The thesis develops singly-periodic mathematical models for complex laminar wakes which are formed behind vortex-shedding bluff bodies. These wake structures exhibit a variety of patterns as the bodies oscillate or are in close proximity of one another. The most well-known formation comprises two counter-rotating vortices in each shedding cycle and is popularly known as the vk vortex street. Of the more complex configurations, as a specific example, this thesis investigates one of the most commonly occurring wake arrangements, which consists of two pairs of vortices in each shedding period. The paired vortices are, in general, counter-rotating and belong to a more general definition of the 2P mode, which involves periodic release of four vortices into the flow. The 2P arrangement can, primarily, be sub-classed into two types: one with a symmetric orientation of the two vortex pairs about the streamwise direction in a periodic domain and the other in which the two vortex pairs per period are placed in a staggered geometry about the wake centerline. The thesis explores the governing dynamics of such wakes and characterizes the corresponding relative vortex motion. In general, for both the symmetric as well as the staggered four vortex periodic arrangements, the thesis develops two-dimensional potential flow models (consisting of an integrable Hamiltonian system of point vortices) that consider spatially periodic arrays of four vortices with their strengths being +/-1 and +/-2. Vortex formations observed in the experiments inspire the assumed spatial symmetry. The models demonstrate a number of dynamic modes that are classified using a bifurcation analysis of the phase space topology, consisting of level curves of the Hamiltonian. Despite the vortex strengths in each pair being unequal in magnitude, some initial conditions lead to relative equilibrium when the vortex configuration moves with invariant size and shape. The scaled comparisons of the model results with experiments conducted in a flowing soap film with an airfoil, which was imparted with forced oscillations, are satisfactory and validate the reduced order modeling framework. The experiments have been performed by a collaborator group at the Department of Physics and Fluid Dynamics at the Technical University of Denmark (DTU), led by Dr. Anders Andersen. Similar experiments have also been run at Virginia Tech as part of this dissertation and the preliminary results are included in this treatise. The thesis also employs the same dynamical systems techniques, which have been applied to study the 2P regime dynamics, to develop a mathematical model for the P+S mode vortex wakes, with three vortices present in each shedding cycle. The model results have also been compared favorably with an experiment and the predictions regarding the vortex circulation data match well with the previous results from literature. Finally, the thesis introduces a novel concept of clean and renewable energy extraction from vortex-induced vibrations of bluff bodies. The slow-moving currents in the off-shore marine environments and riverine flows are beyond the operational capabilities of the more established hydrokinetic energy converters and the discussed technology promises to be a significant tool to generate useful power from these copiously available but previously untapped sources. / Ph. D.

Significant effort in the current technological development of aircraft is aimed at improving engine efficiency, while reducing fuel burn, emissions, and noise levels. One way to achieve these is to better integrate airframe and propulsion system. Tighter integration, however, may also cause adverse effects to the flow entering the engines, such as total pressure, total temperature, and swirl distortions. Swirl distortions are angular non-uniformities in the flow that may alter the functioning of specific components of the turbomachinery systems. To investigate the physics involved in the ingestion of swirl, pre-determined swirl distortion profiles were generated through the StreamVane method in a low-speed wind tunnel and in a full-scale turbofan research engine. Stereoscopic particle image velocimetry (PIV) was used to collect three-component velocity fields at discrete planes downstream of the generation of the distortions with two main objectives in mind: identifying the physics behind the axial development of the distorted flow; and describing the generation of the distortion by the StreamVane and its impact to the flow as a distortion generating device. Analyses of the mean velocity, velocity gradients, and Reynolds stress tensor components in these flows provided significant insight into the driving physics. Comparisons between small-scale and full-scale results showed that swirl distortions are Mach number independent in the subsonic regime. Reynolds number independence was also verified for the studied cases. The mean secondary flow and flow angle profiles demonstrated that the axial development of swirl distortions is highly driven by two-dimensional vortex dynamics, when the flow is isolated from fan effects. As the engine fan is approached, the vortices are axially stretched and stabilized by the acceleration of the flow. The flow is highly turbulent immediately downstream of the StreamVane due to the presence of the device, but that vane-induced turbulence mixes with axial distance, so that the device effects are attenuated for distances greater than a diameter downstream, which is further confirmed by the turbulent length scales of the flow. These results provide valuable insight into the generation and development of swirl distortion for ground-testing environments, and establishes PIV as a robust tool for engine inlet investigations. / Ph. D.

