VORTEX INDUCED CABLE VIBRATIONS Shyang-Yu Baur* Min-Chih Huang** Chia-Chuen Kao*** ABSTRACT In this paper the experimental results of vortex induced vibrations (strumming) of bare cables made of nylon synthetic fiber ropes are presented. Measured vortex shedding 8trouhal numbers and foree coefficients for strumming cabIes ln a steady unlform f low are obtained from a seIf excitation experiments. The mean vortex shedding 8trouhal numbers are 0.171 and 0.168 for the cable under 20 and 10 kg pretensions , respectively. In general the vortex shedding results in alternating lift forces at the fundamental 8trouhal frequencies , while the inline drags alternate at the second harmonics. Large transverse f?rces with lift coefficient as high as 3.897can be induced by cab le strummlng.These force coefficIents , bbtalned from SUITIIRIn9the force amplitudes in an interval centered at the spec1:rum peaks , can be used to simulate the measured force for most cases with narrowband spectrums. INTRODUCTION 8trouhal (1878) first discovered a relati.onship between the vortex sheddi.ng frequency (fo) , cyliner diameter (0) , and the velocity of the ambient fl ow (U).A dimerlslonl ess number can be used to def ine thls re 1at Ionship as So =fo D/U, where So is known as strouhal number.The alternate vortex shedding resul ts in an oscl l latingtransverse force in a di rection away from ?he last detached vortex on the ri9MIy fixed cylinder.Thls transverse lift 主orce exists practically at all Reynolds number. At subcritical Reynolds numbers ,the strouhalnumber is about 0.2for smooth cylInder.At crlt icaI regime , when the cylinder is free to oscillate , the 8trouhal number remains at a value nearly equal to that found at subcritical Reynoldsnumbers [Lienhard (1966)]. *Graudate Student ,Department of Hydraulic & Ocean Engineering ,National Cheng Kung University MASS?ciate Professor , Department Of Naval Archltecture&Manne Engineering , National Cheng Kung Univers'ity ⋯::::;立 λ:::::;: 一 Oepa巾 ent of Hydraulic &06ean Engine 叫 \g ,National 一 635 一
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VORTEX INDUCED CABLE VIBRATIONS - tsoe.org.t · IJ'!I"""' VORTEX INDUCED CABLE VIBRATIONS Shyang-Yu Baur* Min-Chih Huang** Chia-Chuen Kao*** ABSTRACT In this paper the experimental
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In this paper the experimental results of vortex induced vibrations
(strumming) of bare cables made of nylon synthetic fiber ropes are presented.
Measured vortex shedding 8trouhal numbers and foree coefficients for strumming
cabIes ln a steady unlform f low are obtained from a seIf excitationexperiments.
The mean vortex shedding 8trouhal numbers are 0.171 and 0.168 for the
cable under 20 and 10 kg pretensions ,respectively. In general the vortex
shedding results in alternating lift forces at the fundamental 8trouhal
frequencies ,while the inline drags alternate at the second harmonics. Large
transverse f?rces with lift coefficient as high as 3.897can be induced by
cab le strummlng.These force coefficIents ,bbtalned from SUITIIRIn9the force
amplitudes in an interval centered at the spec1:rum peaks ,can be used to
simulate the measured force for most cases with narrowband spectrums.
INTRODUCTION
8trouhal (1878) first discovered a relati.onship between the vortex
sheddi.ng frequency (fo) ,cyliner diameter (0),and the velocity of the ambient
fl ow (U).A dimerlslonl ess number can be used to def ine thls re 1at Ionship as
So =fo D/U,where So is known as strouhal number .The a lternate vortex
shedding resul ts in an oscl l l atingtransverse force in a di rection away from
?he last detached vortex on the ri9MIy fixed cylinder.Thls transverse lift
主orce exists practically at all Reynolds number. At subcritical Reynolds
numbers,the strouhalnumber is about 0.2for smooth cylInder.At crlt icaI
regime ,when the cylinder is free to oscillate ,the 8trouhal number remains at
a value nearly equal to that found at subcritical Reynoldsnumbers [Lienhard(1966)].
