Showing papers on "Vortex-induced vibration published in 1996"

The Effects of Turbulence and Unsteadiness on Vortex Shedding From Sharp-Edged Bluff Bodies

Abstract: Motivated by a desire to evaluate vortex shedding flow meters for measuring velocity in unsteady turbulent flow applications, the objective of our work was to study the effects of flow disturbances on vortex shedding from sharp-edged bluff bodies. In particular, the combined effects of turbulence and unsteadiness were examined, as well as their separate effects using controlled wind tunnel tests. After causing an initial and sudden decrease in the Strouhal number, increasing turbulence intensity from 2.5 to 10 percent resulted in only a 2.4 percent increase in the Strouhal number, for turbulence with a length scale of 0.5 bluff body diameters. Turbulence integral length scale had a significant influence on the Strouhal number, with the greatest effect exhibited for length scales near 3 bluff body diameters. Turbulence of this length scale caused a 26 percent decrease in the Strouhal number, as compared to a low-turbulence base case. Fluctuating pressure amplitude and signal-to-noise ratio were also affected by turbulence, and decreased significantly when the integral length Kali was increased from 0.25 to 0.75 bluff body diameters for a turbulence intensity of 10 percent. Unsteadiness caused lock-on for forcing at the Strouhal frequency, twice and four times the Strouhal frequency, while no lock-on was observed for forcing at half the Strouhal frequency. The range of lock-on increased with increasing perturbation amplitude and was asymmetric about the resonant frequency. For the cases investigated, the effects of combined turbulence and unsteadiness were additive, with the turbulence shifting the Strouhal frequency, and the unsteadiness causing lock-on about the shifted Strouhal frequency. The results of this study suggest that vortex shedding flow meters should be calibrated in turbulent flows and turbulence length scale must be controlled at the bluff body. Lock-on can be avoided by sizing the bluff body so that the shedding frequency is always much greater than any disturbance frequency in the flow.

Method for low velocity measurement of fluid flow

Abstract: An apparatus is described which determines the velocity of a fluid flow stream by measuring the frequency of vortex shedding from an obstruction in the fluid flow stream A constant known as the Strouhal number is used to relate the measured vortex shedding frequency to the fluid flow velocity The Strouhal number is a function of the Reynolds number for the obstruction, which is also a function of fluid flow velocity Hence, an iterative technique is provided, first using an estimated Strouhal number to estimate the fluid flow velocity, using the estimated fluid flow velocity to estimate the Reynolds number, then using the Reynolds number to get a better estimate of the Strouhal number The apparatus repeats this cycle until the estimate of the fluid flow velocity converges

Flow-Induced Vibration of Tubes in Cross-Flow

Abstract: This paper presents an unsteady flow theory for flow-induced vibration of tubes in crossflow. It includes a general description of motion-dependent fluid forces, characteristics of fluid-force coefficients, and mathematical models. The detailed results are presented for the constrained mode in the lift direction of various tube arrangements.

Large Eddy Simulation of Turbulent Flow around a Stationary and an Oscillating Rectangular Cylinder.

Abstract: The application of large eddy simulation (LES) to practical engineering problems has been attempted by many researchers. In recent years, flow fields that have not only simple geometry but also complicated flow structures have been simulated by LES. In this paper, turbulent flow around a stationary and an oscillating rectangular cylinder at high Reynolds numbers of 2.2×104 and 7.14×104 has been analyzed by LES. The main objectives of this work are to compare computed results with experimental values and to prove the applicability of LES to flow induced vibration problems. Drag coefficients, base pressure coefficients and Strouhal numbers were in fairly good agreement with experimental values, while the classical Smagorinsky model was used as a subgrid scale (SGS) model. In cases of flow around an oscillating rectangular cylinder, we successfully simulated the lock-in phenomenon whereby the vortex-shedding frequency equals the oscillating frequency. The width of the lock-in region and phase angles between cylinder displacement and lift coefficient at various oscillating frequencies were in good agreement with experimental results. Both lock-in and nonlock-in states of flow were observed in the vicinity of the boundary frequency of the lock-in region.

Effects of Weir Plate Inclination Angle on Flow-Induced Vibrations of Long-Span, Shell-Type Gates

Abstract: Here we present detailed experimental results concerning flow-induced vibrations of long-span, shell-type gates in which the upstream gate face consists of vertical and inclined skin plates (also referred to as weir plates). Such shell-type gates possess two degrees of freedom, one each in the streamwise (horizontal) and vertical directions, due to bending flexibility in those two directions. The streamwise and vertical vibrations can become closely coupled with each other through the hydrodynamic forces acting on the weir plates, resulting in severe self-excited vibrations. A two-dimensional laboratory model of a long-span, shell-type gate was operated with underflow only (i.e., no overflow) at small gate openings with several different inclined weir plate geometries, ranging from a 17.5°to a 65°inclination angle (relative to the horizontal) for the inclined weir plate. By measuring the gate's vertical and horizontal displacements, it was possible to determine the vibration frequency, the excitation ratio (negative damping ratio) and the trajectories of gate motion. These results show that the inclination angle of the inclined weir plate angle plays a significant role in determining the gate's susceptibility to this type of dynamic instability. Long-span, shell-type gates with an inclined weir plate angle of about 60°relative to the horizontal were found to be the most unstable.

