TN no. N41499

tile CABLE STRUMMINGSUPEIO

author: B. E. Haf en and D. J. Meggitt

date: September 1977

~i: sponsor: Naval Facilities Engineering Command

program nos: YF52.556.091.0l.201B

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(2. TN1499~ ,12 GOVT ACCESSI OG NUMBER - -GNU:BERe-

ABLE STRUMMING SUPPRESSION Not final ApLr"Z i Jun 17

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B. E,/Hafen~nd D. J.Aeggit

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Naval Construction Battalion Center V62759N; YF52.556.091.01.201BPort Hueneme, California 93043

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Al1 KEY WORDS (Continueon f 000 .1d. ifý .... $-y sod id-nitfy bn 6106 ... be,)

Cable fairing; strumming; cables.

20. ABSTRACT (Conti~o nu .on -e.. std. It neoo..efy end identif~y by 1006 .. ooLet)

- This report presents a consolidation of existing data on various devices used to suppressvortex-induced motions of cables and circular cylinders in the ocean. The types of devicesdiscussed herein include "fringe," "hair," and ribbon flexible fairings and helical ridges. Ingeneral, the available data show that all of these methods do, in fact, suppress vortex-inducedvibrations to a greater or lesser degree. However, because of the diverse ways in whichsuppression effectiveness has been measured, comparisons among different types of devicesare difficult to make. Criteria for such comparisons are suggested. Relatively few 7

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20. Continued

measurements of the effects of strumming suppression devices on the drag of a cable or

cylinder have been reported. The available data indicate that a large drag penalty may be

incurred by use of such devices, depending on the configuration employed.

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Civil Engineering LaboratoryCABLE STRUMMING SUPPRESSION, by B. E. Hafenand D. J. MeggittTN-1499 102 pp illus September 1977 Unclassified

1. Cable strumming 2. Strumming suppression devices 1. YF52.556.091.01.201B

This report presents a consolidation of existing data on various devices used to suppress

vortex-induced motions of cables and circular cylinders in the ocean. The types of devicesdiscussed herein include "fringe," "hair," and ribbon flexible foirings and helical ridges. Ingeneral, the available data show that all of these methods do, in fact, suppress vortex-inducedvibrations to a greater or lesser degree. However, because of the diverse way in which suppressioneffectiveness has been measured, comparisons among different types of devices are difficult tomake. Criteria for such comparisons are suggested. Relatively few measurements of the effectsof strumming suppression devices on the drag of a cable or cylinder have been reported. Theavailable data indicate that a large drag penalty may be incurred by use of such devices,

depending on the configuration employed.

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CONTENTS

page

INTRODUCTION................... .. .. ..... . . ..... .. .. .. ......

VORTEX-INDUCED CABLE VIBRATIONS. ........ ........................2

SUPPRESSION DEVICES .. ................ ..........................3

General. ...... .................... ........................3

Fringe Fairing. .. ................ ..........................4

Nylon Rope Thongs. ........ ...................... ....... 5

Tufted Fibers .. .................. ...................... 5

Polyvinyl Chloride Fibers .. .................... .......

Polypropylene Fringe.. ....... ..........................6

Helix Wrap. .. ...................... .................... 6

Hair Fairing. .. .................. ..........................7

Ribbon Fairing. .. .................. ........................9

Helical Ridge. .......... ...................... ............ 11

DISCUSSION. .. .................. .................... .......... 14

Criteria for Suppression Device Comparison. .. ................14

Suppression Effectiveness .. .. ...................... ........ 15

Fringe Fairing. .. ...................... ................16

Hair Fairing. .. ...................... ..................16

Ribbon Fairing. .... .................... ................16

Helical Ridge.. 17

ACCFSSION forCONCLUSION .. ............................. NTIS . . .i 1 Ic*soeIction 1

REFERENES............................

