Showing papers on "Vortex-induced vibration published in 1986"

Flow-induced vibration of fuel rods.

Abstract: A study on flow-induced vibration of fuel rods in a reactor was first reported by Bergreen in 1958. Since then theoretical and experimental researches have been made by many investigators. This is a review of the studies on vibration of the fuel rod excited by axial flow. The fuel rod vibration is understood to be caused by the self-excited vibration of the rod and the random vibration of the fluid. Some parameters which influence the fuel rod vibration, such as velocity, fluid properties, rod conditions etc. are also discussed.

Flow induced vibration of magnetic head suspension in hard disk drive

Abstract: Measurements of fluctuations in the flying height of a magnetic head-slider and in the strain of its suspension in a 14" hard disk drive reveal two dominant suspension vibration components. One is a 2 kHz bending mode, and the other is a 2.9 kHz torsional mode. The bending mode vibration causes the flying height fluctuations. The vortex street behind a section model of the suspension are visualized using the smoke-wire method. The frequency of the street is higher than the frequencies of the two modes. In a free jet, the amplitudes of the two components are both in proportion to the square of the flow velocity, and their frequencies remain constant. A natural vibration mode analysis of the suspension gives similar modes with similar frequencies. These results indicate the two components are forced vibration with the natural vibration frequencies of the suspension due to flow force acting on it. To predict the flow force, a computer program using the discrete vortex method is developed. The calculated drag coefficient is about 70% larger than the one obtained experimentally from the visualized vortex street behind the section model.

Flow-induced vibration of steam generator tubes. Final report

Abstract: This report presents results concerning flow-induced vibration in a square pitch tube bundle successively subjected to air-water flow of density ranging from 40 to 400 kg m/sup -3/. The most prominent vibration excitation mechanism observed was fluidelastic instability. Air-water and steam-water critical flow velocities are adequately predicted using the classical Connors' proportionality factor of 7. Turbulent excitation of the tube bundle was also investigated. 156 figs., 39 tabs.

Flow-induced vibration - 1986

Abstract: This book presents the papers given at a conference which examined mechanical vibrations in pipes caused by fluid flow. Topics considered at the conference included fluid excitation forces, axial flow induced vibration, fluid damping, crossflow induced vibration of multiple cylinders, two-phase flow, a computer program for vibration analysis, hydrodynamics, heat exchangers, and flow-induced vibrations of condenser tubes.

Vortex-Induced Vibration Experienced on Sea Chest

Abstract: A rare kind of structural vibration induced by flow was experienced on sea chests of high speed ships. Model experiments in cavitation tunnel revealed the cause of vibration and the mechanism of excitation. The results showed that the free shear layers flowed into the cavity of sea chest were rolled up periodically into vortices and the vortex excitation resonated with the natural frequency of the vibration system composed of the sea chest structure and the water involved in it. Countermeasures reducing the excitation were also obtained by the model test.Energy method was applied for the estimation of the natural frequency of sea chest vibration system. The calculated results by this method were verified to correlate well with the results of vibration test on sea chest model, together with the measured results on actual ship.

Calculations of axisymmetric vortex sheet roll-up using a panel and a filament model

Abstract: A method for calculating the self-induced motion of a vortex sheet using discrete vortex elements is presented. Vortex panels and vortex filaments are used to simulate two-dimensional and axisymmetric vortex sheet roll-up. A straight forward application using vortex elements to simulate the motion of a disk of vorticity with an elliptic circulation distribution yields unsatisfactroy results where the vortex elements move in a chaotic manner. The difficulty is assumed to be due to the inability of a finite number of discrete vortex elements to model the singularity at the sheet edge and due to large velocity calculation errors which result from uneven sheet stretching. A model of the inner portion of the spiral is introduced to eliminate the difficulty with the sheet edge singularity. The model replaces the outermost portion of the sheet with a single vortex of equivalent circulation and a number of higher order terms which account for the asymmetry of the spiral. The resulting discrete vortex model is applied to both two-dimensional and axisymmetric sheets. The two-dimensional roll-up is compared to the solution for a semi-infinite sheet with good results.

