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Made available in DSpace on 2014-06-11T19:23:29Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-06-28Bitstream added on 2014-06-13T19:50:16Z : No. of bitstreams: 1 simoes_rp_me_bauru.pdf: 20203043 bytes, checksum: b1791cdffb4735eb5cba45bb735dc2f7 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O estudo dos regimes dinâmicos de vórtices em supercondutores do tipo II tem atraído grande interesse do ponto de vista teórico e experimental, por um lado pela riqueza de comportamentos que é apresentado e, por outro, pela compreensão dos mecanismos de pinning, o qual é utilizado para aumentar e manter a densidade de corrente crítica, permitindo seu uso em aplicações práticas. Usando simulações de dinâmica molecular, analisamos os efeitos de pinning artificiais na dinâmica de vórtices em filmes finos supercondutores do tipo II. Além da distribuição aleatória de centros de pinning, dois tipos de arranjos periódicos foram considerados: hexagonal e kagomé. Para o arranjo kagomé analisamos a dinâmica para a força externa aplicada nas direções transversal e longitudinal em relação à simetria da rede. Em cada caso são observados comportamentos dinâmicos distintos da rede de vórtices, uma vez que existem simetrias diferentes para cada direção. Diferentemente dos resultados obtidos para a distribuição aleatória e hexagonal de pinnings, para o arranjo de kagomé, a rede de vórtices apresenta uma dinâmica e um segundo pico na resistência diferencial ainda não descritos na literatura. Quando a força é aplicada na direção longitudinal, são observados regimes dinâmicos complexos da rede de vórtices. Para esse caso, além dos regimes ordenados típicos de redes periódicas de pinnings, os vórtices se movem em canais desordenados quando a corrente de transporte é aumentada, lembrando o regime smectic visto em simulações com pinning aleatório. Subsequentemente, para altas correntes, os vórtices atingem um regime dinâmico onde eles novamente se movem em canais bem definidos, revelando uma ordem transversal. Para o arranjo hexagonal, os vórtices sempre se movem em linhas retas. Apresentamos também um estudo da densidade de corrente crítica para diferentes valores de densidade de vórtices. / The study of current-driven vortex lattices in type II superconductors has attracted a great interest from both experimental and theoretical point of view, on the one side for the riches of behaviors that is showed and, otherwise, for the comprehension of pinning mechanisms, which has been used to increase and to maintain the critical current density, to allow their use in practical applications. Using molecular dynamics simulations, we analyze the effects of artificial pinning on the vortex dynamics of type II superconductors thin films. Yonder random distribution of pinning centers, two types of periodic pinning arrays are considered: the hexagonal and kagomé. For the kagomé array, we analyze the dynamics for the applied external force in the transversal and longitudinal directions to the lattice symmetry. For each case are observed different dynamical behaviors of vortex lattice, once there are different symmetries for each direction. Differently than the results obtained for the random and hexagonal distribution of pinnings, for the kagomé array, the vortex lattice show a dynamic and a second peak in the differential resistance still not reported in the literature. When the force is applied in the longitudinal direction, the dynamical behavior of vortices is much more complex. For this case, yonder the ordered regimes, typically observed in systems with periodic pinning, the vortices moving in disordered channels when the transport current is increased, resembling the smetic dynamical regime that was found in simulations for random pinning distributions. Subsequently, for high currents, the vortices reach dynamical regime where they move in well defined channels again, revealing transversal ordering. For the hexagonal array, the vortices always move in straight trajectories. We also show a study of critical current density for different values of vortex density. For all densities, the hexagonal pinning network shows higher critical currents.

