Showing papers on "Vortex-induced vibration published in 2007"

Cavitation Influence on von Kármán Vortex Shedding and Induced Hydrofoil Vibrations

Abstract: The present study deals with the shedding process of the von Karman vortices at the trailing edge of a 2D hydrofoil at high Reynolds number. This research focuses mainly on the effects of cavitation and fluid-structure interaction on the mechanism of the vortex generation. The vortex shedding frequency, derived from the flow-induced vibration measurement, is found to follow the Strouhal law provided that no hydrofoil resonance frequencies are excited, i.e., lock-off. For such a regime, the von Karman vortices exhibit strong spanwise 3D instabilities and the cavitation inception index is linearly dependent on the square root of the Reynolds number. In the case of resonance, the vortex shedding frequency is locked onto the hydrofoil eigenfrequency and the spatial coherence is enhanced with a quasi-2D shape. The measurements of the hydrofoil wall velocity amplitude and phase reveal the first torsion eigenmotion. In this case, the cavitation inception index is found to be significantly increased compared to lock-off conditions. It makes clear that the vortex roll-up is amplified by the phase locked vibrations of the trailing edge. For the cavitation inception index, a new correlation relationship that encompasses the entire range of Reynolds numbers, including both the lock-off and the lock-in cases, is proposed and validated. In contrast to the earlier models, the new correlation takes into account the trailing edge displacement velocity. In addition, it is found that the transverse velocity of the trailing edge increases the vortex strength linearly. This effect is important in the context of the fluid-structure interaction, since it implies that the velocity of the hydrofoil trailing edge increases the fluctuating forces on the body. It is also demonstrated that cavitation developing in the vortex street cannot be considered as a passive agent for the turbulent wake flow. In fact, for fully developed cavitation, the vortex shedding frequency increases up to 15%, which is accompanied by the increase of the vortex advection velocity and reduction of the streamwise vortex spacing. In addition, a significant increase of the vortex-induced vibration level is found at cavitation onset. These effects are addressed and thought to be a result of the increase of the vorticity by cavitation.

A Study on Positioning Error Caused by Flow Induced Vibration Using Helium-Filled Hard Disk Drives

Abstract: In this paper, we compare the positioning accuracy of helium- and air-filled drives. We report the PES spectra of the both drives and calculate the mechanical disturbance. The results reveal that almost all disturbances in higher rpm drives are flow induced vibrations and that disturbances in helium-filled drives are dramatically smaller. The experimental positioning error of helium-filled drives is less than half of that of air-filled drives. The static and dynamic fluid computations explaine these experimental results.

Drag force control of flow over wavy cylinders at low reynolds number

Abstract: Three-dimensional numerical simulations on the laminar flow around a circular cylinder with different diameter along the spanwise leading to a type of sinusoidal waviness, named wavy cylinder are performed at low Reynolds number. A series of wavy cylinders with different combinations of spanwise wavelength and wave amplitude are conducted at a Reynolds number of 100. The optimal range of wavelength and the effect of wave amplitude are obtained. The results show that the 3-D free shear layers from the cylinder are more difficult to roll up to vortex and hence the wake formation lengths of some typical wavy cylinders are larger than that of the circular cylinder and in some cases the free shear layers even do not roll up into vortex behind the cylinder. The mean drag coefficients of the typical wavy cylinders are less than that of a corresponding circular cylinder with the same mean diameter; also the fluctuating lift coefficients are reduced. The reduction of mean drag coefficient and fluctuating lift coefficient of wavy cylinder increases with the value of wavy amplitude. Furthermore, a typical wavy cylinder model at Re=150 is also simulated and found that the control of flow induced vibration by modifing the spanwise wavelength of cylinder has a relationship with the variation of Reynolds number.

Systems and methods for reducing drag and/or vortex induced vibration

Abstract: There is disclosed a system for reducing drag and/or vortex induced vibration of a structure, the system comprising a fairing defining a plurality of perforations, wherein the fairing is suitable for placement around the structure, the perforations defining a porosity of the fairing of at least 1%.

