Showing papers on "Vortex-induced vibration published in 1984"

Vortex shedding from oscillating bluff bodies

Abstract: When placed ih a fluid stream, some bodies generate separated flow over a substantial proportion of their surface and hence can be classified as bluff. On sharp-edged bluff bodies, separation is fixed at the salient edges, whereas on bluff bodies with continuous surface curvature the location of separation depends both on the shape of the body and the state of the boundary layer. At low Reynolds numbers, when separation first occurs, the flow around a bluff body remains stable, but as the Reynolds number is increased a critical value is reached beyond which instabilities develop. These instabilities can lead to organized unsteady wake motion, dis­ organized motion, or a combination of both. Regular vortex shedding, the subject of this article, is a dominant feature of two-dimensional bluff-body wakes and is present irrespective of whether the separating boundary layers are laminar or turbulent. It has been the subject of research for more than a century, and many hundreds of papers have been written. In recent years vortex shedding has been the topic of Euromech meetings reported on by Mair & Maull (1971) and Bearman & Graham (1980), and a comprehensive review has been undertaken by Berger & Wille (1972). Vortex shedding and general wake turbulence induce fluctuating pres­ sures on the surface of the generating bluff body, and if the body is flexible this can cause oscillations. Oscillations excited by vortex shedding are usually in a direction normal to that of the free stream, and amplitudes as large as 1.5 to 2 body diameters may be recorded. In addition to the generating body, any other bodies in its wake may be forced into oscillation. Broad-band force fluctuations, induced by turbulence produced in the flow around a bluff body, rarely lead to oscillations as severe as those caused by vortex shedding. Some form of aerodynamic instability, such that move-

Dynamics of a vortex filament in a shear flow

Abstract: Motions of a single vortex filament in a background flow are studied by numerical simulation of a set of model equations. The model, which in essence is due to Hama, treats the self-interaction of the filament through the so-called ‘localized-induction approximation’ (LIA). Interaction with the prescribed background field is treated by simply advecting the filament appropriately. We are particularly interested in elucidating the evolution of sinuous vortices such as the ‘wiggle’ seen by Breidenthal in the transition to three-dimensionality in the mixing layer. The model studied embodies two of the simplest ingredients that must enter into any dynamical explanation: induction and advection. For finite-amplitude phenomena we make contact with the theory of solitons on strong vortices developed by Betchov and Hasimoto. In a shear, solitons cannot exist, but solitary waves can, and their interactions with the shear are found to be key ingredients for an understanding of the behaviour of the vortex filament. When sheared, a soliton seems to act as a ‘nucleation site’ for the generation of a family of waves. Computed sequences are shown that display a remarkable morphological similarity to flow-visualization studies. The present application of fully nonlinear dynamics to a model presents an attractive alternative to the extrapolations from linearized stability theory applied to the full equations that have so far constituted the theoretical basis for understanding the experimental results.

Vortex-Induced Vibrations of a Flexible Cylinder Near a Plane Boundary Exposed to Steady and Wave-Induced Currents

Abstract: Model tests were carried out in a wave tank to determine the effect of combined steady and wave-induced currents and/or the proximity of a plane boundary (seabottom) on the vortex-induced vibrations of a flexible pipe. The response of the center of the pipe span was measured using a biaxial accelerometer. The results show that the proximity of the plane boundary and/or superposition of waves on the steady flow have a pronounced effect on the amplitude and frequency response in both the transverse and in-line directions.

Aero-Hydroacoustics for Ships. Volume 1

Abstract: : In this two-volume monograph on the fundamentals of the generation of flow-induced sound and vibration, the subject area is developed from the essentials of fluid mechanics, vibration, and sound generation. This book provides a comprehensive and unified treatment of the mechanisms of flow-generated sound that occurs on ships and in marine machinery. Often the control of these mechanisms involves the essentials of both fluid mechanics and structural dynamics. Dynamical properties of various types of flow and of various structural elements that are typical of ship application are examined in detail beginning with the fundamentals of each physical source. Volume I provides for the treatment of elementary sources of flow noise and the principles of random vibration. Normal mode analysis is the cornerstone of the methods used to describe the behavior of flow induced vibration. Naval applications that involve turbulent boundary layer and lifting surface flows are discussed in Volume 2 (ADA150673). Aerodynamic noise sources, in so far as they occur anogously in underwater acoustics, are examined in detail for low Mach number marine application.

Deflection criteria for wind induced vibrations in cantilever highway sign structures

Abstract: This study considers the vibratory characteristics of cantilever highway sign structures. During the course of the study four structures were instrumented to experimentally determine their vibration characteristics. In addition analytical studies were performed using finite element techniques. The analysis considers several types of wind loading on the structure. It was found from both the experimental and analytical work that flow induced vibration due to vehicular movement under the sign structure and also vibration due to vortex shedding from the structural elements does not result in stress levels which could be damaging to the structure. However the analysis did show that high stress levels can be expected in these structures under severe wind conditions (v = 100 mph). Experimental validation of the analytical results for this condition was not obtained. (FHWA)

Dynamics of entangled vortex filaments

Abstract: The mechanics of two co‐rotating, entangled, slender vortex filaments are analyzed. An equation governing the motions of the vortices is derived on the assumption that their curvature is small. Particular solutions are obtained for two flow situations: one with and one without an external strain field.

A model of the excitation of orderly structures in a shear layer

Abstract: The artificial excitation of shear layers is investigated theoretically and experimentally. The present paper describes quantitatively the coupling between exciting sound field and shear layer fluctuations. The mathematical model is restricted to low Strouhal numbers at which large scale structures are occurring. The theory does not contain any empirical constants and it is confirmed by the experiments in the expected validity range.