Resumo: O estudo dos regimes dinâmicos de vórtices em supercondutores do tipo II tem atraído grande interesse do ponto de vista teórico e experimental, por um lado pela riqueza de comportamentos que é apresentado e, por outro, pela compreensão dos mecanismos de pinning, o qual é utilizado para aumentar e manter a densidade de corrente crítica, permitindo seu uso em aplicações práticas. Usando simulações de dinâmica molecular, analisamos os efeitos de pinning artificiais na dinâmica de vórtices em filmes finos supercondutores do tipo II. Além da distribuição aleatória de centros de pinning, dois tipos de arranjos periódicos foram considerados: hexagonal e kagomé. Para o arranjo kagomé analisamos a dinâmica para a força externa aplicada nas direções transversal e longitudinal em relação à simetria da rede. Em cada caso são observados comportamentos dinâmicos distintos da rede de vórtices, uma vez que existem simetrias diferentes para cada direção. Diferentemente dos resultados obtidos para a distribuição aleatória e hexagonal de pinnings, para o arranjo de kagomé, a rede de vórtices apresenta uma dinâmica e um segundo pico na resistência diferencial ainda não descritos na literatura. Quando a força é aplicada na direção longitudinal, são observados regimes dinâmicos complexos da rede de vórtices. Para esse caso, além dos regimes ordenados típicos de redes periódicas de pinnings, os vórtices se movem em canais desordenados quando a corrente de transporte é aumentada, lembrando o regime smectic visto em simulações com pinning aleatório. Subsequentemente, para altas correntes, os vórtices atingem um regime dinâmico onde eles novamente se movem em canais bem definidos, revelando uma ordem transversal. Para o arranjo hexagonal, os vórtices sempre se movem em linhas retas. Apresentamos também um estudo da densidade de corrente crítica para diferentes valores de densidade de vórtices. / Abstract: The study of current-driven vortex lattices in type II superconductors has attracted a great interest from both experimental and theoretical point of view, on the one side for the riches of behaviors that is showed and, otherwise, for the comprehension of pinning mechanisms, which has been used to increase and to maintain the critical current density, to allow their use in practical applications. Using molecular dynamics simulations, we analyze the effects of artificial pinning on the vortex dynamics of type II superconductors thin films. Yonder random distribution of pinning centers, two types of periodic pinning arrays are considered: the hexagonal and kagomé. For the kagomé array, we analyze the dynamics for the applied external force in the transversal and longitudinal directions to the lattice symmetry. For each case are observed different dynamical behaviors of vortex lattice, once there are different symmetries for each direction. Differently than the results obtained for the random and hexagonal distribution of pinnings, for the kagomé array, the vortex lattice show a dynamic and a second peak in the differential resistance still not reported in the literature. When the force is applied in the longitudinal direction, the dynamical behavior of vortices is much more complex. For this case, yonder the ordered regimes, typically observed in systems with periodic pinning, the vortices moving in disordered channels when the transport current is increased, resembling the smetic dynamical regime that was found in simulations for random pinning distributions. Subsequently, for high currents, the vortices reach dynamical regime where they move in well defined channels again, revealing transversal ordering. For the hexagonal array, the vortices always move in straight trajectories. We also show a study of critical current density for different values of vortex density. For all densities, the hexagonal pinning network shows higher critical currents. / Orientador: André Luiz Malvezzi / Coorientador: Pablo Antonio Venegas Urenda / Banca: Ladir Candido da Silva / Banca: Paulo Noronha Lisboa Filho / O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp / Mestre

Abstract In this thesis, the structure and the dynamics of vortices are studied from the standpoint of the hydrodynamical theory of superfluids. In the hydrodynamical theory a superfluid is described by a continuous order parameter field. In the case of superfluid helium-4 this field is a complex-valued function of position and time. However, in superfluid helium-3 the order parameter is a complex-valued 3 × 3 matrix. The first part of this work consists of studies on structures that appear in the order parameter field, when a vessel filled with superfluid helium-3 in the A phase (3He-A) is rotated in an external magnetic field. Among the most common of these structures are the so-called continuous vortices. They exist in several different forms. In addition to vortices, other possible structures include the vortex sheet of 3He-A that was discovered at the Low Temperature Laboratory of Helsinki University of Technology (currently Aalto University) in late 1993. In this thesis, these structures were studied by finding stationary vortex configurations that minimize the free energy of the superfluid. An algorithm for minimizing the free energy was implemented by writing a computer program. This program was then used to study the structure of a few vortex types, inferred to be the most probable ones. In addition, regular lattices formed by these vortices, including the vortex sheet, were studied. A phase diagram for vortex lattices was constructed by comparing the free energy of various lattice structures as a function of rotational velocity and external magnetic field. The study of vortex structures also lead to a discovery of a new type of vortex in 3He-A, later named the LV3 vortex. In the second part of the work, the dynamics of vortices was studied using a filament model of vortex motion, which also has its theoretical justification in the hydrodynamical model of superfluids, but where the detailed structure of the vortex core is not relevant. The specific problem under consideration here was the motion of a quantized vortex in a rotating elongated cylinder filled with superfluid, and how the motion of the vortex depends on temperature and the rotational velocity of the vessel. The study of vortex motion was simplified using scaling laws. A new type of scaling law was discovered, which both simplified the specific problem under study, and made the results more general. In summary, the research in this thesis touched upon two somewhat complementary areas, i.e. the structure of continuous vortices in 3He-A and the dynamics of thin vortex lines, which is more applicable to superfluid 4He or to the B phase of superfluid helium-3 (3He-B). However, these areas complement each other in advancing the general scientific understanding about the properties of superfluids.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第22970号 / 理博第4647号 / 新制||理||1668(附属図書館) / 京都大学大学院理学研究科数学・数理解析専攻 / (主査)教授 坂上 貴之, 教授 泉 正己, 教授 國府 寛司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