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INTRODUCTION

The Civil Engie..ering Laboratory (CEL), under the sponsorship ofthe Naval Facilities Engineering Command, (NAVFAC), is engaged in aprogram of research concerning the dynamics of cables and cablestructures in the deep ocean. The program considers two specificproblem areas: (1) the relatively small amplitude, high frequencycable vibrations due to periodic lift forces induced by vortex shedding(generally termed "strumming"); and (2) the large displacement,relatively low frequency or transient response due to disturbancesduring implantment or while in place on the ocean floor. When on theocean floor the disturbances are due to shock waves or unsteady hydro-dynamic forces associated with geostrophic, tidal, inertial, or densityflows. In buth parts of the program, the objective is the developmentof effective me:hods for the analysis and design of subsurface cablestructures.

The vortex-excited vibration of cables is a ,;ommonly observedphenomenon in the ocean. This motion frequently results in degradedacoustic or environmental sensor performance and accelerated fatigueof structural elements. Further, the drag of a strumming cable issignificantly higher than that of a nonvibrating cable, producinghigher stresses in the elements and greater distortion of an array ina given current field. An integral part of the research into cablestrumming in this program is the development of effective techniquesto suppress the flow-excited motions of cables.

An extensive survey of existing literature on the suppression ofvortex-induced motion was made recently at CEL [1] and an annotatedbibliography was prepared. The treatment of the analysis of cablestrumming suppression to date has been largely empirical with little orno theoretical zonsideration. Therefore, this initial effort hasbeen extended to analyze the available reports to determine the abilityof the various devices to suppress cable strumming, to compare theireffectiveness in doing so, and to assess the effects of these deviceson the drag of the cables.

Of primary concern in determining the characteristics of a deviceto suppress cable strumming are the environmental and handling conditionsto be encountered. As defined by the Cable Dynamics Program ResearchPlan [2], the device must be easily handled, durable, cost-effectiverelative to cable cost, and able to suppress strumming in currents upto 1 knot (0.52 m/s.) The devices which presently appear to have the

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best probability of meeting these requirements are: hair fairing*,fringe fairing, ribbon fairing, and longitudinal or helical ridges.

This report presents the following information:1. The flow parameters which influence vortex-induced cable

vibration are briefly discussed. This subject is covered extensivelyin the literature, and no attempt is made to present a complete reviewof cable vibration.

2. Each type of device is discussed separately, relevant availabledata are presented, and the present status of experimentation is given.

3. Experimental data for each of the four classes of devices arepresented to provide a comparison of the behavior of the devices andof the effectiveness of the devices relative to each other.

Existing data generally provide only the vortex-induced accelera-tion or displacement of equivalent faired and unfaired cables and acomparison of these quantities; drag data may or may not be available.In any case, only the overall strumming reduction qualities of aparticular configuration are recorded; no data or presentation of dataattempts to indicate which structural or flow parameuers are beingmodified by the device to suppress strumming. However, it is clearthat interpretation of the modifications to the structural and flowparameters is essential for an understanding of the processes by whichstrumming suppression is achieved. It is the intent of the strummingsuppression portion of the NAVFAC/CEL Cable Dynamics Program todetermine which parameters or combination of parameters control theeffectiveness of a suppression device. The data in this report andthat generated by CEL cable experiments will be used to meet this goal.

VORTEX-INDUCED CABLE VIBRATIONS

As a fluid flows past a submerged body, viscosity causes the fluidat the surface of the body to be at rest with respect to the body and ashear and velocity gradient to exist within a boundary layer. Further,on the lee side of a sutmerged body an adverse pressure gradient existswhich decelerates the flow. The combination of viscosity and anadverse pressure gradient results in the reduction of the velocitygradient normal to the body; in the case of bluff bodies, such as acylinder or cable, this velocity gradient becomes zero at some pointon the surface and the flow separates from the body. The flow

*The term "fairing" for suppression devices is a generic term thatcomes from early drag-reducing cable experiments with streamliningdevices (or fairings).

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separation gives rise to the development behind the body of a wake,the configuration of which is dependent on the dimensionless Reynoldsnumber, which is defined as:

Re = UD

where U is the freestream velocity, D is a characteristic length, andV is the kinematic viscosity of the fluid, in a consistent set of units.

