Vortex-Induced Vibration and Drag Coefficients of Long Cables Subjected to Sheared Flows

1986 ◽  
Vol 108 (1) ◽  
pp. 77-83 ◽  
Author(s):  
Y.-H. Kim ◽  
J. K. Vandiver ◽  
R. Holler
Keyword(s):  
Field Experiments ◽  
Broad Band ◽  
Single Mode ◽  
Response Spectra ◽  
Flow Speed ◽  
Current Profile ◽  
Vortex Induced Vibration ◽  
Sheared Flow ◽  
Typical Experiment ◽  
Lock In

The vortex-induced vibration response of long cables subjected to vertically sheared flow was investigated in two field experiments. In a typical experiment, a weight was hung over the side of the research vessel by a cable that was instrumented with accelerometers. A typical experiment measured the acceleration response of the cable, the current profile, the tension, and angle of inclination at the top of the cable. Total drag force was computed from the tension and angle measurements. Two braided Kevlar cables were tested at various lengths from 100 to 9,050 ft. As a result of these experiments, several important conclusions can be drawn: (i) the wave propagation along the length of the cable was damped, and therefore, under most conditions the cable behaved like an infinite string; (ii) response spectra were quite broad-band, with center frequencies determined by the flow speed in the region of the accelerometer; (iii) single mode lock-in was not observed for long cables in the sheared current profile; (iv) the average drag coefficient of long cables subjected to sheared flow was considerably lower than observed on short cables in uniform flows; (v) the r.m.s. response was higher in regions of higher current speed. A new dimensionless parameter is proposed that incorporates the properties of the cable as well as the sheared flow. This parameter is useful in establishing the likelihood that lock-in may occur, as well as in estimating the number of modes likely to respond.

Numerical Simulation of Vortex Induced Vibration on an Elastically Mounted Cylinder

2004 ◽  
Author(s):  
Juan B. V. Wanderley ◽  
Carlos A. Levi
Keyword(s):  
Stokes Equations ◽  
Oscillation Amplitude ◽  
High Sensitivity ◽  
Flow Speed ◽  
Navier Stokes ◽  
Experimental Measurements ◽  
Navier Stokes Equations ◽  
Vortex Induced Vibration ◽  
Set Up ◽  
Lock In

The Vortex-induced vibration on a circular cylinder is investigated by the numerical solution of the unsteady Reynolds Average Navier-Stokes equations and results are compared with experimental measurements obtained by different authors. The Beam and Warming implicit factored scheme is used to solve the governing equations and the Baldwin and Lomax model is used to simulate the turbulent flow in the wake of the cylinder. The cylinder is laterally supported by a spring and a damper and is free to oscillate in the transversal direction in an initially uniform flow for the first flow speed investigated. For the subsequent speeds, the final condition obtained for the previous speed is used as initial condition to reproduce the actual experimental set up. In that case, the measurements are done by progressive increments of the flow speed retaining the fluid memory effect. The complexity and high sensitivity of the flow phenomenon at this configuration requires a very accurate and robust numerical model. Most of the known algorithms failed to duplicate the available experimental measurements. The numerical results for the transversal oscillation amplitude are compared to experimental data showing a fairly precise agreement at the difficult to simulate regime of the lock-in phenomenon.

scholarly journals Vortex-Induced Vibration of Catenary Riser: Reduced-Order Modeling and Lock-In Analysis Using Wake Oscillator

2009 ◽  
Author(s):  
Narakorn Srinil ◽  
Marian Wiercigroch ◽  
Patrick O’Brien ◽  
Rae Younger
Keyword(s):  
Interaction Model ◽  
Single Mode ◽  
Dynamic Responses ◽  
Ocean Current ◽  
Flexible Cylinder ◽  
Vortex Induced Vibration ◽  
Reduced Order ◽  
Wake Oscillator ◽  
Lock In ◽  
Multi Mode

A novel reduced-order fluid-structure interaction model for the vortex-induced vibration of catenary riser subject to the ocean current is developed and systematically investigated. The semi analytical-numerical approach accommodates multi-mode nonlinear dynamic responses and accounts for the effect of varying initial curvature of the inclined flexible cylinder. The geometrically nonlinear equations of riser motion are based on a pinned-pinned beam-cable model with bending and extensibility stiffness. The empirical hydrodynamic model is based on a distributed van der Pol wake oscillator which approximates the space-time varying fluid forces. In this initial study, the incoming current flow is assumed to be steady, uniform, unidirectional and perpendicular to the riser initial plane of curvatures. Thus, emphasis is placed on evaluating the riser cross-flow responses due to fluctuating lift forces. A preliminary validation of model and analysis results has been performed. Several insights into the vortex-induced vibration of catenary risers are highlighted through a series of parametric studies. These include the characterization of single-mode vs. multi-mode lock-in, the limitations of a single-mode solution through a convergence analysis which accounts for a varying number of considered riser modes, the prediction of riser maximum response amplitudes, the quantitative/qualitative behaviors of tension- or beam-dominant catenary risers and the overall influence of fluid-riser parameters. Moreover, recent industrial concepts of modes switching/sharing are discussed along with the meaningful effect of Reynolds number.