Flow-induced vibration of long structures

Abstract: When a body is exposed to a flowing fluid, oscillations can occur due to one or more of several different mechanisms. The resulting large amplitudes of motion and fatigue are potential sources of structural failure. Furthermore, the drag force on a structure can be increased due to the effectively larger cross-sectional area presented to the flow from the oscillation. A complete understanding of the nature of such vibration is essential in the design of many civil and mechanical engineering systems. Previous solutions to the vortex-induced vibration problem were primarily based on modal analysis, using a one- or two-mode approximation. Use of modal analysis implies a "locked-in" condition: the vortex shedding frequency and a natural frequency of the system are coincident. Observations made on long cable systems indicate that the amplitude of response is smaller than is predicted by a conventional modal analysis. The drag forces on such structures are therefore overestimated by current design approaches. In very long structures, typical of those found in ocean applications, modes are closely spaced, and it is not reasonable to assume total spanwise correlation in the fluid forces or response. The approach used herein attempts to avoid the limitations associated with the modal solution of such problems by implementing a solution based on traveling waves. The technique draws on earlier theoretical and empirical models for the complex vortex-shedding phenomenon, and incorporates these into a new method for analyzing the structural response problem. The traveling wave approach can be used to model effectively spanwise variable flow environments by summing the calculated responses of adjacent active sections of cable. Until this method was developed, there was no suitable method available for modeling flow characteristics of this type. Modal analysis is effectively limited to systems with uniform flow over all or part of the system.

Flow-induced vibration in shell-and-tube heat exchangers with double-segmental baffles

Abstract: One of the techniques used by designers of shell-and-tube heat exchangers when they encounter a potential flow-induced vibration problem is to shift from a tube bundle with segmental baffles to one with double-segmental baffles. This results in a split of the flow into either half of the shell with lower velocities and the possibility to reduce the unsupported tube span length while keeping below a given allowable pressure drop. Tests performed as a part of a systematic study of water flow-induced vibration in industrial-sized heat exchangers have demonstrated the possibility of exciting vibration in tubes exposed to localized high velocities as a consequence of inherent flow maldistributions in the end zone regions. Results for nine different double-segmental bundle configurations are presented. Comparison with similar results for segmental baffled bundles, where available, indicates that vibration is normally observed at somewhat higher total flows for double-segmental baffled bundles.

Effect of homogeneous stratification of the fluid on the dynamics of a kármán vortex street behind a circular cylinder

Abstract: The experimental results of studying the effect of homogeneous stratification of the fluid on the conditions of generation of a Karman vortex street [1] developing in the wake of a cylinder in steady horizontal motion are described. In a homogeneous medium at Reynolds numbers Re >5 two symmetrical regions of vorticity of opposite sign are formed behind the cylinder and move together with the latter. As the speed of the cylinder increases, the link between the vortices and the cylinder grows weaker, the vortices are stretched out along the flow and at Re > 40 begin to separate alternately, forming a vortex street in the wake. At first, the frequency of vortex separation increases sharply with increase in Re, but then levels off. It is found that in a uniformly stratified fluid the onset of vortex separation from the moving cylinder is delayed. The dependence of the critical Reynolds number (onset of vortex separation) on the internal Froude number is obtained. The effect of stratification of the fluid on the frequency of separation of the vortices in the Karman street is investigated. The effect of the Froude number on the dependence of the Strouhal number on the Reynolds number is established.

Modelling of trailing edge flow tones in elastic structures

Abstract: The hydroelastic coupling of turbine blade vibration modes and trailing edge vortex shedding from the blade is a well known phenomenon that leads to intense audible tones and shortened fatigue life. Historically, most researches on such turbo-blade “singing” have focussed on the prediction of the vortex shedding frequency and on the reduction of offending vibration levels by reshaping the trailing edges. The associated control measures effectively permit the “detuning” of the vortex shedding from the blade modes, and the effectiveness of such measures is more or less qualitatively known. This paper builds on and extends a different line of research that is aimed at merging the controling fluid dynamic and structural dynamic factors of these flow-induced tones into a quantitative semiempirical model of the non linear flow-induced vibration of turbo machine blading. The elements of the model development involve the use of measured nonlinear “wake impedances” by the forced motion of trailing edges, the development of a mathematical model for the response of blade structures that are coupled to the wake dynamics, and finally the testing of the modeling against measured flow-induced vibration of simple hydrofoils. The parameters used in both the modeling and the experimental verification include structure mass density, hydrofoil geometry, damping, and trailing edge geometry.