Current-Induced Fluidelastic Instabilities of a Multi-Tube Flexible Riser: Theoretical Results and Comparison With Experiments

1993 ◽  
Vol 115 (4) ◽  
pp. 206-212 ◽  
Author(s):  
S. J. Price ◽  
M. P. Pai¨doussis ◽  
A. M. Al-Jabir
Keyword(s):  
Free Stream ◽  
Cross Flow ◽  
Quantitative Agreement ◽  
Circular Cylinders ◽  
Flexible Riser ◽  
Tunnel Model ◽  
Flow Induced Vibrations ◽  
Static Fluid ◽  
The Stability ◽  
Theoretical Results

Previous experiments on a five-riser cluster in steady cross-flow have indicated that for certain orientations of the cluster the peripheral, or wing, risers can undergo violent flow-induced vibrations. It was shown that these vibrations were not due to vortex shedding; furthermore, it was suggested that they are due to a classical self-excited fluidelastic instability. In the present paper, a previously developed quasi-steady fluidelastic stability analysis for a group of circular cylinders in steady crossflow is modified to enable the stability of a flexible riser in a five-riser bundle to be analyzed. As input to the theoretical model, the static fluid force coefficients on a peripheral riser, and the manner in which they vary with displacement, are required. These were measured for a number of orientations of the cluster with respect to the free-stream current, using a wind tunnel model. Using this data in the analysis, the stability of a five-riser cluster was investigated. Instability is predicted to occur for the same orientations of the riser as obtained from the experimental results; however, the quantitative agreement between experimental and theoretical critical flow velocities for instability to occur is not as good. Theory also predicts the system to be stable for those orientations where no instability was obtained experimentally.

On The Influence Of Porous Coating Thickness On Supersonic Boundary Layer Stability

2015 ◽  
Vol 10 (3) ◽  
pp. 41-47
Author(s):  
Vladimir Lysenko ◽  
Sergey Gaponov ◽  
Boris Smorodsky ◽  
Yuri Yermolaev ◽  
Aleksandr Kosinov ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Coating Thickness ◽  
Free Stream ◽  
Plate Boundary ◽  
Quantitative Agreement ◽  
Supersonic Boundary Layer ◽  
Linear Stability Theory ◽  
Porous Coating ◽  
Free Stream Mach Number ◽  
The Stability

Theoretical and experimental investigation of the influence of porous-coating thickness on the stability of the supersonic flat-plate boundary layer at free-stream Mach number M = 2 have been performed. Good quantitative agreement of experimental data obtained with artificially generated disturbances performed on models with various porous inserts and calculations based on the linear stability theory has been achieved. It is shown that the increase of the porous-coating thickness leads to the boundary layer destabilization.

Flow-Induced Vibrations in Two-Phase Flow

1991 ◽  
Vol 113 (2) ◽  
pp. 234-241 ◽  
Author(s):  
S. S. Chen
Keyword(s):  
Two Phase Flow ◽  
Cross Flow ◽  
Axial Flow ◽  
Circular Cylinders ◽  
Phase Flow ◽  
Two Phase ◽  
Flowing Fluid ◽  
Flow Induced Vibrations ◽  
Excitation Mechanisms ◽  
Quiescent Fluid

Two-phase flow exists in many shell-and-tube heat exchangers and power generation components. The flowing fluid is a source of energy that can induce small-amplitude subcritical oscillations and large-amplitude dynamic instabilities. In fact, many practical system components have experienced excessive flow-induced vibrations. This paper reviews the current understanding of vibration of circular cylinders in quiescent fluid, cross-flow, and axial flow, with emphasis on excitation mechanisms, mathematical models, and available experimental data. A unified theory is presented for cylinders oscillating under different flow conditions.