KONDO EFFECT AND SPIN-CHARGE SEPARATION IN QUANTUM DOTS

2001 ◽  
Vol 15 (10n11) ◽  
pp. 1426-1442
Author(s):  
L. I. GLAZMAN ◽  
F. W. J. HEKKING ◽  
A. I. LARKIN
Keyword(s):  
Quantum Dot ◽  
Charge Separation ◽  
Coulomb Blockade ◽  
Kondo Effect ◽  
Broad Band ◽  
Single Mode ◽  
Kondo Temperature ◽  
Average Charge ◽  
Opposite Case ◽  
Anderson Impurity Model

The Kondo effect in a quantum dot is discussed. In the standard Coulomb blockade setting, tunneling between the dot and the leads is weak, the number of electrons in the dot is well-defined and discrete; the Kondo effect may be considered in the framework of the conventional one-level Anderson impurity model. It turns out however, that the Kondo temperature TK in the case of weak tunneling is extremely low. In the opposite case of almost reflectionless single-mode junctions connecting the dot to the leads, the average charge of the dot is not discrete. Surprisingly, its spin may remain quantized: s=1/2 or s=0, depending (periodically) on the gate voltage. Such a "spin-charge separation" occurs because, unlike an Anderson impurity, a quantum dot carries a broad-band, dense spectrum of discrete levels. In the doublet state, the Kondo effect develops with a significantly enhanced TK. Like in the weak-tunneling regime, the enhanced TK exhibits strong mesoscopic fluctuations. The statistics of the fluctuations is universal, and related to the Porter-Thomas statistics of the wave function fluctuations.

scholarly journals Final design and on-sky testing of the iLocater SX acquisition camera: broad-band single-mode fibre coupling

2020 ◽  
Vol 501 (2) ◽  
pp. 2250-2267
Author(s):  
J Crass ◽  
A Bechter ◽  
B Sands ◽  
D King ◽  
R Ketterer ◽  
...  
Keyword(s):  
Adaptive Optics ◽  
Near Infrared ◽  
Broad Band ◽  
Single Mode ◽  
Injection System ◽  
Single Mode Fibre ◽  
Astronomical Instruments ◽  
Final Design ◽  
Camera Design ◽  
And Performance

ABSTRACT Enabling efficient injection of light into single-mode fibres (SMFs) is a key requirement in realizing diffraction-limited astronomical spectroscopy on ground-based telescopes. SMF-fed spectrographs, facilitated by the use of adaptive optics (AO), offer distinct advantages over comparable seeing-limited designs, including higher spectral resolution within a compact and stable instrument volume, and a telescope independent spectrograph design. iLocater is an extremely precise radial velocity (EPRV) spectrograph being built for the Large Binocular Telescope (LBT). We have designed and built the front-end fibre injection system, or acquisition camera, for the SX (left) primary mirror of the LBT. The instrument was installed in 2019 and underwent on-sky commissioning and performance assessment. In this paper, we present the instrument requirements, acquisition camera design, as well as results from first-light measurements. Broad-band SMF coupling in excess of 35 per cent (absolute) in the near-infrared (0.97–1.31 ${\mu {\rm m}}$) was achieved across a range of target magnitudes, spectral types, and observing conditions. Successful demonstration of on-sky performance represents both a major milestone in the development of iLocater and in making efficient ground-based SMF-fed astronomical instruments a reality.

Fatigue Damage From High Mode Number Vortex-Induced Vibration

2006 ◽  
Author(s):  
J. Kim Vandiver ◽  
Susan B. Swithenbank ◽  
Vivek Jaiswal ◽  
Vikas Jhingran
Keyword(s):  
Fatigue Damage ◽  
Field Experiments ◽  
Higher Harmonics ◽  
Vortex Induced Vibration ◽  
Third Harmonic ◽  
The Third ◽  
Damage Rate ◽  
Surface Vessels ◽  
Vortex Shedding Frequency ◽  
First Time

This paper presents results from two field experiments using long flexible cylinders, suspended vertically from surface vessels. The experiments were designed to investigate vortex-induced vibration (VIV) at higher than tenth mode in uniform and sheared flows. The results of both experiments revealed significant vibration energy at the expected Strouhal frequency (referred to in this paper as the fundamental frequency) and also at two and three times the Strouhal frequency. Although higher harmonics have been reported before, this was the first time that the contribution to fatigue damage, resulting from the third harmonic, could be estimated with some certainty. This was enabled by the direct measurement of closely spaced strain gauges in one of the experiments. In some circumstances the largest RMS stress and fatigue damage due to VIV are caused by these higher harmonics. The total fatigue damage rate including the third harmonic is shown to be up to forty times greater than the damage rate due to the vibration at the fundamental vortex-shedding frequency alone. This dramatic increase in damage rate due to the third harmonic appears to be associated with a narrow range of reduced velocities in regions of the pipe associated with significant flow-induced excitation.