*Graudate Student ,Department of Hydraulic & Ocean Engineering ,NationalCheng Kung University
MASS?ciate Professor ,Department Of Naval Archltecture&Manne Engineering ,National Cheng Kung Univers'ity⋯::::;立λ:::::;:一Oepa巾 ent of Hydraulic & 06ean Engine叫 \g,National
一635一
structura1 resonance often occurs when the vortex shedding frequency is
near the natura1 frequency of the structure. Numerous experiments have shown
that when the vortex shedding frequency brackets the natura1 frequency of an
e1astica11y mounted structure ,the structure takes contro1 of the shedding in
apparent vio1ation of the Strouha1 re1ationship ,where the interaction between
the structura1 osci11ation and the f1uid action is non1inear. This phenomenon
often encompass a range of 25 to 30 屯 of the natura1 vortex shedding frequency
[Sarpkaya & Isaacson (1981)].
Cab1es are high1y non1inear structures. The non1inearities are due to
inherent properties of cab1e responses ,such as 1arge disp1acement , 1ack of
stiffness in f1exure and compression ,and constitutive re1ations. The
adaptive nature of cab1es often makes them more vu1nerab1e to vortex induced
vibrations.
Marine cab1es made of synthetic fiber ropes are constructed by twisting
severa1 strands around each other he1ica11y. This he1ica1 shape introduces
disturbances on the surface which interact with the vortex shedding
mechanicsm , and the wake structure becomes more comp1icated. The vortex
induced cab1e vibrations may be studied experimenta11y by se1f excitation or
forced vibration of the tested cab1e ,since the simi1arities in the wake
structures from these two experiments were va1idated by Griffin (1972).
Recent studies inc1ude those of Pe1tzer (1983) ,Pe1tzer & Rooney (1984) ,Griffin & Vandiver (1984) ,Vandiver ~ Chung (1989).
The osci11ations of marine cab1es caused by vortex shedding , common1y
termed as cab1e strumming ,resu1t in 1arger hydrodynamic forces , increased
fatigue and amp1ified acoustic f10w noise ,and sometimes 1ead to structura1
fai1ures. Since cab1e supported structures become increasing1y important to
the ocean re1ated app1ications ,the phenomenon of cab1e strumming needs to be
studied more extensive1y. In this paper the strumming of bare cab1es made of
ny10n synthetic fiber ropes are discussed.
TEST SETUP AND bATA ANALYSIS
The present experiments were conducted at the horizonta1 circu1ation tank
of the Fishing Boat and Marine Engineering Research Center ,NCKU. The test
section of this tank is 3.8 m 1ong,1.2 m wide and 0.825 m deep. Steady f10ws
with speeds up to 1.5 m/s can be generated in the test section.
The cab1es that were used in the tests are constructed of nylon synthetic
fiber ropes. A pre1iminary test program was carried out to identify the
pertinent strumming characteristics. A three-components dynamometer was he1drigid1y at the center top of the.test section , with an universa1 joint
attached be1ow. Each tested cab1e was connected at the top to the universa1
joint and was pretensioned by a sinker at the bottom. Two sinkers with the
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same outer geometry but.with different weights were used in the tests , one
weights 11.5 kg (in air) whi1e the other weights 23 kg. The hydrodynamic
effect of the two sinkers are the same whi1e giving different stiffness for
the test cab1es. A tota1 of four cab1es with diameters 1.15 ,1.46 ,1.95 and
2.10 cm were used. The test setup is i11ustrated in Fig. 1a.
The dynamometer was pre-ca1ibrated and then used to measure the in1ine
and transverse forces on the cab1e system. The ana10g signals from the
dynamometer were amp1ified and fi1teredthrough a 100 Hz 10w pass fi1ter ,digita1 force measurements of 12 seconds at a samp1ing rate of 50 Hz were then
obtained from an A/D converter and stored for da.ta ana1ysis. A tota1 of 5f2
discretized data points was ana1yzed in FFT a1gorithm to ca1cu1ate the in1ine
and transverse force spectrums from f = 0 to 25 Hz with the frequency
reso1ution of 0.0977 Hz.
The fi1tered ana10g signa1s from the dynamometer were a1so connected to a
dua1 channe1 Ono-Sokki Spectrum ana1yzer where the force spectrums averaged
from 256 consecutive runs were stored and compared with the FFT spectrums.
The averaged smooth spectrums ,compared c1ose1y with the FFT spectrums ,were
used on1y to identify if the f10w conditions were too disturbant or not.