In dilute polymer solution, polymers are able to change the flow structures and suppress the intensity of turbulence, resulting in a considerable friction drag reduction (DR). Despite the extraordinary progress made in the past few decades, some critical questions remain unanswered. This dissertation will try to address two fundamental questions in dilute polymeric turbulence: (I) interactions between polymers and turbulent motions during the qualitative low-extent to high-extent drag reduction (LDR and HDR) transition in inertia-driven turbulence, (II) roles of the inertia- and elasticity-driven turbulent motions in the dynamics of high elasticity polymeric flows. Many studies in the area of DR turbulence have been focused on the onset of DR and the maximum drag rection (MDR) asymptote. Between these two distinct stages, polymeric turbulent flows can also be classified into the qualitative LDR and HDR stages. Understanding the polymer-turbulence interactions during the drastic LDR-HDR transition is of vital importance for the development of efficient flow control technology. However, knowledge regarding this qualitative transition is still limited. In our DNS (direct numerical simulation) study, differences between the LDR and HDR stages are presented by a number of sharp changes in flow structures and statistics. Drag reduction in the flows is thus governed by two different mechanisms. The first is introduced at the onset of DR, which has been well explained by the indiscriminate suppression of turbulent fluctuations during the coil-stretch transition of polymers. The second mechanism starts at the LDR-HDR transition but its physical origin is not clear. Based on instantaneous observations and indirect statistical evidence, we proposed that polymers, after the LDR-HDR transition, could suppress the lift-up process of the near-wall vortices and modify the turbulent regeneration cycles. However, direct evidence to support this hypothesis is not available without a statistical analysis of the vortex configurations. Therefore, a new vortex tracking algorithm -- VATIP (vortex axis tracking by iterative propagation) -- is developed to analyze statistically the configurations and distribution of vortices. Implementing this method in the polymeric turbulence demonstrates that the lift-up process of streamwise vortices in the buffer layer is restrained at HDR, while the generation of hairpins and other three-dimensional vortices is suppressed. In addition, the characteristic lifting angle of conditional eddies extracted by a conditional sampling method is found to be larger in HDR than in the Newtonian turbulence. These observations all support our hypothesis about the mechanism of LDR-HDR transition. Research on the low elasticity turbulence usually considered the flow motions to be Newtonian-like. Turbulence here is driven by the inertial force (and hence called ``inertia-driven'' turbulence (IDT)) while polymers are responsible for dissipating turbulent kinetic energy. In the high elasticity turbulence, recent studies found a completely different turbulent flow type in which turbulence is driven by the elastic force and polymers could also feed energy to the flow. The behaviors of this ``elasticity-driven'' turbulence (EDT) are of significant interest in this area because of its potential connection to the MDR asymptote. However, EDT is difficult to capture by the traditional pseudo-spectral DNS scheme (SM) as a global artificial diffusion (GAD) term is involved in the polymer constitutive equation to stabilize the simulation. In our study, a new hybrid pseudo-spectral/finite-difference scheme is developed to simulate the polymeric turbulence without requiring a GAD. All of the spatial derivative terms are still discretized by the Fourier-Chebyshev-Fourier pseudo-spectral projection except for the convection term in the constitutive equation which is discretized using a conservative second-order upwind TVD (total variation diminishing) finite difference scheme. The numerical study using the hybrid scheme suggests that turbulent flows can be either driven by the inertial or the elastic forces and respectively result in the IDT and EDT flows. A dynamical flow state is also found in the high elasticity flow regime in which IDT and EDT can be sustained alternatively. / Thesis / Doctor of Philosophy (PhD) / Turbulence is known to consume kinetic energy in a fluid system. To enhance the efficiency of fluid transportation, various techniques are developed. Especially, it was found that a small amount of polymers in turbulent flows can significantly suppress turbulent activity and cause considerable friction drag reduction (DR). Extraordinary progress has been made to study this phenomenon, however, some questions still remain elusive. This dissertation tries to address some fundamental questions that relate to the two typical polymeric turbulent motions: the inertia- (IDT) and elasticity-driven turbulence (EDT). In IDT, mechanisms of transitions between the intermediate stages are investigated from the perspective of vortex dynamics. The different effects of polymers at each stage of the flow lead to different flow behaviors. Particularly, starting from the low- to high-extent DR transition, the lift-up process of vortices is suppressed by polymers. The regeneration cycles of turbulence are thus modified, which results in qualitative changes of flow statistics. Numerical study on EDT is enabled by a newly developed hybrid pseudo-spectral/finite-difference scheme. A systematic investigation of the parameter space indicates that EDT is one self-contain turbulence driven purely by the elastic force. It can also interact with IDT and lead to a dynamical flow state in which EDT and IDT can alternatively occur.

