Wall Effect on Fluid-Structure Interactions of a Tethered Bluff Body

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... As this work focus on potential for collision with nearby structures and people, lifted body motion is assumed to be a two-dimensional pendulous like movement. Such simplification has been previously adopted [8]. Further, modules of oil and gas platforms can be composed by various equipment of different shapes, allowing wind to flow through the module. ...
This review summarizes fundamental results and discoveries concerning vortex-induced vibration (VIV), that have been made over the last two decades, many of which are related to the push to explore very low mass and damping, and to new computational and experimental techniques that were hitherto not available. We bring together new concepts and phenomena generic to VIV systems, and pay special attention to the vortex dynamics and energy transfer that give rise to modes of vibration, the importance of mass and damping, the concept of a critical mass, the relationship between force and vorticity, and the concept of “effective elasticity”, among other points. We present new vortex wake modes, generally in the framework of a map of vortex modes compiled from forced vibration studies, some of which cause free vibration. Some discussion focuses on topics of current debate, such as the decomposition of force, the relevance of the paradigm flow of an elastically mounted cylinder to more complex systems, and the relationship between forced and free vibration.
Despite the practical significance of studying the case of the tethered sphere in a steady flow, there are almost no laboratory investigations for such a problem, and it was previously unknown whether such a system would tend to oscillate or not. It is also common ocean engineering practice to assume no oscillation effects in predictions of drag and tether angle of a tethered body. The present work demonstrates that a tethered sphere will oscillate remarkably vigorously at a saturation amplitude of close to two diameters peak-to-peak. The oscillations induce an increase in drag and tether angle of the order of around 100% over what is predicted using nonoscillating drag measurements. Analysis of in-line and transverse natural frequencies indicate that these frequencies should have the same value. Our experiments show that the in-line oscillations become phase locked with the transverse oscillations and vibrate attwicethe frequency of the transverse motion. The above results suggest that oscillations are highly significant to predictions of sphere response in a steady flow, and should not be neglected. Finally, although response amplitudes show large disparity when plotted against Reynolds number, under a range of different sphere mass ratios (M*)and tether length ratios(L*),we find an excellent collapse of data for the different experiments by plotting the amplitudes versus the reduced velocityVR=U/fnD.This result shows that, for very small structural damping, the response amplitude may be considered as a function of the (normalized) natural frequency, and is only a function of the mass ratio and length ratio in so much as these parameters influence the natural frequency itself.
One of the most basic examples of fluid-structure interaction is provided by a tethered body in a fluid flow. The tendency of a tethered buoy to oscillate when excited by waves is a well-known phenomenon; however, it has only recently been found that a submerged buoy will act in a similar fashion when exposed to a uniform flow at moderate Reynolds numbers, with a transverse peak-to-peak amplitude of approximately two diameters over a wide range of velocities. This paper presents results for the related problem of two-dimensional simulations of the flow past a tethered cylinder. The coupled Navier–Stokes equations and the equations of motion of the cylinder are solved using a spectral-element method. The response of the tethered cylinder system was found to be strongly influenced by the mean layover angle as this parameter determined if the oscillations would be dominated by in-line oscillations, transverse oscillations or a combination of the two. Three branches of oscillation are noted, an in-line branch, a transition branch and a transverse branch. Within the transition branch, the cylinder oscillates at the shedding frequency and modulates the drag force such that the drag signal is dominated by the lift frequency. It is found that the mean amplitude response is greatest at high reduced velocities, i.e., when the cylinder is oscillating predominantly transverse to the fluid flow. Furthermore, the oscillation frequency is synchronized to the vortex shedding frequency of a stationary cylinder, except at very high reduced velocities. Visualizations of the pressure and vorticity in the wake reveal the mechanisms behind the motion of the cylinder.
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