The formation of vortices in the wake of a circular cylinder hasbeen theoretically presented as the "vorte': street" formed by thealternate shedding of vortices with a definite periodicity, which isdependent on the Reynolds number of the flow. This periodicity in thewake is quantified in terms of the Strouhal number,

S DU

where f is the shedding frequency. In the range 4 x 10 < Re < 2 x 105the vortex shedding is regular and the Strouhal number is approximately0.21. It has been shown 13], that a periodic wake appears at Re = 40;the wake is stable and regular up to Re = 150; between Re = 150 andRe = 300 the vortices gain energy and begin to interact; and aboveRe = 300 an irregular wake exists.

As a consequence of the wake formation there is a momentum loss

which results in a drag force. Further, due to the periodic vortexformation, an instantaneous pressure differential exists, resultingin a periodic lift force perpendicular to the freestream direction offluid flow. The strumming of a cable is the elastic structural responseto this periodic lift force. The primary objective of the cablestrumming portion of the Cable Dynamics Program is to investigate andpredict the interaction between the elastic cable and the vortex-

4 induced forces.

SUPPRESSION DEVICES

General fAs discussed previously, the four types of suppression devices

which appear to meet the program requirements (hair, fringe, ribbon,ridge) have been studied for their strumming suppression effectivenessby various investigators. The annotated bibliography [1] discussesa variety of suppression devices; however, many of the devices are notadaptable to a cable, and others are not feasible from a handling andlogistics point of view when long cables - up to 20,000 feet (6096 m) -

are being used. Ideally, the suppression device should be attachedalong the entire length of cable or a portion thereof during manufacture;

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Ln addition, it ia de54rable to develop the capability to suppressstrumming on an existing cable. Fringe fairing, hair fairing, ribbons,and helical ridges meet the requirements of a suppression device formoored arrays; therefore, a clcz lr look at these devices was warranted.

The vibrations of a cylinder or cable can be substantially reducedby utilizing any one of the above devices. A helical device, whethera ridge, hair, fringe, or ribbons, tends to break up the spanwisecoherence of the vortices by causing a variable location of theseparation point. The adverse pressure gradient may also be reducedif the boundary layer is induced to turbulence. The trailing - andto some extent, helical-fringe, hair, and ribbons interfere with thevortex interaction in the near wake and disrupt the vortex formationlength. The exact manner by which strumming is reduced is not generallywell-established since there is a very complex pattern to the vortexdisturbances; however, suppression effectiveness usually can beincreased or decreased, depending on the geometry of each device.

Table I Asts the geometric and material parameters which can bevaried for each device. Structural and fluid dynamic parameters whiLhare varied or determined experimentally are listed in Table 2.

The variation of the parameters listed in Tables 1 and 2 affectsthe ability of a device to suppress strumming; therefore, the para-meters are important for determining the mechanism of strumming suppres-sion and for comparision of suppression results with other devices orconfigurations of the same device.

The majority of tests conducted to date have measured theacceleration at various points along the cable when both the fairedand unfaired cables are at the bare cable resonance condition. Thisignores the change in resonant frequency of the faired cable from thatof the unfaired cable. Consideration of changes in virtual mass or thelogarithmic decrement of damping have not been addressed in studies todate, although the change in drag due to fairing has been reported.Generally, tGsts have not been made to determine the parameters whichinfluence suppression, but rather to compare the strumming suppressionqualities of the various devices. The sections which follow presentdetails of the studies which have been conducted utilizing fringefairing, hair fairing, ribbons or helical ridges.

Fringe Fairing

The term "fringe" refers to a fairing which has bunched tufts ofstrands of flexible material (such as polypropylene or nylon) attachedto the cable helically or longitudinally. A typical longitudinalattachment is illustrated in FIgure 1. This configuration is referredto as "trailing" fringe since the fringe is nominally along the down-strean side of the cable and "trails" in the flow.