With the aim to understand the synergistic roles played by kinematics, aerodynamics, and inertia in flapping wing maneuvers, this thesis first investigates the plunging motion of a simple flat plate as it is a fundamental motion in the kinematics of many flying animals. A wide range of frequency (k) and amplitude (h) is investigated to account for a robust kinematic characterization in the form of plunge velocity (kh). Leading Edge Vortices (LEVs) are found to be responsible for producing thrust while Trailing Edge Vortices (TEVs) produce drag. The vortex dynamics becomes nonlinear for higher kh and three main vortex-vortex interactions (VVI) are identified in the flow-field. To estimate the sole effect of LEVs on thrust coefficient, TEVs are eliminated by introducing a splitter plate. This resulted in reduced non-linearity in VVI and facilitated a parametrization of aerodynamic thrust coefficient with key kinematic features, frequency (k) and amplitude (h) [C_T= A.k^1.4 h-B where A and B are constants]. This is followed by investigating the more direct problem of bio-inspired MAV research – the interplay of kinematics, aerodynamics, and inertia on maneuvering bat flights. At first, an ascending right turn of a H. pratti bat is investigated to elucidate on the kinematic features and aerodynamic mechanisms used to effectuate the maneuver. Deceleration in flight speed, an increase in flapping frequency, shortening of the upstroke, and thrust generation at the end of the upstroke is observed during this maneuver. The turn is initiated by the synergisytic implementation of roll and yaw rotation where the turning moments are generated by drawing the inside wing closer to the body, by introducing phase lags in force generation between the two wings and by redirecting force production to the outer part of the wing outside of the turn. Upon comparison with a similar maneuver by a H. armiger bat, some commonalities as well as differences were observed. This analysis was followed by a comparative study among different maneuvering flights (a straight flight, two ascending right turns, and a U-turn) in order to establish the complete motion dynamics of a maneuver in action. The individual effects of aerodynamics and wing inertia for maneuvering flights of a H. armiger and H. pratti are investigated. It is found that for both, translation and rotation the overall trajectory trend is mostly driven by the aerodynamic forces and moments, whereas inertial effects drive the intricate intra-cycle fluctuations as well as the vertical velocity and altitude gain during ascent. Additionally, inertial moments play a dominant role for effecting yaw rotations where the importance of the Coriolis and centrifugal moments increase with increasing acuteness of the maneuver, with the largest effect of centrifugal moments being evidenced in the U-turn. / Doctor of Philosophy / The study of flapping wing is of paramount interest in the field of small aerial and aquatic vehicle propulsion. The intricate mechanisms acting behind a flapping wing maneuver can be explained by the synergistic roles played by 3 main components; details of the wing motion or the kinematics, how the air reacts to the wing motion or the aerodynamics, and the effort or force required to move the wings or wing inertia. This dissertation systematically reports the contribution of these components to a flapping flight maneuver. At first, the plunging motion of a simple flat plate is investigated as it is a fundamental motion in the flapping flight of many flying animals. A wide range of frequency and amplitude is investigated and their effect is characterized by a single parameter called "plunge velocity". It is found that, the resultant flow field becomes disorderly for higher plunge velocities which can be characterized by three different types of vortex interactions. The observed results facilitated a robust parametrization of aerodynamic thrust production with key kinematic features, frequency and amplitude. After this, the dissertation focuses on the bio-inspiration aspect of flapping flight by investigating the interplay of kinematics, aerodynamics, and inertia of maneuvering bat flights. At first, an ascending right turn of one species (H. pratti) is investigated to elucidate on the kinematic features and aerodynamic mechanisms used to effectuate the maneuver. Some characteristic features observed are – lowering of flight speed, increase in flapping rate, shortening of upstrokes, and generation of a forward force at the end of the upstroke. It is observed, that the bat turns by using synergistic body rotations in multiple directions which are effected by various techniques such as - drawing the wing inside the turn closer to the body, and changing the timing and location of the forces produced between the two wings. Upon comparison with a similar maneuver by a H. armiger bat, some commonalities as well as differences were observed in the maneuver mechanisms. This analysis was followed by a comparative study among different maneuvering flights (a straight flight, two ascending right turns, and a U-turn) to establish the complete motion dynamics of a maneuver. The individual contributions of aerodynamics and wing inertia for maneuvering flights of a H. armiger and H. pratti are investigated. It is found that for both, translation and rotation the overall trajectory is mostly influenced by the aerodynamic forces and moments, whereas inertial effects are responsible for trajectory fluctuations during a flapping cycle as well contributing to altitude gain during ascent for the H. armiger bat.