An Approach to Include Observed VIV Likelihood in Drilling Riser Fatigue Analyses

2009 ◽  
Author(s):  
Michael A. Tognarelli ◽  
Rene D. Gabbai ◽  
Mike Campbell
Keyword(s):  
Fatigue Damage ◽  
Field Measurements ◽  
Scaling Analysis ◽  
Current Profile ◽  
Vortex Induced Vibration ◽  
Probability Of Occurrence ◽  
Industry Standard ◽  
Annual Probability ◽  
Analysis Computer ◽  
Viv Suppression

Field measurements of the response of a number of drilling risers indicate that vortex-induced vibration (VIV) occurs significantly less often than predicted by the industry-standard fatigue analysis computer program SHEAR7 V4.4. Several comparisons to model tests and field data, including one published by BP and 2H in 2007 [1], demonstrate that this analysis program is generally quite conservative, given that VIV occurs. Furthermore, this conservatism does not take into account those situations in which VIV fatigue is predicted but none is observed in the field, which adds yet another layer of “hidden” conservatism to design analyses. In an effort to address this and reduce conservatism to a more appropriate level, the probability of occurrence of vortex-induced vibration (VIV) is examined using full-scale measured data. The data has been collected over the past several years from five drilling risers without VIV suppression devices. These risers are on rigs under contract to BP at high-current-susceptible sites worldwide. Collectively, the data correspond to 9,600 10-minute field measurements, equivalent to 0.18 years of continuous monitoring. The riser response is obtained from motion loggers placed at selected positions along the riser as described in [1]. Each logger measures 3D accelerations and 2D angular rates. Through-depth currents are measured via Acoustic Doppler Current Profilers (ADCP). By comparison of measurements to computer predictions based on the observed current profile, a relationship is developed between the intensity of the fatigue damage predicted and the probability that VIV is observed in the field. Subsequently, an approach is proposed for scaling analysis predictions to reflect the relative likelihood of VIV. The database of measured and SHEAR7 maximum predicted fatigue damage rates is statistically characterized to determine how it may be used to determine factors of safety (FOS) for VIV design. A worked example for a deepwater drilling riser in the GoM is used to show how the FOS methodology can be applied in the case of multiple design currents each with a different annual probability of occurrence.

Vortex-Induced Vibration and Frequency Lock-In of an Airfoil at High Angles of Attack

2015 ◽  
Vol 138 (1) ◽  
Author(s):  
Fanny M. Besem ◽  
Joshua D. Kamrass ◽  
Jeffrey P. Thomas ◽  
Deman Tang ◽  
Robert E. Kielb
Keyword(s):  
Secondary Flows ◽  
Common Source ◽  
Structural Failure ◽  
Vortex Induced Vibration ◽  
Fluid Instability ◽  
Unsteady Force ◽  
Computational Fluid Dynamics Cfd ◽  
Shedding Frequency ◽  
Lock In ◽  
High Angles Of Attack

Vortex-induced vibration is a fluid instability where vortices due to secondary flows exert a periodic unsteady force on the elastic structure. Under certain circumstances, the shedding frequency can lock into the structure natural frequency and lead to limit cycle oscillations. These vibrations may cause material fatigue and are a common source of structural failure. This work uses a frequency domain, harmonic balance (HB) computational fluid dynamics (CFD) code to predict the natural shedding frequency and lock-in region of an airfoil at very high angles of attack. The numerical results are then successfully compared to experimental data from wind tunnel testings.

scholarly journals Experimental Study on the Effects of a Vertical Jet Impinging on Soft Bottom Sediments

2020 ◽  
Vol 12 (9) ◽  
pp. 3775 ◽  
Author(s):  
Wei Fan ◽  
Weicheng Bao ◽  
Yong Cai ◽  
Canbo Xiao ◽  
Zhujun Zhang ◽  
...  
Keyword(s):  
Theoretical Model ◽  
Side Effects ◽  
Field Experiments ◽  
Sediment Resuspension ◽  
Ecological Engineering ◽  
Phosphorus Release ◽  
Flow Speed ◽  
Critical Conditions ◽  
Water Tank ◽  
Flow Speeds

Artificial downwelling, which is an ecological engineering method, potentially alleviates bottom hypoxia by bringing oxygen-rich surface water down below the pycnocline. However, the downward flow is likely to disturb sediments (or induce sediment resuspension) when reaching the bottom and then have unwanted side effects on the local ecosystem. To evaluate this, our paper presents a theoretical model and experimental data for the sediment resuspension caused by artificial downwelling. The theoretical model considers the critical conditions for sediment resuspension and the scour volume with the downwelling flow disturbing sediment. Experiments with altered downwelling flow speeds, discharge positions relative to the bottom, and particle sizes of sediment were conducted in a water tank, and the results were consistent with our theoretical model. The results show that the critical Froude number (hereinafter Fr) for sediment resuspension is 0.5. The prevention of sediment resuspension requires the downwelling flow speed and the discharge position to be adjusted so that Fr < 0.5; otherwise a portion of sediment is released into the water and its volume can be predicted by the derived formulation based on the Shields theory. Furthermore, sediment resuspension has side effects, such as a water turbidity increase and phosphorus release, the magnitudes of which are discussed with respect to engineering parameters. Further study will focus on field experiments of artificial downwelling and its environmental impacts.

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