No abstract

This paper presents experimental results concerning the response of circular cylinders with and without strakes. The longitudinal and transverse fluid forces (drag and lift), amplitude response and wake structures of plain and helically straked cylinders are compared. Six different configurations of straked cylinders with pitches (p) equal to 5D, 10D and 15D and heights (h) equal to 0.1D and 0.2D are investigated. Measurements on the dynamic response oscillations of an isolated plain and straked cylinders and flow visualization employing a PIV system are shown. Fixed cylinder drag measurements are also shown. The models are mounted on an elastic base fitted with flexor blades and instrumented with strain gauges or in an air bearing base. The base is fixed on the test-section of a water channel facility. The flexor blades possess a low-damping and the flexor blades base an the air bearing base are free to oscillate only in the cross-flow direction. The Reynolds number of the experiments ranges from 2000 to 10000, and reduced velocities, based on natural frequency in still water, vary up to 13. The drag coefficient is increased by 20% for the h = 0.1D cylinder, and 60% for the h = 0.2D cylinder, comparing both with the plain cylinder. The smaller height strokes (h = 0.1D) do not prevent vortex formation in the region very close to the body, resulting in a decrease of about 50% of the amplitude response compared with the plain cylinder. Lowest amplitude response was found to the p = 10D and h = 0.2D case. The analysis of the vorticity contours shows that the shear layer does not roll close to the body (same result for the other cases with h = 0.2D).

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O ambiente urbano está em constante transformação e seu planejamento adequado é fundamental para garantir a qualidade de vida da população. Uma ferramenta útil no estudo do ambiente urbano é o túnel de vento, que permite, por meio de ensaios em modelos reduzidos, avaliar condições de conforto ambiental, de dispersão de contaminantes e, também, de carga de vento nas estruturas. Esse trabalho tem foco em conforto de pedestres devido à ação mecânica do vento. O objetivo foi elaborar um compêndio de algumas técnicas experimentais utilizadas no túnel de vento do IPT para o estudo da ação do vento no nível dos pedestres em ambientes abertos, cada qual com suas limitações e vantagens de acordo com o problema a ser estudado. As técnicas analisadas e as conclusões obtidas foram: a erosão de areia é útil em uma análise preliminar e qualitativa de zonas planas de grande extensão; a anemometria de fi o quente pode ser utilizada sempre e também é importante na investigação de efeitos de rajada, embora as medições sejam intrusivas e demoradas; os sensores Irwin, que foram desenvolvidos especifi camente para o estudo de conforto de pedestres, devem ser cuidadosamente fabricados e calibrados, mas podem ser instalados em vários pontos ao longo do piso do modelo.

Flow induced vibrations play an important role in the process of fatigue in cylindrical elements of offshore platforms. One of the main sources of vibration is due to vortex shedding. Such vortices are originated from the separation of the boundary layer and the low-pressure regions cause the hydrodynamic forces to oscillate. This kind of vibration is known in literature as vortex-induced vibration (VIV), and it has some points that are not well understood yet. When a circular cross section is considered, the point of separation changes with the Reynolds number, increasing the complexity of the phenomena. Additionally, when two bluff bodies are disposed near each other, the vortices shed by one of them can reach the other, interfering with the vortex formation close to the later, and constituting another source of vibration. This work is an effort to provide a better comprehension of the vortex-induced vibration phenomena in a flow around groups of cylinders. In this paper, a detailed computational study of the flow around two rigid cylinders in a tandem arrangement is carried out. The spectral/hp element method is used in the simulations, with a modal decomposition in the span direction. The three-dimensional structures in the wake are analysed, and the level of synchronization of the flow along the span is verified for a gap of five diameters between the cylinders.

No abstract

The offshore helideck wind flow is usually subject to many interferences. The helideck airspace velocity and turbulence fields are important issues to promote safe helicopter take-off and landing operations. The current work brings some CFD results of a helideck wind flow 3D-field defined by the local conditions and constrained by the FPSO structure. A discussion about the chosen CFD boundary conditions is also presented. These CFD results are compared with the wind tunnel model-scale velocity and turbulence measurements. The wind tunnel measurements were performed with use of two different techniques: Particle Image Velocimetry (PIV) and Constant Temperature Anemometry (CTA). The British standard CAP437: Offshore Helideck Design Criteria is examined and suggestions are made herein. The CFD simulations were conducted using the ANSYS CFX software.