One meter upstream of the tested cablE!,a simp1e mechanism was
constructed to 10wer and current meter to any vertica1 position in the tank.
The current meter was a Marsh-McBirney Mode1 523,dua1-ax 土s, e1ectromagnetic
type. The sensor outputs were passed through 10w pass fi1ters. A time
constant of 5 seconds ,with a 3dB corner frequency of 0.0306 Hz,was set
during the tests. The uniform steady currents ~Iere measured 0.1 m be10w the
free surface after force measurements were taken 。
In genera1 ,the resu1ts from this pre1iminary test program revea1 that:
(1) The transverse force spectrums are most1y narrowband.
(2) At transition ,the shedding process are slight1y broadband.
(3) For the sma11er two cab1es at higher tension , transition occurs
approximate1y at Reyno1ds number betweE!n 10500 - 15000; at 10wer
tension ,transition occurs at Reynolds number between 9000 - 12000.
(4) For the 1arger two cab1es at higher tension , no transition was
observed; at 10wer tension ,transition occurs at Reyno1ds number
between 20500 - 24500.
(5) At 10w tensions ,the tested cab1es were more vu1nerab1e to strumming.
At subcritica1 Reyno1ds numbers ,the shedding proc
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transition the upper and 10wer bonds of Strouhal. number are p10tted instead ofthe average va1ues.
As summarized by Vandiver & Chung (1989) ,the prediction of the response
of a cab1e system to vortex shedding may be thought of as consisting of fourmajor components:an excitation mode1,a structura1 model ,a damping mode1,and
a solution technique. For examp1e,wake osci11ator mode1 for transverse
osci11ations was deve10ped by Griffin et a1. (1975) for predicting the 1ift
coefficient as a function of time. This paper emphasizes the measurement and
prediction of excitation mode1. The information provided by the pre1iminary
tests are however 1imited ,e. g. ,the effect of the sinker on the force
coefficients can not be accurate1y accounted for.
The test arrangement was therefore improved by using a bending type 10ad
ce11 (Kyowa Model LUB-50KB) rigid1y mounted at the tank bottom ,a swive1 was
connected to the 10ad ce1l where the tested cab1e can be simp1y connected ,as
i11ustrated in Fig. 1b. The cab1e was pretensioned and adjusted at the top
through a turnbuck1e. The cab1e that was used in the tests has a diameter of
2.10 cm. During the first test sequence the cab1e tension was origina11y
adjusted at 20 kg,then the tests were carried out from 10wer f10w speeds to
higher speeds. A residua1 tension of 9.9 kg was measured after the tests with
no f1ow. During the second test sequence the cab1e tension was origina11y
adjusted at 10 kg and a residua1 tension of 7.5 kg was measured after the
tests. Tab1e 1 summarizes the mean tension on the test cab1e obtained from
FFT ana1ysis of the 10ad ce11 output for each test runs. Resu1ts from these
two test sequences are discussed in the next section.
RESULTS AND DISCUSSION
The transverse force amp1itude spectrums for the tested cab1e under 20
kg pretension at various speeds are i11ustrated in Figs. 3a - 3c. The
transverse force amp1土tude spectrums for the tested cab1e under 10 kg
pretension at various speeds are i11ustrated in Figs. 4a - 4c. Each figure
contains six spectrums to i11ustrate the variation of vortex shedding process
with increasing current speeds. It is c1ear1y shown in these figures that the
shedding frequency increases when the current speed is increased. Transition
occurs approximate1y at Reynolds number between 25500 - 31300 (corresponding
to fi1es C194 ~ C198) for cab1e under 20 kg pretension ,and occurs at Reyno1ds
number between 23000 - 27000 (corresponding to fi1es C211 - C214) for cab1eunder 10ιkg pretension.
The variation of Ströuha1 number with Reyno1ds number from the improvedexperiments is i11ustrated in Fig. 5,where at critica1 regime an average
Strouha1 number was ca1cu1ated fromeach force spectrum to quantify the
broadband shedding process. The mean Strouha1 numbers are 0.171 and 0.168 for
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cab1e under 20 kg and 10 kg pretensions ,respect 土ve1y,throughout the Reyno1ds
number range 12000 to 35000. For comparison ,a va1ue of 0.192 was obtained by
Pe1tzer (1983) for Kev1ar ropes wrapped with a po1yurethane jacket throughout
the Reyno1ds number range 2000 to 42000.