Three-Dimensional Numerical Simulations of Circular Cylinders Undergoing Two Degree-of-Freedom Vortex-Induced Vibrations

Abstract: In an effort to gain a better understanding of vortex-induced vibrations (VIV), we present three-dimensional numerical simulations of VIV of circular cylinders. We consider operating conditions that correspond to a Reynolds number of 10 5 , low structural mass and damping (m*= 1.0, ζ*=0.005), a reduced velocity of U*=6.0, and allow for two degree-of-freedom (X and Y) motion. The numerical implementation makes use of overset (Chimera) grids, in a multiple block environment where the workload associated with the blocks is distributed among multiple processors working in parallel. The three-dimensional grid around the cylinder is allowed to undergo arbitrary motions with respect to fixed background grids, eliminating the need for grid regeneration as the structure moves on the fluid mesh.

An Investigation on the Effect of Tension Variation on VIV of Risers

Abstract: This paper aims at investigating the effect of vertical motion (or equivalently the effect of variable tensioning) of the floating unit on the vortex-induced vibrations of vertical risers. This is done using a numerical procedure, based on modeling assumptions, which, though simple, succeeded in describing some expected dynamic behaviors. The model simulates the riser dynamics using a finite element model coupled to a wake-oscillator model, of the van der Pol type, used to emulate the fluid dynamics. Vertical motion (or dynamic tension) is directly imposed to the top. The transverse amplitudes at each section feed the wake-oscillator, which responds with a transverse force that is applied to the riser. The rigidity matrix is updated at each time integration step. The analysis is also carried out with a commercial simulation code dedicated to riser analysis, with a similar wake-oscillator VIV module. Amplitude envelopes are extracted from the time series, showing response mode jumps. The application of the Hilbert-Huang spectral analysis technique helps distinguishing mode jumps by tracking frequency responses in time. The results of the two different dynamic models are compared with very good agreement.Copyright © 2007 by ASME

Experimental Investigation of Flow-Induced Vibration in a Parallel Plate Reactor Fuel Assembly.

Abstract: This research aims to experimentally investigate the critical flow velocity of light-water coolant in a reactor parallel-plate fuelassembly. The critical flow velocity is the speed at which rectangular fuel-plates will buckle and collapse onto each other as a result of flow-induced vibration and consequent asymmetric pressure distribution. Although fuel plates do not rupture during plate collapse, the excessive permanent lateral deflection (buckling) of a plate can cause flow blockage in the reactor core, which may lead to over-heating. This is an important consideration in reactor core designs with parallel plate fuel assemblies. The Replacement Research Reactor (RRR) currently under construction at the Australian Science and Technology Organisation (ANSTO) is of such a design. A simple physical model of a parallel-plate fuel-assembly composed of two parallel plates was constructed and tested in a closed-loop water tunnel (figure 1). Plate vibration was measured at low flow speeds and the critical flow velocity was recorded. Test results show plate collapse occurring, in 25°C light water, at an average flow velocity range of 11.9 - 12.0m/s. For the first time, cavitation was observed as a result of leadingedge deformation during plate collapse. The experimental results attained support Miller’s [4] critical-velocity calculation for plate collapse. However, it must be stressed that the flow characteristics of the RRR are significantly different to the results reported here due to the presence of a lateral-support comb at the RRR fuel assembly inlet. This comb greatly reduces any vibrations and increases the critical velocity for the fuel assembly.

CFD High L/D Riser Modeling Study

Abstract: Fully three dimensional fluid flow simulations are used with a simple structural model to simulate very long risers. This method overcomes many shortcomings of methodologies based on two dimensional flow simulations and can correctly include the effects of three dimensional structures such as strakes, buoyancy modules and catenary riser shapes. The method is benchmarked against laboratory and offshore experiments with model risers of length to diameter ratios up to 4,000. RMS values of vortex induced vibration motions are shown to be in good agreement with measurements. The resources needed to model ultra deep water drilling and production risers are estimated based on current computer technology.