Le travail présenté dans ce mémoire est une contribution à l'étude numérique des systèmes tourbillonnaires hélicoïdaux qui sont émis dans le sillage des rotors (éoliennes, hélicoptères,...) et de leurs instabilités. Ici, ces écoulements sont localement modélisés par un ensemble de tourbillons à symétrie hélicoïdale. À l'aide d'un code de simulation numérique directe dédié, des solutions de base quasi-stationnaires sont obtenues pour différents systèmes tourbillonnaires. Une caractérisation précise et détaillée de ces solutions est ensuite effectuée : vitesse de rotation, taille et ellipticité du cœur, structure des champs de vitesse et de vorticité... À l'aide d'un algorithme d'Arnoldi couplé à une version linéarisée du code, on détermine les modes dominants d'instabilité ayant la même symétrie que l'écoulement de base, en fonction des paramètres du système: nombre de vortex, pas hélicoïdal, taille de cœur, nombre de Reynolds et présence d'un vortex de moyeu. En dessous d'un certain pas hélicoïdal critique, l'instabilité est dominée par un mode de déplacement global analogue au mode d’appariement d'une allé infinie de points vortex ou d'anneaux tourbillonnaires. En régime non linéaire, ce mode est à l'origine d'une dynamique complexe du système: dépassements, saute-mouton et fusion. On utilise un autre code linéarisé pour déterminer les modes instables qui brisent la symétrie hélicoïdale de l'état de base, caractérisés par une longueur suivant l'axe. À faible nombre d'onde, ces modes induisent localement des rapprochements entre portions de spires voisines. À grand nombre d'onde, on observe un autre type de mode qui déforme les cœurs tourbillonnaires via l'instabilité elliptique. / The work presented in this manuscript is a contribution to the numerical study of helical vortex systems and their instabilities, as encountered in the near wake of rotors (wind turbines, helicopters,~...). In this work, such flows are locally modelled within the framework of helical symmetry. Using a dedicated DNS code, helical quasi-stationary basic state solutions are obtained for several configurations, and accurate tools for their characterisation are developed: angular velocity, core size and ellipticity, structure of the velocity and vorticity fields... An Arnoldi algorithm is then coupled to a linearised version of the code. The dominant instability modes with the same symmetry as the base flow are extracted as a function of the system parameters: number of vortices, helical pitch, core size, Reynolds number, presence of a central hub vortex. Under a critical helical pitch, the instability is dominated by a global displacement mode analogous to the pairing mode of an infinite array of point vortices or vortex rings. In the nonlinear regime, this mode gives rise to complex dynamics: overtaking events, leapfrogging and merging. Another linearised code is then used to extract modes characterised by a wavelength along the helix, which break the helical symmetry of the base flow. At low wavenumbers, these modes induce local displacements of the vortices and bring together portions of neighbouring coils. At large wavenumbers, another type of mode is found, which deforms the vortex cores through the elliptical instability mechanism.