An experimental study of an aerodynamic ramp (aeroramp) injector was conducted at Virginia Tech. The aeroramp consisted of an array of two rows with two columns of flush-wall holes that induce vorticity and enhance mixing. For comparison, a single-hole circular injector with the same area angled downstream at 30 degrees was also examined. Test conditions involved sonic injection of helium heated to 313 K, to safely simulate hydrogen into a Mach 4 air cross-stream with average Reynolds number 5.77 e+7 per meter at a jet to freestream momentum flux ratio of 2.1. Sampling probe measurements were utilized to determine the local helium concentration. Pitot and cone-static pressure probes and a diffuser thermocouple probe were employed to document the flow. The main results of this work was that the mixing efficiency value of this aeroramp design which was optimized at Mach 2.4 for hydrocarbon fuel was only slightly higher than that of the single-hole injector at these flow conditions and the mass-averaged total pressure loss parameter showed that the aero-ramp and single-hole injectors had the same overall losses. The natural extension of the investigation was then to look in detail at two major physical phenomena that occurs in a complex injector design such the Aeroramp: the jet-shock interaction and the interaction of the vortical structures produced by the jets injection into a supersonic cross flow. Experimental studies were performed to investigate the effects of impinging shocks on injection of heated helium into a Mach 4 crossflow. It was found that the addition of a shock behind gaseous injection into a Mach 4 crossflow enhances mixing only if the shock is closer to the injection point where the counter-rotating vortex pair (always associated with transverse injection in a crossflow) is not yet formed, and the deposition of baroclinic generated of vorticity is the highest. The final investigation concerned with the interaction of the usual vortex structure produced by jet injection into a supersonic crossflow and an additional axial vortex typical of those that might be produced by the inlet of a scramjet or the forebody of a vehicle to be controlled by jet interaction phenomena. The additional axial vortices were generated by a strut-mounted, diamond cross-section wing mounted upstream of the injection location. The wing was designed to produce a tip vortex of a strength comparable to that of one of the typical counter-rotating vortex pair (CVP) found in the plume of a jet in a crossflow. The profound interaction of supersonic vortices supported by a quantitative description and characterization of the flowfield has been demonstrated. / Ph. D.

The wake of a bluff body is a representative issue in vortex dynamics that plays a central role in civil engineering, ocean engineering and thermal engineering. In this work, a flowing soap film was used to investigate the wakes of multiple stationary circular cylinders and of a single oscillating cylinder. Corresponding computer simulations were also conducted. Vortex formation of a stationary circular cylinder was analyzed by proper orthogonal decomposition (POD). The POD analysis was used to define an unsteady vortex formation length, which suggests a relationship between the vortex formation length of a single cylinder and the critical spacing of two cylinders in a tandem arrangement. A systematic parametric study of the wake structure was conducted for a controlled transversely oscillating cylinder. Neural network and support vector machine codes assisted the wake classification procedure and the identification of boundaries between different wake regimes. The phase map of the vortex shedding regimes for the (quasi) two-dimensional experiment qualitatively agrees with previous three-dimensional experiments. The critical spacings of two identical tandem circular cylinders in a flowing soap film system were determined using visual inspections of the wake patterns and calculations of the Strouhal frequencies. The dimensionless spacing was both increased and decreased quasi-statically. Hysteresis was observed in the flow patterns and Strouhal numbers. This study appears to provide the first experimental evidence of critical spacing values that agree with published computational results. The wake interaction between a stationary upstream circular disk and a free downstream circular disk was also investigated. With the ability to tie together the wake structure and the object motion, the relationship between energy generation and flow structure in the simplified reduced order model system was studied. The research results find the optimal efficiency of the energy harvesting system by a parametric study. / PHD / The wake of a bluff body is a classic issue in vortex dynamics that has been the subject of much research in civil engineering, ocean engineering and thermal engineering. Bluff bodies, especially circular cylinders, can be found extensively in heat exchangers, cooling systems and offshore structures. Flow-induced vibration of a bluff body due to the formation of a wake is an important problem in many fields of engineering. Flow-induced vibration determines the oscillation of flexible pipes that transfer oil from the seabed to the surface of the ocean, for example [71]. In civil engineering, flow-induced vibration affects the design of