Showing papers on "Vortex-induced vibration published in 1989"

Application of a General Technique for Prediction of Riser Vortex-Induced Response in Waves and Current

Abstract: This paper describes applications of an extension to a recently developed calculation method for vortex-induced response of risers and tethers. The vortex-induced vibration response for the excited modes is generated using a semi-empirical formulation which is applied within a time domain calculation. This makes it possible to take account of the variation of flow properties along the riser length, as well as in time. The calculation method has been verified by comparison with model test data for vortex-induced response to surface vessel motions in still water. This paper presents applications of the technique for vortex shedding due to the combined action of current, surface vessel motions and waves. The mathematics of the extended prediction technique is described and results are illustrated by presenting typical vortex-induced responses for single-tube risers. The results of the prediction method are used to highlight the relative magnitudes and nature of vortex-induced response excited by currents, surface vessel motions and waves.

Effect of Strouhal Number on Flow Characteristics in a Symmetric Sinusoidal Wavy-Walled Channel for Oscillatory Flow

Abstract: The effect of the Strouhal number on several flow characteristics in a wavy-walled channel was numerically and experimentally studied for oscillatory flow. Although oscillatory flow is characterized by a vortex formation/ejection cycle for any Strouhal number, the peak of vortex strength appears at an intermediate Strouhal number for both instantaneous flow and steady streaming. The time required for erosion of the ejected vortex at the centerline of the channel is closely related to the vortex strength at the instant of zero flow rate.

Computation of vortex induced vibration and the effect of correlation on circular cylinders in oscillatory flow

Abstract: A numerical method for the computation of vortex shedding induced vibration is presented. The method is a discrete vortex method based on operator splitting, random walk and vortex-in-cell techniques originally developed by Stansby and Dixon (1983) at University of Manchester. Simulation of the oscillatory flow around a flexibly mounted rigid cylinder is carried out. The results are compared with experiments (Bearman et Hall 1987). The 2-dimensional fluid loading method is then applied by a strip theory formulation for a number of sections along the same flexibly mounted rigid cylinder. The different sections are hydrodynamically uncoupled. When the cylinder starts to oscillate, the correlation between the transverse forces for each section increases, due to the motion of the cylinder only. The effect of correlation on vibration amplitudes is shown. The method is also compared with experiments for prescribed displacement of a model riser in still water (Lyons and Patel 1987).

A full-scale experimental study of the effect of shear current on the vortex-induced vibration and quasi-static configuration of a long tow cable

Abstract: The analysis of data of a unique experiment on the quasi-statics and vortex-induced dynamic of a long vertical tow cable. The measured steady-state configuration of the cable was used to calculate the cable's average drag coefficient which was 2.2 plus or minus 0.24. It was observed during the sea trials that the vortex-induced motion of the cable was modulated by a beat due to the presence of a shear current. This is confirmed here by spectral and time domain analysis of the acceleration records. The modulated amplitude caused the RMS motion of the cable to be lower than that in a uniform flow. The modulation phenomenon that is caused by the presence of a shear current thus accounts for the lower drag amplification that is observed here an has been reported elsewhere by others. The RMS motion of the cable decreases with depth which indicates that the drag coefficient of the cable also decreases with depth.

A study on vortex shedding from spheres in a uniform flow.

Abstract: Vortex shedding from spheres at Reynolds numbers from 3×102 to 6×104 in a uniform flow was experimentally investigated. Standard hot-wire techniques were used to measure the vortex shedding frequency form spheres in a low-speed wind tunnel. Flow-visualization experiments were carried out in a water channel. Important findings resulting from the investigation were that (i) the variation of the Strouhal number St (=fD/U0, U0 : free-stream velocity, D : diameter of sphere, f : vortex shedding frequency) with the Reynolds number Re (=U0D/ν, ν : kinematic viscosity) can be classified into four regions, (ii) the critical Reynolds number at which the horseshoe-shaped vortex loops are shed periodically is about 300, (iii) the vortex loops diffuse very rapidly and the wake structure of the sphere is not shown clearly when a Reynolds number of about 800 is reached, and (iv) at Reynolds numbers ranging from 8×102 to 1.5×104, the higher and lower frequency modes of the Strouhal number which are caused by the periodic fluctuation in the vortex sheet separating from the surface of the sphere and in a wake with progressive wave motion are coexisted.

Computation of vortex induced fluid loading and response interaction of marine risers

Abstract: A numerical method for fully integrated hydrodynamic loading and structural response interaction of marine risers is presented. The empirical Morison's formula is replaced by a numerical technique for computation of vortex induced forces on circular cylinders subjected to an arbitrary incoming flow. The method is a discrete vortex method based on operator splitting, random walk and vortex-in-cell techniques originally developed by Stansby et al at University of Manchester. Vortex shedding induced nonsteady in-line and transverse forces on the structure are computed taking into account ambient flow from waves, current and the motion response of the compliant structure itself. The 2-dimensional fluid loading program is applied by a strip theory formulation in a number of sections along the riser, and the loads are distributed on the elements within each section. The method is tested for a marine steel riser subjected to current, waves and top-point displacement in 310m water depth. Results in form of time histories of displacements, trajectory plots, deformed shapes, power density spectra of force and response are presented. In addition a comparison with the use of the traditional Morison formula wave and current loading is shown.

Study and analysis on the flow induced vibration of the core barrel of pwr

Abstract: This paper describes the deduction of the resemblance criterion and the design of the test model by applying flow-solid coupling theory. The model analyses of a core barrel both in the air and stationary water were performed in a 1:10 model, thus obtaining the dynamic characteristic. In a 1:5 reactor model with a hydraulic closed loop, the inner structure and support were modeled for performing hydraulic vibration test of the core barrel. The flow induced pulse pressure of the core barrel and corresponding response were obtained by using miniature pressure capsule, strain gauge and accelerometer. Power spectrum, correlation functions, transfer function and amplitudes under different flow velocities were calculated. The hydraulic vibration test shows that the core barrel will be in safety during its 30-year life time.

流出渦と連成した長径間ゲートの流体関連振動 : 第1報,振動発生の条件と流体励振力の強さおよび流体付加質量

Abstract: Submerged long span gates which dam a wide river undergo violent streamwise vibrations caused by vortex shedding beneath the gate. This study presents flow-induced vibration characteristics which were obtained in a model gate test. From the measured vibration frequencies and damping ratios in air and water, respectively, the level of fluid excitation and the added mass for small amplitude gate vibrations were calculated and reduced to a dimensionless form, and thus the vibration onset criteria were obtained. In addition to the average value of the maximum amplitude of gate vibration, was measured. The results of these experiments, taken as a whole, suggest that the flow-induced vibration characteristics of the long-span gates are well predicted by the dimensionless parameters, such as the reduced gate opening height and the reduced velocity.