Local Force Measurements on Finite-Span Cylinders in a Cross-Flow

1987 ◽  
Vol 109 (2) ◽  
pp. 136-143 ◽  
Author(s):  
V. K. Sin ◽  
Ronald M. C. So
Keyword(s):  
Free Stream ◽  
Cross Flow ◽  
Circular Cylinders ◽  
Two Dimensional ◽  
Force Measurements ◽  
Unsteady Forces ◽  
Free Stream Turbulence ◽  
Finite Span ◽  
Present Technique ◽  
Unsteady Force

A technique employing a three-axis piezoelectric load cell is developed to measure local unsteady forces induced on cylinders placed in a cross flow. Verification of the technique is carried out with a two-dimensional circular cylinder. All measurements are made at a Reynolds number of ∼4.8 × 104 and a free-stream turbulence of ∼1.5 percent. The local two-dimensional unsteady lift measurement is found to be in excellent agreement with spanwise-averaged data reported in the literature, thereby validating the feasibility of the present technique. Steady and unsteady force measurements on finite-span circular cylinders are reported and compared with available data in the literature.

Experimental Investigation of Flow-Induced Vibrations Interference Between Two Circular Cylinders in Tandem Arrangements

2005 ◽  
Author(s):  
Gustavo R. S. A´ssi ◽  
Julio R. Meneghini ◽  
Jose´ A. P. Aranha ◽  
Peter W. Bearman ◽  
Bruno S. Carmo ◽  
...  
Keyword(s):  
Flow Direction ◽  
Water Channel ◽  
Cross Flow ◽  
Elastic Base ◽  
Circular Cylinders ◽  
Interference Phenomenon ◽  
Flow Induced Vibrations ◽  
Set Up ◽  
Flow Interference ◽  
Future Work

This paper presents experimental results concerning flow-induced oscillations of rigid-circular cylinders in tandem. Preliminary results are presented: new measurements on the dynamic response oscillations of an isolated cylinder and flow interference of two cylinders in tandem are shown. The oscillations are due to vortex-induced vibrations (VIV). Models are mounted on an elastic base fitted with flexor blades and instrumented with strain gages. The base is fixed on the test section of a water channel facility. The flexor blades possess a low damping characteristic [ζ ≈ 0.008 and less] and they are free to oscillate only in the cross-flow direction. The Reynolds number of the experiments is from 3,000 to 13,000 and reduced velocities, based on natural frequency in still water, range up to 12. The interference phenomenon on flow-induced vibrations can be investigated by conducting experiments in two ways: first, the upstream cylinder is maintained fixed and the downstream one is mounted on the elastic base; subsequently, an investigation will be carried out letting both cylinders oscillate transversally. The results for an isolated cylinder are in accordance with other measurements in the literature for m* ≈ 2 and m* ≈ 8. For the tandem arrangement (m* ≈ 2), the trailing cylinder oscillation presents what previous researchers have termed interference galloping behaviour for a centre-to-centre gap spacing ranging from 3·0D to 5·6D. These initial results validate the experimental set up and lead the way for future work; including tandem, staggered and side-by-side arrangements with the two cylinders free to move.

scholarly journals Experimental studies of a single flexibly-mounted rod in a triangular rod bundle in cross-flow

2018 ◽  
Vol 148 ◽  
pp. 09002
Author(s):  
Sabine Upnere ◽  
Normunds Jekabsons ◽  
Sergejs Dementjevs ◽  
Michael Wohlmuther
Keyword(s):  
Experimental Studies ◽  
Cross Flow ◽  
Flow Induced Vibration ◽  
Rod Bundle ◽  
Rod System ◽  
Flow Induced Vibrations ◽  
The Stability ◽  
Paul Scherrer ◽  
Fixed Array ◽  
Rod Array

Experiments on flow-induced vibrations using a closely-packed triangular rod array with a pitch-todiameter ratio of 1.1 in water cross-flow was carried out at Paul Scherrer Institute. The bundle consists of 21 row of five rods in each one. Single flexibly-mounted test rod (TR) is in the fourth row in an otherwise fixed array. The test rod can freely move in the transverse and in-line direction. Two accelerometer sensors were attached at both ends of the TR to measure the rod response on the fluid flow. The effect of flow rate on the stability of the flexibly-mounted TR has been analysed. During experiments, it reveals a set of conditions and tendencies for the flow-induced vibration in the closely-packed multi-rod system.