bluff structures in wind such as bridges, chimneys and buildings [62]. Flow-induced vibration caused by vortices being shed from a bluff body is also a promising way to extract energy from geophysical flows [10]. FIV energy harvesting systems are especially suitable for slow flow speeds in the range 0.5-1.5m/s which cannot be efficiently harvested by traditional hydroelectric power technologies. When a pair of tandem cylinders is immersed in a flow, the downstream cylinder can be excited into wake-induced vibrations (WIV) due to the interaction with vortices coming from the upstream cylinder. In this work, a flowing soap film was used to investigate the flow-induced vibration of the downstream cylinder of a tandem pair. With the ability to tie together the wake structure and the object motion, we investigate the relationship between energy generation and flow structure in the reduced order model system. The research results find the optimal efficiency of the energy harvesting system by a parametric study. To get deep physical understanding of the flow-induced vibration, wake structures of a circular cylinder undergoing controlled motion and the critical spacing of two identical tandem circular cylinders were also investigated in this research. These research results can help not only the optimization of energy harvesting systems based on flow-induced vibration of the circular-cylinder system, but also will benefit the understanding of wake interactions between multiple bluff bodies such as schooling fish, natural draft cooling towers and wind turbine farms.

Vortices interacting with the solid surface of aerodynamic bodies are prevalent across a broad range of geometries and applications, such as dynamic stall on wind turbine and helicopter rotors, the separated flows over flapping wings of insects, birds and micro-air vehicles, formation of the vortex wakes of bluff bodies, and the lift-producing vortices formed by aircraft leading-edge extensions and delta wings. This study provides fundamental insights into the formation and evolution of such vortices by considering the leading-edge vortices formed in variations of a canonical flapping wing problem. Specifically, the vorticity transport within three distinct experimental cases--2D plunging airfoil, 3D plunging airfoil and 2D plunging airfoil with suction applied at the leading edge--were analyzed in order to characterize the formation and evolution of the leading-edge vortex (LEV). Three-dimensional representations of the velocity and vorticity fields were obtained via multi-plane particle image velocimetry (PIV) measurements and used to perform a vorticity flux analysis that served to identify the sources and sinks of vorticity within the flow. Time-resolved pressure measurements were obtained from the surface of the airfoil and used to characterize the flux of vorticity diffusing from the solid surface, and a method for correcting dynamic pressure data was developed and validated for the application within the current study. Upon characterizing all of the sources and sinks of vorticity, the circulation budget was found to be fully accounted for. Interpretation of the individual vorticity balance terms demonstrated vorticity generation and transport characteristics that were consistent among all three cases that were investigated. Three-dimensional vorticity fluxes were found to be an almost negligible contributor to the overall circulation budget, mostly due to the individual terms canceling each other out. In all cases, the diffusive flux of vorticity from the surface of the airfoil was shown to act primarily as a sink of LEV vorticity, with a magnitude roughly half that of the flux of vorticity emanating from the leading-edge shear layer. Inspection of the chordwise distribution of the diffusive flux within the 2D case showed it to correlate very well with the evolution of the flow field. Specifically, the diffusive flux experienced a major increase during the phase interval in which the LEV remained attached to the downstream boundary layer. It was also noted that the accumulation of secondary vorticity near the leading edge prevented the diffusive flux from continuing to increase after the roll-up of the LEV. This result was validated within the 3D case, which demonstrated that maintaining an LEV that stays attached to the downstream boundary layer produces a larger diffusive flux of vorticity--presumably enhancing both lift and thrust. Through the use of a spanwise array of suction ports, the suction case was able to successfully alter the total circulation of the flow by removing positive vorticity from the opposite-signed vortex (OSV) that formed beneath the LEV. This removal of positive vorticity produced a measured increase in the total lift, and it was noted that weakening this region of secondary vorticity allowed the LEV to impose more suction on the surface of the airfoil. However, it was also noted that weakening the OSV resulted in a loss of thrust, which was attributed to the loss of suction that occurred near the leading edge when the removal of secondary vorticity caused the energetic OSV to be reverted into a low energy region of separated flow. The physical insights provided by this work can form the basis of novel flow control strategies for enhancing the aerodynamic loads produced in unsteady, separated flows.