Numerical Simulation of Vortex-induced Vibration of a Square Cylinder

Abstract: Vortex-induced vibration (VIV) of a square cylinder in a cross flow is examined numerically. Both the rigid and elastic cases are simulated at a low Reynolds number of 100. The approach solves the unsteady flow field using a finite element method with a deforming grid to accommodate the moving cylinders. As for the cylinder motions, a two-degree-of-freedom structural dynamics model is invoked. Fluid-structure interactions are resolved through iteration at the same time step. The calculated results for the case of rigid cylinder indicated that the non-dimensional vortex shedding frequency (or the Strouhal frequency) of a square cylinder at rest is 0.13, which is in good agreement with the published results. For the elastic case, with the change of the cylinder’s natural frequency, “lock-in” and “beat” phenomena were successfully captured. The phenomena of resonance and galloping can also be indicated.

Identifying the Power-In Region for Vortex-Induced Vibrations of Long Flexible Cylinders

Abstract: The primary objective of this research is to locate the source of the vortex-induced vibrations (VIV) for long flexible cylinders at high-mode number and to help determine the source region for future predictions. The two Gulf Stream tests were conducted to collect data on a scale-model pipe that was excited at high-mode numbers. The high density of the sensors on the pipe allowed for analysis that had not previously been done. Two methodologies are presented to locate the area of the region that is the source of the vibration. In VIV, the current which causes the vibrations is important, because the speed of the current will determine the frequency of the vibration. Therefore, one important question is which section of the pipe will be the source of the vibrations for a known current profile. This source region is known as the power-in region. Regions on the pipe that are not a source of power instead damp the structural vibrations. Once the region where the vibration originated has been found, the different phenomena that effect the location of the power-in region that were discovered are shown. Four different factors are presented that effect the locations of the power-in region: the angle of the pipe with respect to the vertical, the gradient of the current direction, the current profile, and the end effects at high mode number. A dimensionless parameter is presented which help in the prediction of VIV given a current profile. The power-in factor predicts the region where the source of the vibration occurs using a combination of the current velocity and the source region length.© 2007 ASME

Hydrodynamic Coefficients for In-Line Vortex Induced Vibrations

Abstract: The paper presents results from an experimental investigation of hydrodynamic forces on a cylinder under forced in-line motions. Measured forces are decomposed into added mass, driving force and average drag components. From a large set of experiments it has been possible to draw a complete map for in-line force coefficients as function of arbitrary combinations of motion amplitude and frequency. The paper presents test set-up, data processing and how the coefficients can be used in an empirical force coefficient model for calculation of in-line vibrations of slender marine structures with arbitrary damping. Such analyses are in particular important for free spanning pipelines, where damping from pipe/seafloor interaction will reduce the response amplitudes and hence also stresses and fatigue damage.Copyright © 2007 by ASME

Open loop control of vortex-induced vibration of a circular cylinder

Abstract: In this paper both numerical and experimental investigations have been carried out to suppress the vortex-induced vibration (VIV) of a circular cylinder in an electrically low-conducting fluid. The electromagnetic forces (Lorentz forces) in the azimuthal direction were generated through the mounted electrodes and magnets locally on the surface of the cylinder, which have been proved having an accelerating effect to the fluid on the surface of the cylinder. Results of computations are presented for synchronous vibration phenomenon of a cylinder at Re = 200, which are in good agreement with previous computational results. With the Lorentz forces loaded, the VIV of the cylinder has been suppressed successfully. Experimental results have also shown the same tendency and are in reasonable agreement with the numerical results.