The present study describes the flow characteristics of swirling flows induced by twisted tape inserts in circular pipes. The study is focused on the secondary flow which is investigated experimentally and with numerical models. The results are expected to improve the paper manufacturing process by identifying and removing the detrimental secondary flow. Experimental tests show for the first time the existence of two co-rotating helical vortices superimposed over the main swirling flow, downstream of twisted tapes. The close proximity of the two co-rotating vortices creates a local counter-rotating flow at the pipe centerline. The flow is analyzed using LDV measurements and high speed camera visualization with fine air bubbles seeding which confirm that the helical vortices are stable. After extracting the characteristic tangential velocity profiles of the main vortex and of the two secondary vortices, it was observed that the maximum tangential velocity of all three vortices is the same, approximately half of the bulk velocity. The winding of the helical vortices is in the swirl direction and the pitch of the helical vortices is found to be independent of the inlet velocity. The experimental findings are confirmed by numerical simulations. The numerical results show that the helical vortices originate inside the swirler and evolve from single co-rotating vortices on each side of the tape. The flow characteristics are analyzed in detail. Swirlers with multiple twists and multiple chambers are shown to have less stable secondary motion and could be employed in applications were the secondary motion is detrimental.

Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2017-02-15T13:24:58Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertação_Mestrado_FaniaCM.pdf: 2231632 bytes, checksum: bb26fa0342704fd41ad5ba76719deac0 (MD5) / Made available in DSpace on 2017-02-15T13:24:59Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertação_Mestrado_FaniaCM.pdf: 2231632 bytes, checksum: bb26fa0342704fd41ad5ba76719deac0 (MD5) Previous issue date: 2015-06-30 / FACEPE / Recentes progressos na fabrica¸c˜ao de nanoestruturas fornecem a possibilidade de produzir filmes finos que contenham defeitos artificiais como s´ıtios de aprisionamento com tamanho, geometria e disposi¸c˜ao espacial bem definidas. O estudo dessa introdu¸c˜ao de um arranjo regulardedefeitosemfilmesfinossupercondutoresdotipoIItemsidoativamentedesenvolvido desde o ponto de vista te´orico e experimental, por um lado pela riqueza de comportamentos est´aticos e dinˆamicos que apresenta e, por outro, pela compreens˜ao dos mecanismos para estabilizareancorararededev´orticescontraasfor¸casmotorasexternas. Umdosfenˆomenos mais not´aveis destes sistemas ´e chamado efeito matching, o qual ocorre quando o n´umero de v´ortices ´e um m´ultiplo inteiro do n´umero dos centros de aprisionamento, sendo observados picos na corrente cr´ıtica (resposta est´atica) ou atenua¸c˜oes na mobilidade dos v´ortices (resposta dinˆamica) nos valores de campo onde esta condi¸c˜ao ´e satisfeita. Nesses campos comensur´aveis, os v´ortices podem formar uma rede altamente ordenada onde as intera¸c˜oes v´ortice-v´ortice que reduzem o ancoramento efetivo s˜ao minimizadas. Usando simula¸c˜oes dedinˆamicamolecular,analisamosasconfigura¸c˜oesdosv´orticeseosefeitosdepinning artificialnadinˆamicadev´orticesemumfilmefinosupercondutordotipoII,comumadistribui¸c˜ao peri´odica de centros de pinning. A dinˆamica de v´ortices e suas configura¸c˜oes estruturais, foram simuladas usando o algoritmo de dinˆamica de Langevin. As intera¸c˜oes entre v´ortices foram modeladas dentro da aproxima¸c˜ao de London com condi¸c˜oes de contorno peri´odicas. Na parte inicial do estudo, calculamos as configura¸c˜oes de equil´ıbrio para a rede de v´ortices, encontrando resultados similares aos estudados em trabalhos anteriores. Inclusive, observamos em nossas simula¸c˜oes que o n´umero de satura¸c˜ao dos centros de ancoragem, i.e. o n´umero m´aximo de v´ortices aprisionados por cada centro, ´e fun¸c˜ao do campo externo. de aprisionamento. Esse estudo revela que os tipos de comensurabilidades encontradas nesse problema dinˆamico s˜ao, em geral, diferentes daqueles encontrados no problema est´atico. Portanto, a pr´atica usual na literatura de se inferir sobre as configura¸c˜oes est´aticas dos v´ortices a partir de medidas de magnetorresistˆencia deve ser revista. / Recent progress in manufacturing of nanostructures provides the possibility to produce thin films containing artificial defects as pinning sites with well-defined size, geometry and spatial arrangement. The study of the introduction of a regular arrangement of defects in type II superconducting thinfilmshasbeenactivelydevelopedfromatheoreticalandexperimentalpointofview,ononeside due to the richness of static and dynamic behaviors that it presents and, second, by understanding themechanismstostabilizeandanchorthenetworkofvorticesagainstexternaldrivingforces. One