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Junction flows originate from the interaction between a fluid moving over a wall with an obstacle mounted on the same surface. Understanding the physics of such flows is of great interest to engineers responsible for the design of systems consisting of wall-body junctions. From aerodynamics to turbomachinery and electronics to bridge hydraulics, a number of phenomena (drag, heat transfer, scouring) are driven by the behavior of the most prominent feature of junction flows: the horseshoe vortex system (HVS). Focusing on turbulent flows, the complex dynamics of the HVS is established through its unsteadiness and non-uniformity. The fundamentals of this dynamically-rich phenomenon have been described within the body of a rapidly-expanding literature. Nevertheless, important aspects remain inadequately understood and call for further scrutiny. This study emphasized three of them, by investigating the effects of: model scale, wall roughness, and bed geometry. High-resolution experiments were carried out using Particle Image Velocimetry (PIV). Statistical analyses, vortex identification schemes, and Proper Orthogonal decomposition were employed to extract additional information from the large PIV datasets. The time-averaged topology of junction flows developing over a smooth and impermeable wall was independent of the flow Reynolds number, Re (parameter that expresses the effects of scale). On the contrary, time-resolved analysis revealed a trend of increasing vorticity, momentum, and eruptions of near-wall fluid with Re. New insights on the modal dynamics of the HVS were also documented in a modified flow mechanism. Wall roughness (modeled with a permeable layer of crushed stones) diffused turbulence and vorticity throughout the domain. This effect manifested with high levels of intermittency and spatial irregularity for the HVS. Energetic flow structures were also identified away from the typical footprint of the HVS. Finally, a novel implementation of PIV allowed for unique velocity measurements over an erodible bed. It was demonstrated that, during the initial stages of scouring, the downflow at the face of the obstacle becomes the dominant flow characteristic in the absence of the HVS. Notwithstanding modeling limitations, the physical insight contributed here could be used to enhance the design of systems with similar flow and geometrical characteristics. / Ph. D.

Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2016-08-18T13:04:44Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertacao-Version-Final.pdf: 16533060 bytes, checksum: 920aa7a96edb1e78d330ff1b83033a68 (MD5) / Made available in DSpace on 2016-08-18T13:04:44Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertacao-Version-Final.pdf: 16533060 bytes, checksum: 920aa7a96edb1e78d330ff1b83033a68 (MD5) Previous issue date: 2015-07-29 / CAPEs / CNPq / Neste trabalho é realizado um estudo sobre os efeitos das rugosidades nas propriedades magnéticas de nanofios de níquel para os modos de reversão curling e transversal. Para o estudo adotamos a simulação micromagnética como ferramenta fundamental e para ser implementada utilizamos OOMMF. Para mudar a rugosidade utilizamos uma cadeia de elipsóides e uma forma de variar este parâmetro foi fixando o comprimento dos fios em 1 μm e mudando o número de elipsóides na cadeia. Dessa forma a relação de aspecto dos elipsóides foi modificada para ser entendida como câmbios na rugosidade. Nas análises realizamos estudos dinâmicos e estáticos da reversão dos momentos. A simulação se baseia fundamentalmente na resolução da equação LLG. Nos estudos dinâmicos monitoramos a dependência temporal dos mapas de momentos a das componentes transversais da magnetização depois de ser invertido o campo aplicado. Foram simulados os ciclos de histerese através da minimização da energia livre de Gibbs. Nos estudos estáticos monitoramos a dependência ângular do campo coercitivo, campo de comutação e a magnetização remanente. Observamos em modo geral que há grandes efeitos das rugosidades sobre as propriedades magnéticase que nossos resultados reproduzem os reportados na literatura assim como as curvas experimentais. / This work is a study on the effects of roughness on the magnetic properties of nickel nanowires for their reversal modes (curling and transversal). For the study we adopted the micromagnetic simulation as a fundamental tool and we used OOMMF to implemented. To change the roughness use ellipsoids chain and a way to vary this parameter was securing the length of the wires in 1 μm and changing the number of ellipsoids in the sequence. Thus the ellipsoids of the aspect ratio has been modified to be understood as the exchange roughness. In the analyzes we perform static and dynamic studies of the reversal of moments. The simulation is based largely on the resolution of the LLG equation. In dynamic studies we monitor the time dependence of the maps of magnetic moments and the transverse components of the magnetization after being reversed the field applied. The hysteresis cycles were simulated by minimization of Gibbs free energy.In static studies we monitor the angular dependence of the coercive field, the switching field and remanent magnetization. We observe in general that there are major effects of roughness on the magnetic properties and that our results reproduce the reported in the literature as well as the experimental curves.