Drilling Riser Fairing Tests at Prototype Reynolds Numbers

Abstract: Tow tests have been performed on flexible circular cylinders, with and without short weathervaning fairings, towed in a basin at critical and supercritical Reynolds numbers. The tests were conducted in the David Taylor Model Basin and the Rotating Arm Facility, at the Carderock Division, Naval Surface Warfare Center, in West Bethesda, Maryland. Measurements were made of both the drag and acceleration (due to vortex-induced vibration) of the cylinder. A 5-9/16-inch diameter PVC pipe was used to achieve Reynolds numbers ranging from about 7×105 to 1.5×106 , in uniform flow, for straight tow tests with the pipe experiencing first mode bending vortex-induced vibration. Fiberglass pipes with a 2.5 inch diameter were used to achieve high mode number vortex-induced vibration, in sheared flow, at Reynolds numbers as high as about 3.75×105 . The test results illustrate the importance of conducting tests at prototype Reynolds numbers for drilling riser as well as the importance of conducting tests in sheared flows and at higher mode numbers to fully understand the performance of a suppression device.Copyright © 2007 by Shell Global Solutions

Vortex-induced vibration of submerged floating tunnel tethers in shear current

Abstract: The engineering analysis method for vortex-induced transverse vibration of submerged floating tunnel(SFT) tether is presented,based on the modified wake oscillator model.The effects of specific gravity ratio and the shear current's characteristics on the vortex-induced vibration of tether are investigated. The calculated results show that the distribution of lock-in regions and the vortex-induced vibration of tether vary with the specific gravity ratios of SFT.The increase of the shear parameter makes the amplitude of vortex-induced vibration decrease.Furthermore,if the shear current is replaced by the uniform current to calculate the vortex-induced vibration of tether,its response amplitude may be overestimated.

Fluid flow on interacting deformable surfaces

Abstract: Fluid simulation on interacting deformable surfaces is a challenging problem that has many applications. In this paper, we present a framework in which artistic as well as physically realistic flows can be generated on surfaces during deformation and collision. Our simulation system provides comprehensive control over the motion and deformation of an object as well as the movement and density of the fluid on the surface. At the heart of our system is a numerical solver that allows viscous and incompressible flows to be directly generated on surfaces using concepts from differential geometry, such as geodesic polar maps and parallel transport. This solver is fast and stable even when the object undergoes deformation or collides with other surfaces. We also propose rules that allow deformation and collisions to impact fluid flows in a physically realistic manner. By combining these rules with a set of comprehensive design functionalities, we develop a system in which the user can specify shape deformation, collision, and fluid flow in a unified framework. We demonstrate the capability of our system with a number example scenarios. CR Categories: I.3.7 [COMPUTER GRAPHICS]: ThreeDimensional Graphics and Realism—Animation.

Hydrodynamic Force Identification From Vortex Induced Vibration Experiments With Slender Beams

Abstract: The characteristics of hydrodynamic forces are much more complicated for a long flexible cylinder under vortex induced vibration (VIV) than that for a rigid pipe. However, it is difficult to get force measurement directly from experiment with flexible beams. The dynamic behavior of the cylinder under VIV is recorded by accelerometers or transducers for bending moment measurement along the cylinder. The present work provides a modal approach to estimate the hydrodynamic force from measurement. The method is able to estimate the force at the fundamental vortex shedding frequency and the third order harmonic force component as well.Copyright © 2007 by ASME

High-precision vortex flow meter

Abstract: A high-precision vortex flow meter includes a blunt body having a predetermined dimension and being arranged inside a fluid channel to serve as a vortex shedder. The vortex flow meter also includes a temperature detecting device for detecting the vortex shedder temperature and a temperature control element for adjusting the vortex shedder temperature. A frequency measuring device is arranged in the downstream section of the blunt body for detecting the vortex shedding frequency. From the measured temperatures of the upstream fluid flow and that of the blunt body, an effective temperature and a temperature ratio are calculated. The kinematic viscosity of the fluid is looked up from database. By using the relationship between the Strouhal number and Reynolds number, the fluid flow rate is calculated. By employing different blunt body temperature, the measurement range of the flow meter can be broadly extended.