ofthemostremarkablephenomenaofthesesystemsiscalledmatching effectwhichoccurswhenthe number of vortices is a multiple integer of the number of pinning centers, with peaks observed in the cr´ıtical current (static response) or the attenuation of the vortices mobility (dynamic response) values in field where this condition is satisfied. These measurable fields, the vortices can form a highly ordered network where the vortex-vortex interactions that reduce the effective anchoring are minimized. Using molecular dynamics simulations, we analyzed the settings of the vortices and the effects of artificial pinning at the dynamics of vortices in a type II superconducting thin film with a periodic distribution of pinning centers. The dynamics of vortices and their structural configurations were simulated using the Langevin dynamics algorithm. hm. The interactions between vortices were modeled using London’s approach with periodic boundary conditions. In the early part of the study, we calculated the balance settings for the vortex network, finding results similar to those studied in previous works. In fact, we observed in our simulations that the number of saturation of the anchoring centers, i.e. the maximum number of vortices trapped by each center, is a function of the external field. This result had already been predicted in calculations performed within the Ginzburg-Landau formalism. Here, we show that the same effect can be reproduced at the London scheme. In the second part of the study, we analyzed the dynamics when an external force is applied in the longitudinal direction relative to the symmetry of the network of traps. We show that this system displays dynamic commensurability effects when the settings of the vortices coincide with the periodicity of the arrangement, through the magnetoresistance curves that have a number of features in the different fields of matching. Certain field values, it falls in tension are absent due to the particular configuration of vortices and the interaction between interstitial vortices with the vortices occupying the pinning centers. This study shows that the types of measurability found in this dynamic problem are generally different from those found in the static problem. Therefore, the usual practice in the literature to infer the static settings of vortices from magnetoresistance measures should be reviewed.

L'objet de ce travail de recherche est d'estimer le rôle des structures tourbillonnaires pour améliorer l'efficacité des refroidisseurs microfluidiques. Cette problématique a été étudiée en deux parties. L'objectif de la première partie est de quantifier les transferts thermiques engendrés par des tourbillons lors de leur interaction avec une paroi chaude. Ceci a été réalisé numériquement en deux étapes. La première étape a pour vocation d'améliorer la compréhension des processus de transferts thermiques d'un tourbillon transporté dans un écoulement laminaire. Deux processus majeurs ont été mis en avant : l'advection des particules froides vers la paroi chaude et le mélange par advection.Suite à ces observations, les transferts thermiques induis par une structure tourbillonnaire ont été optimisés en fonction de ses caractéristiques initiales. Cette optimisation a permis d'augmenter l'efficacité d'un système de refroidissement microfluidique de plus de 50% dans le cas d'une allée tourbillonnaire.La seconde partie couple une étude expérimentale à un modèle numérique. Cette étude se focalise sur le formation des structures tourbillonnaires à l'échelle micrométrique. Les structures tourbillonnaires sont générées par le couplage d'un micro jet synthétique à un écoulement transverse. Dans cette configuration, plusieurs topologies de structures tourbillonnaires ont été observées en fonction du nombre de Reynolds du jet et de l'écoulement. Ces différentes topologies ont été cartographiées en fonction des nombres de Reynolds du système. Finalement, pour une certaine topologie de tourbillon il a été possible de lier leurs caractéristiques aux paramètres de contrôle du système. Grâce à cette relation, il est possible de contrôler le type de structures formées par le système de refroidissement / The purpose of this research paper is to determine the role of vortical structures in the improvement of microfluidic cooling systems’ efficiency.This study is presented in two parts. The aim of the first part is to measure heat transfers produced by vortices while interacting with a hot wall. This part was carried out numerically following two steps. The aim of the first step was to improve the understanding of the processes involved during heat transfers in a vortex convected in a laminar flow. As a result, two major processes can be highlighted : the advection of cold particles to the hot wall and an advective mixing. Following these observations, heat transfers produced by a vortical structure were optimized in accordance with its initial characteristics. Consequently, the optimization of heat