Effects of Internal Flow on Vortex-Induced Vibration and Fatigue Life of Submarine Pipelines

Abstract: With the rapid development of the offshore oil industries, submarine oil / gas pipelines have been widely used. Under the complicated submarine environmental conditions, the dynamic characteristics of pipelines show some new features due to the existence of both internal and external flows. The paper is intended to investigate the vortex-induced vibration of the suspended pipeline span exposed to submarine steady flow. Especially, the effects of the flow inside the pipeline are taken into account. Its influences on the amplitude of pipeline response, and then on the fatigue life, are given in terms of the velocity of the internal flow.

An experimental study on vortex-induced vibration of circular cylinder towers at a high wind speed

Abstract: 円形断面を有する塔状構造物においては, 通常の渦励振共振風速よりも高風速域で渦励振と類似した応答が現れるとの報告があるが, その発生メカニズムは解明されていない. 本研究では円柱塔状1自由度ロッキング模型を用いた風洞実験を行い, 一様流中においてその現象を確認した. それを「高風速渦励振」と呼び, 気流や構造減衰を変化させて特性を調べた. 応答測定と後流変動風速の多点測定結果から高風速渦励振の発現特性を検討した結果, 自由端部付近に現れる渦が高風速渦励振を励起することが判明した. また, 後流変動風速の時系列にウェーブレット変換を適用した結果, 高風速渦励振の応答振幅に弱い変動があるのは, 励振源となっている自由端付近に現れる渦の発生がやや間欠的なためであることがわかった.

Numerical Simulation of Flow around a Circular Cylinder in a Solid-Liquid Two-Phase Flow Using a Vortex Method.

Abstract: In this study, a two-dimensional solid-liquid two-phase flow past a circular cylinder was simulated using the boundary element method combined with the vortex method. As a result of calculation, the flow pattern and the motion of particles were shown. It was found that the motion of particles did not follow the fluid movement in the Karman vortex street. In the cases of mixing ratios 0.25%, 1.0% and 1.5%, there is no difference in the Strouhal number of vortex shedding between the two-phase flow and the single-phase flow. In the case of mixing ratio 2.3%, two patterns of vortex shedding, that is Karman vortex type and twin vortex type, were shown, and the value of the Strouhal number was approximately 40% lower than the Strouhal number in the single-phase flow. The spin speed and size of the particle had an effect on vortex shedding.

弾性支持された円柱群まわりの流れに及ぼす円柱間隔比の影響 : 第1報, 並列1列配置

Abstract: This paper describes the effect of the spacing between two adjoining circular cylinders on the flow around two-dimensional circular cylinders supported by wires. The experiment was carried out in an N.P.L. blow-down unit wind tunnel with a working section of 500×500×2000mm, at the Reynolds number 9.4×103. The frequency of vortex shedding from the circular cylinder, displacement of the vibrating circular cylinder, and the time-mean and fluctuating surface pressure distributions on the circular cylinder were measured. The present results were compared with those for the case of fixed support Vortex shedding from circular cylinders occurs in the range of spacing ratio S/D≥2.3. The variations of Strouhal number, displacement, surface pressure distributions, drag coefficient, lift coefficient, fluctuating drag coefficient and fluctuating lift coefficient with the spacing ratio were obtained.

弾性支持された円柱群まわりの流れに及ぼす円柱間隔比の影響(第2報,並列3列配置)

Abstract: In this paper we describe the effect of spacing between two adjoining circular cylinders on flow around three rows of two-dimensional circular cylinder bundles supported by wires. The experiment was carried out in an N.P.L. blow-down-type wind tunnel with a working section of 500mm×500mm / 2000mm and with the Reynolds number of 9.4×103. The frequency of shedding vortices from the circular cylinder, displacement of the vibrating circular cylinder, and the time mean and fluctuating surface pressure distributions on the circular cylinder were measured. And the results were compared with those for the case of fixed support. Consequently it was found that (i) shedding vortices from circular cylinders exist in the range of spacing ratio S/D≥3 for the first row, S/D≥2.3 for the second row and S/D≥2.0 for the third row, (ii) the fluid elastic vibration occurs in the range of S/D<2.O for the first row and S/D<2.3 for the second row, though it does not occur for the third row, and (iii) the variation of fluctuating drag coefficient and fluctuating lift coefficient corresponds to that of displacement of a vibrating cylinder.

Prediction of Flow Induced Vibration of a Circular Cylinder Using LES

Abstract: The vortex induced vibration of a 2D circular cylinder in turbulent ow is studied at a Reynolds number of 13000 using the large eddy simulation technique by solving the incompressible Navier-Stokes equations and the continuity equations together. The computational tool used is Ansys uent 14. SIMPLE is the pressure-velocity coupling used for the solution with a xed time step of 5e-04. Numerical simulations of both rigid and moving cylinders are carried out. The results obtained from the LES for the rigid cylinder are compared with the experimental values. To simulate the vibrations of the cylinder,’Dynamic Mesh’ setup was used associated with the six-degree of freedom (SDOF) tool. The motion of the moving cylinder is obtained by solving the equation of motion in the direction transverse to the ow for zero and a few values of structural damping with the help of a user dened function (UDF) associated with the moving body. The computed response of the moving cylinder is compared with experimental data along with the aerodynamic Lift and Drag coecients, vortex shedding frequency and vibrating frequency which showed a good agreement.