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Nylon Rope Thongs. Kelly and Goff [4] utilized nylon rope thongsof various lengths end spacings (Figure 2) in an attempt to reducecable vibrations. Their particular configuration was designed forsystems towed at high speeds. A normal drag coefficient for eachconfiguration was determined from the length and diameter of the cable,measured depth of the outboard end, weight, design lift-drag ratio,lift coefficient of the depressor, and drag depth of the recorder. Avisual observation of the vibration amplitude of the -able was madeduring the initial testing and a vibration analyzer was used tomeasure the predominant frequency in subsequent tests. The resultsof the tests of this fairing are shown in Table 3, where CD is thenormal coefficient of drag. The data are limited in their usefulnessand provide only a qualitative assessment of this fairing.

Tufted Fibers. In 1970, the Naval Underwater Systems Center (NUSC)initiated a cable development program for suspended sensor systems [5].NUSC design criteria for a general family of suspended sensor systemsrequired high reliabillty, stability, and quietness from the cable.Drag reduction was desirable, but not mandatory. The initial effortutilized a ribbon fairing, but the development of Kevlar cablesindicated that a fringe-type fairing could be woven into the outerjacket during bra:.ding. Wall Rope Works has developed a techniqueto incorporate tufts of yarn up to 7 inches (177.8mm) long at 1-inch(25.4mm) spacings in the cable outer braid. To date, polypropylene,nylon and monofilament polyester fibers have been used at a reportedcost of $1.00/foot (304.8mm) to fair a cable. An "acceptable levelof strumming" was reported in three 1,000-foot (305 m) lengths of0.66-inch (17 mm) diameter, double-armored, steel tow cable withWall Rope fringe fairing tested in the summer of 1974 by NUSC and WoodsHole Oceanographic Institute (WHOI) [5]. Acoustic and mechanicalperformance of a 0.75-inch (19.1 mm) diameter fringe-faired Kevlar 29cable-3-inch-(7.6 m-) long polypropylene tufts spaced 1 inch (25.4 mm)apart - used in 15,000-foot (4572 m) WHOI and 4,600-foot (1402 m)NTSC arrays have been reported to be excellent (5). WHOI is presentlypreparing a report on the performance of the 15,000-foot (4572 m)moored array. The fairing system has now been used to reduce strummingca the Moored Acoustic Buoy System (MABS) and the Telemetering AcousticBuoy System (TABS). The NUSC cable program has not attempted to modifythe fringe fairing to attain maximum strum reduction with the leastamount of materials and drag.

Polyvinyl Chloride Fibers. WHOI has tested faired and unfairedcables suspended in 60 feet (18.3 m) of water off the WHOI dock [61 intidal citrrents to 1.5 knots (0.77 m/s). Bundles of polyvinyl chloride(PVC) fibers were woven into the outer jacket of a 0.375-inch (9.5 mm)diameter Kevlar cable; the fairing was 5.5 inches (140 mm) long, andthe tufts were spaced 0.5 inches (12.7 mm) apart. Tests to determinestrumming reduction dependence on tuft pattern density were conducted

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by removing portions of the tufts. Figure 3 shows the results of theexperimentation. The data show a great deal of scatter; however, theresults do indicate that the fringe fairing does reduce cable strumming.

Polypropylene Fringit. The drag associated with the Wall RopeWorks faired cable tested by WHOI and NUSC was investigated in theMassachussets Institute of Technology (MIT) water tunnel [7]. Acantilevered steel rod with a fringed dacron jacket was used tosimulate a cable. A pclyprorylene fringe 6.5 inches (165.1 mm) longwith 1.0-inch (25.4 mm) spacing was used in the tests. Drag, vibrationfrequency, and vibration amplitude were measured at various flowvelocities-16 velocity runs from 0.54 ft/s to 15.75 ft/s (0.17 m/s to4.8 m/s). At each velocity the cylinder was rotated up to a maximumof 765 degrees to simulate the possible wrapping that may occur in theocean. Figures 4, 5, 6, and 7 present the data from this experiment.In the range of velocities where the bare cylinder strummed, 5 to 13ft/s (1.5 to 4 m/s), the drag on the nonrotated faired cylinder(angle of rotatiolL is 000 degrees) is less than that of the bare cylinder,and the maximum amplitude of vibration relative to the bare cylinderwas reduced by 300%; i.e., from 2.25 to 0.75 diameters. The drag atother angles of rotation increased above that for the nonrotatedcylinder, but in all casea the fairing reduced the maximum amplitudeof vibration relative to that of the bare cylinder. In some cases theamplitude of vibration for angles of rotation greater than zero wassmaller than at an angle of 000 degrees (see Figure 5).

