabilities of floating OWCs. This code is then validated against physical experiments to verify its effectiveness in predicting the device's motions. This publicly accessible code, known as the Floating OWC Iterative Time Series Solver (FlOWCITSS), showed error ranging from 1.5 % - 37% for the most important motions that are relevant to energy harvesting and power generation. These errors could be caused by the numerical models used, or uncertainties in experimental data. The presence of smaller error values shows potential for FlOWCITSS to achieve consistently higher fidelity results as the code undergoes further developments. To demonstrate the use of FlOWCITSS, geometry variations of floating OWCs are explored.

Laser Doppler velocimetry, a non-intrusive flow measurement technique, was employed to experimentally investigate two-dimensional airfoil-vortex interaction. Vortices were generated by sinusoidally oscillating a NACA 0012 airfoil about its quarter-chord at a reduced frequency of k = 2.05 and an amplitude of ±10° angle of attack. The target airfoil, a NACA 63₂A015, was immersed in the wake, two chord lengths downstream of the vortex generators trailing edge. Phase-averaged velocity measurements of the flow around the target airfoil were made with the airfoil at angles of attack of α = 0° and α = 10°. A close encounter with a counterclockwise rotating vortex was observed for both angles of attack, and a head-on collision, which split the counterclockwise rotating vortex in two, was observed for α = 10°. Vorticity fields were constructed from the velocity measurements and the circulation of the vortex was evaluated throughout the interaction. The surface pressure fluctuations on the airfoil were determined by substituting the measured velocities into the Navier-Stokes equations and numerically integrating the resulting pressure gradients. Furthermore, an extensive investigation of the undisturbed wake of the oscillating airfoil was performed in order to determine the effect of oscillation frequency and amplitude on the wake development. / Ph. D.

Adsorption layers of soluble surfactants enable and govern a variety of phenomena in surface and colloidal sciences, such as foams. The ability of a surfactant solution to form wet foam lamellae is governed by the surface dilatational rheology. Only systems having a non-vanishing imaginary part in their surface dilatational modulus, E, are able to form wet foams. The aim of this thesis is to illuminate the dissipative processes that give rise to the imaginary part of the modulus. <br><br> There are two controversial models discussed in the literature. The reorientation model assumes that the surfactants adsorb in two distinct states, differing in their orientation. This model is able to describe the frequency dependence of the modulus E. However, it assumes reorientation dynamics in the millisecond time regime. In order to assess this model, we designed a SHG pump-probe experiment that addresses the orientation dynamics. Results obtained reveal that the orientation dynamics occur in the picosecond time regime, being in strong contradiction with the two states model. <br><br> The second model regards the interface as an interphase. The adsorption layer consists of a topmost monolayer and an adjacent sublayer. The dissipative process is due to the molecular exchange between both layers. The assessment of this model required the design of an experiment that discriminates between the surface compositional term and the sublayer contribution. Such an experiment has been successfully designed and results on elastic and viscoelastic surfactant provided evidence for the correctness of the model. <br><br> Because of its inherent surface specificity, surface SHG is a powerful analytical tool that can be used to gain information on molecular dynamics and reorganization of soluble surfactants. They are central elements of both experiments. However, they impose several structural elements of the model system. During the course of this thesis, a proper model system has been identified and characterized. The combination of several linear and nonlinear optical techniques, allowed for a detailed picture of the interfacial architecture of these surfactants. / Amphiphile vereinen zwei gegensätzliche Strukturelemente in einem Molekül, eine hydrophile Kopfgruppe und ein hydrophobe, meist aliphatische Kette. Aufgrund der molekularen Asymmetrie erfolgt eine spontane Adsorption an der Wasser-Luft Grenzfläche. Die Adsorptionsschicht verändert die makroskopischen Eigenschaften des Materials, z.B. die Grenzflächenspannung wird erniedrigt. Amphiphile sind zentrale Bauelemente der Kolloid- und Grenzflächenforschung, die Phänomene, wie Schäume ermöglichen. <br><br> Eine Schaumlamelle besteht aus einem dünnen Wasserfilm, der durch zwei Adsorptionsschichten stabilisiert wird. Die Stabilität der Lamelle wird durch die Grenzflächenrheologie entscheidend geprägt. Die wesentliche makroskopische Größe in diesem Zusammenhang ist das so genannte Grenzflächendilatationsmodul E. Es beschreibt die Fähigkeit des Systems die Gleichgewichtsgrenzflächenspannung nach einer Expansion oder Dilatation der Adsorptionschicht wieder herzustellen. Das Modul E ist eine komplexe Größe, in dem der Imaginärteil direkt mit der Schaumstabilität korreliert. <br><br> Diese Arbeit widmet sich der Grenzflächenrheologie. In der Literatur werden zwei kontroverse Modelle zur Interpretation dieser Größe diskutiert. Diese Modelle werden experimentell in dieser Arbeit überprüft. Dies erfordert die Entwicklung neuer experimenteller Aufbauten basierend auf nichtlinearen, optischen Techniken. Mit diesen Experimenten konnte eines der Modelle bestätigt werden.