The transverse 1ift force coefficients for the strumming cab1es are
presented in Figs. 6a-6b , for cab1e under 20 kg and 10 kg pretensions ,respective1y ﹒The in1ine force coefficients are presented in Figs. 7a-7b ,for
cab1e under 20 kg and 10 kg pretensions ,respective1y. The mean 1ift and drag
coefficients ,CL(O) and CO(O),were ca1cu1ated direct1y from the FFT ana1yses
of force measurements. The fundamenta1 1ift and drag coefficients ,CL(l) and
CO(l) ,were obtained by first 10cating the vortex shedding frequency ,fo ,from
each spectrum; then the force amp1itudes in an interva1 of 40 frequency
increments (i.e. ,3,906 Hz) centered at fo were summed together as the peak
amp1itude for the representative narrowband spectrum. The force coeff 主cients
for the higher harmonics ,e.g. ,CL(2) and CO(2),were ca1cu1ated simi1ar1y ﹒Figs. 6 - 7 indicate that:
(1) In genera1 the vortex shedding resu1ts in a1ternating 1ift forces at
the fundamenta1 frequencies ,whi1e the in1ine drags a1ternate at the
second harmonics.(2) The vortex shedding does not necessari1y resu1t in a symmetric 1ift
force variation ,主.e. ,mean 1ift coefficient ,CL(O),is not necessari1y
zero.(3) The variations of force coefficients are found to be simi1ar for the
two cab1e pretensions studied.
(4) Large 1ift forces can be induced ,e.g. , the fundamenta1 1ift
coefficient is as high as 3.897 for cab1e with 10 kg pretension at f10w
speed U = 72 cm/s (Fi1e C207),whi1e the mean drag coefficient is on1y
1. 607.These force coefficients can be used to simu1ate measured force
variations' for most cab1e strummings with narrowband spectrums. Fig. 8
i11ustrates the c10se agreement between the simu1ated and measured transverse
force for cab1e with 10 kg pretension at f10w speed U = 80 cm/s (Fi1e C209).
Evident1y , the characterization of the vortex shedding proceSB by a simp1e
(fundamenta1) frequency and its harmonics is a practica1 simp1ification ,
especia11y at transitions. An examp1e is given in Fig. 9 for cab1e with 10 kg
pretension at f10w speed U = 104.5 cm/s (Fi1e C214) where the corresponding
force spectrum is broadband.
CONCLUSIONS
In this paper the vortex induced vibration of bare ny10n eab1es are
studied experimenta11y by se1f excitations of the cab1e in steady uniform
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f1ows. In the pre1iminary tests ,cab1es were pretensioned by free pendu10us
sinkers of 23 and 11.5 kg (in air) at bottom. In the improved tests ,cab1es
were simp1y connected at both ends and pretensioned at 20 and 10 kg.respective1y.
Prior to and after transitions ,the vortex shedding process can be
identified by a single fundamenta1 frequency from the measured force spectrum.
At transitions on1y an average va1ue ,or upper and 10wer bonds , for the
shedding process can be defined ﹒The variation of vortex shedding Strouha1
number versus Reyno1ds number obtained from the improved tests 1ie between the
corresponding upper and 10wer bonds obtained from the pre1iminary tests.
Experimenta1 resu1ts from the improved tests indicate that:
(1) The mean vortex shedding Strouha1 numbers are 0.171 and 0.168 for cab1eunder 20 and 10 kg pretensions.
(2) Large transverse forces can be induced by cab1e strumming , therefore ,suppression devices shou1d be considered in severe conditions.
(3) Force coefficients ca1cu1ated from summing the force amp1itudes near
the spectrum peaks can on1y be used to simu1ate the strumming force
vibrations with narrowband spectrum. An improved deterministic mode1
or a statistica1 ana1ysis based on the spectra1 content is required at
transitions in order to better simu1ate the vortex shedding process.
REFERENCES
1. Griffin , O.M﹒1972. F10w near se1f-excited and forced vibrating circu1ar
cy1inders. J. Engrg. for Ind. ,ASME,Vo1.94 ,539-548.
2. Griffin ,O.M.,R.A. Skop &S.E. Ramberg. 1975. The resonant ,vortex-excited
vibration of structures and cab1e systems. Proc﹒ 7th OTC. Paper OTC 2319.