Vortex-Induced Vibration of a Variable Tension Riser

Abstract: The transverse vibratory response of a long, slender vertical top-tension riser, subject to an ocean current, is studied. The problem is treated as a coupled fluid-flow/vibration problem which is solved numerically. The fluid flow part is represented by the 2-D Navier-Stokes equations, with LES and strip theory, which are solved numerically to obtain the flow field and determine the vortex-shedding behavior in the flow. The approach flow is a shear flow ranging in Reynolds number from 8000 to 10,000. Given the flow field and vortex-shedding behavior, the transverse fluid forcing function can be determined at a given instant, which becomes the input to the Euler-Bernoulli beam equation to calculate the displacement of the riser, using a technique that involves the WKB method and modal decomposition. The boundary conditions for the fluid-flow equations are updated each time step as the cylinder moves. The natural frequency of the riser is tension-dominated, not bending stiffness-dominated. With the decrease in tension with increasing depth, the natural frequency is affected. Therefore, the solution will be influenced by the depth-dependent tension. This study has indicated some interesting features regarding the VIV of a variable-tension riser. The vibrational response is greater for a variable-tension riser than for a constant-tension riser, when the variable-tension riser is assumed to have the same top tension as the constant-tension riser. Therefore, it is important to take into account the variable tension when estimating fatigue failures of marine risers.Copyright © 2007 by ASME

Three-Dimensional Numerical Simulations of Flows Past Smooth and Rough/Bare and Helically Straked Circular Cylinders Allowed to Undergo Two Degree-of-Freedom Motions

Abstract: We simulate the flow past smooth and rough rigid circular cylinders that are either bare or outfitted with helical stakes. We consider operating conditions that correspond to high Reynolds numbers of 105 and 106 , and allow for two degree-of-freedom motions when the structure is allowed to respond to vortex-induced cross flow and in-line forces. The computations are performed using a parallelized Navier-Stokes in-house solver using overset grids. For smooth surface simulations at a Reynolds number of 105 , we use a Smagorinsky Large Eddy Simulation (LES) turbulence model and for the Reynolds number cases of 106 we make use of the unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with a two-layer k-epsilon turbulence model. The rough surface modifications of the two-layer k-epsilon turbulence model due to Durbin et al. (ASME J. Fluids Eng., 2001) are implemented to account for surface roughness effects. In all our computations we aim to resolve the boundary layer directly by using adequate grid spacing in the near-wall region. The predicted global flow parameters under different surface conditions are in good agreement with experimental data and significant VIV suppression is observed when using helically straked cylinders.Copyright © 2007 by ASME and Shell Global Solutions, Inc.

An experimental study of flow induced vibration on a tensioned membrane

Abstract: In pursuing noise and wave control with minimal aerodynamic or hydrodynamic sacrifice, a tensioned membrane is used to line the otherwise rigid duct wall. The membrane vibrates in response to the grazing incident waves and the vibration serves to reflect the wave towards its source. The mechanism is identical to what happens in a rig of active wave control, but the difference is that the current rig has no active component. For the purpose of wave control, the device has been tested successfully without flow and with moderate flow conditions commonly found in air ventilation systems. When the flow speed is further increased, flow induced vibration occurs. This study reports the phenomena of such vibration under various flow speeds and membrane tensions. Transient process of exponential vibration growth is recorded and analysed together with the boundary layer measurements. The effects of an external cavity as well as the lateral membrane tension are also found to be significant. Other possible mechanisms for the flow induced vibration are also explored and discussed.

Vortex-induced Vibration Control by Micro Actuator

Abstract: The control of vortex-induced vibration of two side-by-side circular cylinders in a cross flow is carried out experimentally. One cylinder is elastically supported and the other is fix-supported at both ends. The two cylinders vibrate under the action of the unsteady flow-induced forces. A micro actuator is embedded on the surface of each cylinder to perturb the boundary layer. The spacing ratio is set at 1.2. The measurement shows that the structural vibration can be suppressed significantly when the reduced excitation frequency is around 2.655.