In this thesis, we study homogenization of optimal control problems in various oscillatory domains. Specifically, we consider four types of domains given in Figure 1 below. Figure 1: Oscillating Domains The thesis is organized into six chapters. Chapter 1 provides an introduction to our work and the rest of the thesis. The main contributions of the thesis are contained in Chapters 2-5. Chapter 6 presents the conclusions of the thesis and possible further directions. A brief description of our work (Chapters 2-5) follows: Chapter 2: Asymptotic behaviour of a fourth order boundary optimal control problem with Dirichlet boundary data posed on an oscillating domain as in Figure 1(A) is analyzed. We use the unfolding operator to study the asymptotic behavior of this problem. Chapter 3: Homogenization of a time dependent interior optimal control problem on a branched structure domain as in Figure 1(B) is studied. Here we pose control on the oscillating interior part of the domain. The analysis is carried out by appropriately defined unfolding operators suitable for this domain. The optimal control is characterized using various unfolding operators defined at each branch of every level. Chapter 4: A new unfolding operator is developed for a general oscillating domain as in Figure 1(C). Homogenization of a non-linear elliptic problem is studied using this new un-folding operator. Using this idea, homogenization of an optimal control problem on a circular oscillating domain as in Figure 1(D) is analyzed. Chapter 5: Homogenization of a non-linear optimal control problem posed on a smooth oscillating domain as in Figure 1(C) is studied using the unfolding operator.

In nature, many swimming and flying creatures use the principle of oscillatory lift-based propulsion. Often the flapping element is flexible, totally or partially. The flow dynamics because of a flexible flap is thus of considerable interest. We are interested especially in lunate fish propulsion. The present work investigates the effect of trailing edge flexibility on the flow field created by an oscillating airfoil in an attempt to mimic the flow around the flexible tails often found in fish. A flexible flap with negligible mass and stiffness is attached at the trailing edge of NACA0015 airfoil. The flap length is 75% of the rigid chord length. The airfoil oscillates about a hinge point at 30% chord from the leading edge and at the same time it moves in a circular path in stationary water. The parameters varied are frequency, amplitude of oscillation and forward speed. For a given combination of amplitude and frequency of oscillation, the forward speed is chosen such that the Strouhal number comes around 0.3, which falls in the gamut of Strouhal numbers for maximum propulsive efficiency. We visualize the flow with dye and particles and measure velocities using Particle Image Velocimetry (PIV). We use shadow technique and image processing to study the flap dynamics. We do a qualitative and quantitative comparison of the wake flow generated by two airfoil models, one with rigid trailing edge (model -B) and the other with flexible trailing edge (model -A) i.e. with a flexible flap fixed to the trailing edge. We study the flap dynamics, the flow around the flap, evolution of vortices, wake width, circulations around airfoil and vortices, momentum and energy in the wake (which is measure of propulsion efficiency), vortex geometry in the wake in terms of vortex spacing, etc. We also conduct a parametric study for both the models. Flap dynamics plays a prominent role in defining the signature of the wake. The observed flap deflections are quite large and the flap exhibits more than one mode of deflection; this affects the vortex-shedding pattern. The flap tip also executes a near sinusoidal motion with a phase difference between the trailing edge and the flap tip. The dye visualization studies show that a flexible trailing edge induces multiple vortices while in case of a rigid trailing edge, large vortical structures are shed. In case of flexible trailing edge (model -A), the vortices are shed away from the mean path of motion and are arranged in a ‘reverse Karman vortex street’ pattern producing an undulating jet representing a thrust on the airfoil. For the same Strouhal number, in case of rigid trailing edge (model -B), the vortices are shed nearly along the mean path of motion indicating a momentumless wake. The wake structures, particularly in case of model -A, are nearly insensitive to variations in amplitude and frequency. The wake of model -B shows some variable flow patterns for different amplitudes of oscillation. Although the total chord of model -A is 1.75 times more than the chord of model -B, the wake width is nearly the same for the two models when the amplitude of oscillation is same. The addition of the flap to the airfoil keeps the wake flow two-dimensional or symmetric about the center plane for longer times and longer downstream distances in comparison with the wake flow generated by rigid trailing edge. For 15o and 20o amplitudes of oscillations, the flow separates over the airfoil itself; the interaction of the separated flow with the flexible flap is quite interesting, which needs further investigations. The wake generated by the airfoil with flexible flap at the trailing edge has some common features with the wakes generated by the flow over a flapping filament (which is the one-dimensional representation of a fluttering flag), an accelerating mullet fish (a carangiform swimmer) and a steadily swimming eel (an anguilliform swimmer).