transfers increased microfluidic cooling systems’ efficiency by more than 50% in the case of vortex streets.The second part combines an experimental study with a numerical model. This study focuses on the creation of vortical structures on a microscopic scale. For this purpose, vortical structures are produced by combination of a synthetic micro jet with a crossflow. The outcome is that several topologies of vortical structures can be observed depending on the Reynolds number of the jet and the crossflow. These topologies can be mapped according to the Reynolds numbers of the system. As a result, a connection can be made between characteristics of some topologies of vortices and control parameters of the system. As a conclusion, the type of structures produced by the cooling system can be controlled

When a high-temperature superconductor transitions in the presence of a magnetic field, the resistance does not immediately go to zero. Instead, the transition is smoothed by the resistive effects of moving vortices so that perfect conductivity isn't reached until several K below Tc, where the vortices have frozen into a glass state. The properties of this glass depend mainly on the amount and correlation between impurities in the superconductor. This report describes a study of the glass transition in an YBCO thin film rich in uncorrelated impurities. The aim was to use sensitive resistive measurements to determine which of two models of this transition, the vortex glass model and the vortex molasses model, is more accurate. After evaluation, both models are found lacking and a third model is put forth. This model, which resembles the normal distribution, excellently describes the YBCO thin film case. When the new model is tested against data from similar measurements, an YBCO single crystal and a TlBaCaCuO thin film, the results are mixed. Further study is suggested to determine the model's range of validity.

No description available.

Immersed boundary (IB) methods are attractive due to their ability to simulate flow over complex geometries on a simple Cartesian mesh. Unlike conformal grid formulation, the mesh does not need to conform to the shape and orientation of the boundary. This eliminates the need for complex mesh and/or re-meshing in simulations with moving/morphing boundaries, which can be cumbersome and computationally expensive. However, the imposition of boundary conditions in IB methods is not straightforward and numerous modifications and refinements have been proposed and a number of variants of this approach now exist. In a nutshell, IB methods in the literature often suffer from numerical oscillations, implementation complexity, time-step restriction, burred interface, and lack of generality. This limits their ability to mimic conformal grid results and enforce Neumann boundary conditions. In addition, there is no generic IB capable of solving flow with multiple potentials, closely/loosely packed structures as well as IBs of infinitesimal thickness. This dissertation describes a novel 2$ ^{\text{nd}} $ order direct forcing immersed boundary method designed for simulation of two- and three-dimensional incompressible flow problems with complex immersed boundaries. In this formulation, each cell cut by the IB is reshaped to conform to the shape of the IB. IBs are modeled as a series of 2D planes in 3D space that connect seamlessly at the edges of the cut cells, in a way that mimics conformal grid. IBs are represented in a continuous and consistent fashion from one cell to another, thus eliminating spatial pressure oscillations originating from inconsistent description of the IB as well as the traditional stair-step problem, leading to a more accurate resolution of the boundary layer. Boundary conditions are enforced at the exact location of the IB devoid of interpolation, which guarantees sound simulations even on grids with high aspect ratio, and enables simulations of flow packed with multiple IBs in close proximity. Boundary conditions for each phase across the IB are enforced independently, yielding a unique capability to solve flows with zero-thickness IBs. Simulations of a large number of 2D and 3D test cases confirm the prowess of the devised immersed boundary method in solving flows over multiple loosely/closely-packed IBs; stationary, moving and highly morphing IBs; as well as IBs with zero-thickness. Extension of the proposed scheme to solve flow with multiple potentials is demonstrated by simulating transfer and transport of a passive scalar from an array of side-by-side and tandem cylinders in cross-flow. Aquatic vegetation represented by a colony of circular cylinders with low to high solid fraction is simulated to showcase the prowess of the current numerical technique in solving flow with closely packed structures. Aquatic vegetation studies are extended to a colony of flat plates with different orientations to show the capability of the developed method in modeling zero-thickness structures.