Wake-induced galloping of two interfering circular cylinders

1984 ◽  
Vol 146 ◽  
pp. 383-415 ◽  
Author(s):  
A. Bokaian ◽  
F. Geoola
Keyword(s):  
Relative Position ◽  
Free Stream ◽  
Dynamic Instability ◽  
Fluid Dynamic ◽  
Continuous Functions ◽  
Circular Cylinders ◽  
Static Tests ◽  
Free Stream Turbulence ◽  
Drag Forces ◽  
The Stability

Measurements are presented of fluid-dynamic instability of a smooth circular cylinder, free to oscillate laterally against linear springs in the wake from an identical stationary neighbouring body. The observations also encompassed determination of static forces on the downstream cylinder as functions of relative position of the cylinder pair. Most of the experiments were performed under two conditions of free-stream turbulence. Static tests indicated that both the drag coefficient and the Strouhal number of the downstream body are continuous functions of its relative position. The drag forces were found to be negative at small gaps. It was observed that the transverse extent of the force field increases with increasing streamwise gap.In the dynamic experiments, depending on the cylinders’ separation and structural damping, the cylinder exhibited a vortex-resonance, or a galloping, or a combined vortex-resonance and galloping, or a separated vortex-resonance and galloping. Whilst the characteristics of wake-excited motion were found to be essentially unaffected by a limited change in free-stream turbulence intensity, the galloping amplitudes were observed to be sensitive to the cylinders’ aspect ratio. An increase in the stability parameter caused significant effects on the cylinder response in amplitude domain. Wake observations behind the oscillating body indicated that in vortex lock-in the frequency of vortex-shedding locked to vibration frequency, but during small-amplitude galloping motion the shedding frequency behaved as if the cylinder was stationary.

Receptivity coefficients at excitation of cross-flow waves by free-stream vortices in the presence of surface roughness

2013 ◽  
Vol 716 ◽  
pp. 487-527 ◽  
Author(s):  
V. I. Borodulin ◽  
A. V. Ivanov ◽  
Y. S. Kachanov ◽  
A. P. Roschektaev
Keyword(s):  
Surface Roughness ◽  
Free Stream ◽  
Cross Flow ◽  
Leading Edge ◽  
Vortex Street ◽  
Swept Wing ◽  
Model Surface ◽  
Wing Model ◽  
Roughness Elements ◽  
The Stability

AbstractThe present experimental study is devoted to examination of the vortex receptivity mechanism associated with excitation of unsteady cross-flow (CF) waves due to scattering of unsteady free-stream vortices on localized steady surface non-uniformities (roughness). The measurements are carried out in a low-turbulence wind tunnel by means of a hot-wire anemometer in a boundary layer developing over a $25\textdegree $ swept-wing model. The harmonic-in-time free-stream vortices were excited by a thin vibrating wire located upstream of the experimental-model leading edge and represented a kind of small-amplitude von Kármán vortex street with spanwise orientation of the generated instantaneous vorticity vectors. The controlled roughness elements (the so-called ‘phased roughness’) were placed on the model surface. This roughness had a special shape, which provided excitation of CF-waves having basically some predetermined (required) spanwise wavenumbers. The linearity of the stability and receptivity mechanisms under study was checked accurately by means of variation of both the free-stream-vortex amplitude and the surface roughness height. These experiments were directed to obtaining the amplitudes and phases of the vortex-roughness receptivity coefficients for a number of vortex disturbance frequencies. The vortex street position with respect to the model surface (the vortex offset parameter) was also varied. The receptivity characteristics obtained experimentally in Fourier space are independent of the particular roughness shape, and can be used for validation of receptivity theories.

Instability Mechanisms and Stability Criteria of a Group of Circular Cylinders Subjected to Cross-Flow. Part I: Theory

1983 ◽  
Vol 105 (1) ◽  
pp. 51-58 ◽  
Author(s):  
S. S. Chen
Keyword(s):  
Dynamic Instability ◽  
Cross Flow ◽  
Circular Cylinders ◽  
Stability Criteria ◽  
Critical Flow Velocity ◽  
Closed Form Solutions ◽  
Fluid Damping ◽  
Functional Forms ◽  
Flow Part ◽  
The Stability

A mathematical model is presented for a group of circular cylinders subject to cross-flow. It is found that there are two basic dynamic instability mechanisms: instability controlled by fluid damping and instability controlled by fluidelastic force. Approximate closed form solutions of the critical flow velocity for the two mechanisms are obtained based on constrained-mode analyses. The model has identified the key parameters in the stability criteria and their functional forms and resolved the controversy associated with the empirical stability criteria.

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