Flow-Induced In-Line Oscillation of Two Square Cylinders in Tandem Arrangement

Abstract: Flow-induced in-line oscillation of two tandem circular cylinders has been experimentally studied by free-oscillation tests in a wind tunnel. Only one cylinder composed of two tandem cylinders was elastically supported easily to move in the in-line direction for reduced mass-damping parameter Cn≈1 and the other of the tandem cylinders was fixed to the tunnel sidewalls. The gap distance ratio between upstream and downstream cylinders was changed from 0.3 to 3. We measured the response amplitudes of the oscillatory cylinder in the in-line direction and the vortex-shedding frequency in the wake. The flow around the tandem cylinders was visualized by the smoke-wire method. The results of in-line oscillation of an upstream cylinder are as follows, there was a wide excitation region of reduced velocity, Vγ=1.5 to 2.5 in all ranges of gap ratio of 0.3 to 3. This excited oscillation was mainly induced by symmetric vortex shedding;the other excitation of Vγ= 2.8 to 3.5 at wide gap distance ratios of 1.75 to 3, was induced by alternate Karman vortex shedding, as in the case of the in-line oscillation of a single cylinder. The downstream cylinder had an excitation region in a wide range of Vγ=2 to 4 for the narrow gap distance ratios of 0.3 to 0.75. which seems to be induced by alternate Karman vortex shedding. The other excitation regions due to symmetrical vortices were limited to the region of gap distance ratio of 0.75 to 2. Furthermore, the downstream cylinder oscillated as a buffeting phenomenon influenced by wake-fluctuation of the upstream cylinder, when gap distance ratio was greater than 2.5.

High Mode Vortex Induced Vibration (VIV) Experiments on a Large-Scale Riser

Abstract: The focus of this research is on the development and testing of a large-scale model riser (130 m in length) undergoing high mode vortex induced vibrations (VIV) in the ocean environment. This large scale model will provide an intermediate step between the common riser models (8-10 m in length) that have mainly been used to research VIV to date and the actual 3,000+ m deepwater risers being used in industry today. During offshore drilling operations, marine risers carry mud and debris from below the sea floor, and during production operations they are responsible for transporting oil or gas from the hydrocarbon reservoir to the surface platform. The integrity of a marine riser is therefore critical to the success of offshore drilling and production. The flow of seawater around marine risers is subject to vortex shedding which excites oscillations known as Vortex Induced Vibrations (VIV). When the VIV frequency approaches one of the natural frequencies of the structure, resonance, or lock-in occurs. This results in enhancement of the vibration amplitude of the structure and may have potentially destructive consequences due to high bending stresses and fatigue damage of the riser. At present, the prediction of this phenomenon is one of the most challenging areas in the offshore industry. In experimental investigations of VIV, large aspect ratio risers, namely long cylinders with relatively small diameters, pose a modeling challenge. This paper focuses on the development of a large-scale model riser capable of transmitting data in real-time under realistic operating conditions.

Numerical investigation of vortex-induced vibration of a marine scr

Abstract: In this paper the dynamic response and fatigue analysis of a marine SCR (Steel Catenary Riser) due to vortex shedding is numerically investigated. The riser is divided in two-dimensional sections along the riser length. The discrete vortex method (DVM) is employed for the assessment of the hydrodynamic forces acting on these two-dimensional sections. The hydrodynamic sections are solved independently, and the coupling among the sections is taken into account by the solution of the structure in the time domain by the finite element method implemented in ANFLEX code [1]. Parallel processing is employed to improve the performance of the method. A master-slave approach via MPI (Message Passing Interface) is used to exploit the parallelism of the present code. The riser sections are equally divided among the nodes of the cluster. Each node solves the hydrodynamic sections assigned to it. The forces acting on the sections are then passed to the master processor, which is responsible for the calculation of the displacement of the whole structure. The time histories of stress are employed to evaluate the damage as well as the life expectancy of the structure by the rainflow method to count the cycles in the dynamic response.Copyright © 2007 by ASME