Additive manufacturing (AM) has largely improved design freedom compared with traditional manufacturing processes such as casting and milling. The layer-by-layer workflow makes it possible to produce objects with much more complex shapes and structures. This feature is of particular interest for turbine blade manufacturing since internal cooling channels with higher thermal efficiency can be achieved toimprove the overall efficiency of a gas turbine. One feature of AM, especially for Laser Power Bed Fusion (LPBF) working on metal powders, is the relatively large surface roughness (SR), which will affect both heat transfer and pressure loss. Its geometry is also unique with the very randomly distributed spherical-shaped structures. This randomness makes the correlations for heat transfer and pressure loss based on sand grain roughness not applicable anymore. More in-depth research is needed to investigate the roughness effects. In this study, the AM roughness is modelled by a statistical distribution of spheres with different diameters using an upscale ratio of 77.4. An infrared (IR) camera was used to record the temperature distribution on the rough plates subjected to heated airflow. Three Re in the turbulent region (15000, 20000, 25000) were tested and the data from the IR camera were used to calculate the heat transfer coefficient (HTC) on the rough plate through a 3D finite element calibration solver. The results of averaged HTC agree well with data of the real Inconel 939 AM channel from which the upscaled rough plates are modelled. Also, the general patterns of HTC distributions matched the fluid dynamics analysis. Moreover, the results of arranging smooth and rough plates together shows that the heat transfer enhancement from SR is due to both the induced turbulent flows and the increased surface area. / Additiv tillverkning har i hög grad förbättrat designfriheten jämfört med traditionella tillverkningsprocesser som gjutning och fräsning. Att bygga lager för lager gör det möjligt att tillverka föremål med mycket mer komplexa former och strukturer. Denna egenskap är av särskilt intresse för tillverkning av turbinblad eftersom bättre interna kylkanaler kan uppnås för att förbättra den totala verkningsgraden hos en gasturbin. En egenskap hos additiv tillverkning är den relativt stora ytojämnheten, som påverkar både värmeöverföring och tryckförlust. Dess geometri är också unik med mycket slumpmässigt fördelade sfäriskt formade strukturerna. Denna slumpmässighet gör att de korrelationer för värmeöverföring och tryckförlust som baseras på sandkornens grovlek inte längre är tillämpliga. Mer djupgående forskning behövs för att undersöka grovhetseffekterna. I den här studien modelleras den additiva tillverkningens grovhet med en statistisk fördelning av sfärer med olika diametrar med ett uppskalningsförhållande på 77,4. En infraröd (IR) kamera användes för att registrera temperaturfördelningen på de skrovliga plattorna som utsattes för ett uppvärmt luftflöde. Tre Re i det turbulenta området (15000, 20000, 25000) testades och data från IR-kameran användes för att beräkna värmeöverföringskoefficientenpå den grova plattan genom en 3D finita elementkalibreringslösare. Resultaten av den genomsnittliga värmeöverföringskoefficienten stämmer väl överens med data från den verkliga additivt tillverkade Inconel 939-kanalen från vilken de uppskaladeg rova plattorna är modellerade. Även de allmänna mönstren för fördelningen av värmeöverföringskoefficienter stämmer överens med den fluiddynamiska analysen. Dessutom visar resultaten av att arrangera släta och grova plattor tillsammans att värmeöverföringsförbättringen från ytjämnhet beror på både de inducerade turbulenta flödena och den ökade ytarean.