Helix Wrap. Two series of tests were conducted by General Electricand the U.S. Navy on Wall Rope Works fringe fairing applied longitudi-nally and spirally to a cable. Polyester monofilament and p:lypropy-lene fringe materials were tested. Only the helix wrap of fringefairing was tested in the second series of tests in December 1975.

Table 4 shows the range of paraineteis in the first tests; valuesof the drag coefficient were determined for each cable at bare cableresonance and the amplitude of acceleration of the bare and fairedcables were determined for the first, second, and third harmonics.The acceleration was reduced at all three harmonics, but a greaterreduction was seen for the first and third barmonics. No shifting ofenergy between harmonics was noted. The drag coefficient dataexhibited considerable scatter, ranging from 0.7 to 1.9 for the barecable and from 2.0 to 6.0 for the faired cable, depending on thematerial and geometry.

Based on these experiments, a helix-wrapped polypropylene fringefairing was tested further. Table 5 provides a summary of parametersfor the helix-wrapped fringe fairing, and Figures 8 and 9 compare thedrag coefficient for various fringe lengths and spacings as afunction of Reynolds number. Acceleration data were taken; however,these have not been reduced to date. The drag results indicate thatthe helical wrap of fringe does increase the drag coefficient above

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that of the bare strumming cable; however, the drag coefficient wascomparable to that found in the earlier General Electric experiments[8] for a longitudinally fringe-faired cable. Drag dependency on angleof orientation was not exhibited when the tufts were cut back, butwhen two-thirds cf the tufts were removed, the drag at 60 degrees wasapproximately %wice that at an angle of 90 degrees. These data arebeing analyzed further at the present time.

A The MIP testing program is an extension of earlier tests conductedin 1974 and the summer of 1975 [9]. This work was supported by a,iart from the Ocean Science and Technology Divsilon of the Office of.-.sli Research. It, the first series of tests bare cable strum testsS4.•x conducted in cidal currents up to 3.0 ft/s (0.91 m/s). A 76.5-fc1o" '23 ý-m) cable was suspended parallel to the bottom; tension,acc½_L .doa, and displacement measurements were recorded. As part ofthe L:Yer•:tiet, a faired Kevlar cable was tested simultaneously withan unfaired Kevlar cable to determine the reduction in strumming.Only one test was made with the fairing to gain qualitative informationinto the strum reduction achieved by the fairing in field conditions.Table 6 is a summary of the test parameters.

MIT's second series of experiments were conducted the latter partof June and early July of 1976 [10] utilizing faired cable samples fromWall Rope Works, Philadelphia Resins Corporation, four of which weresupplied by the Civil Engineering Laboratory. The experimentalconfigur&tion was identical to the 1975 experiments; however, the fringefairing was investigated by varying the length and spacing of thefairing. Results from this test have not been published.

Hair Fairing

The term "hair" fairing, as used in this report, includes any fiberfairing which is not bound together in tufts to form a fringe. The hairmay be "fringe" in appearance, but it will be composed of individualfibers attached to the cable. Some of the types of fairings which fallin this class are Environmental Devices Corporation's (Endeco)"Haired Fairing", Philadelphia Resin Corporation's "fuzzy" fairing,and Prodesco, Inc's fairing.

"Haired Fairing," introduced by BRAINCON [11] listed among itsattributes: (1) reduced cable drag, (2) reduced acoustic noise,(3) reduced cable vibration, and (4) reduced cable fatigue. The fairedcable, shown in Figure 10, was designed to be wound on a standardwinch and sheaved over standard cable blocks. Figure 11 shows theresults of tests with the faired cable as published by BRAINCON. Thisfairing is now being manufactured by ENDECO, Marion, Massachusetts.

Kelly and Goff (16) tested BRAINCON's Haired Fairing and a clothShair in a towed configuration at Reynolds numbers of 6.3 x 104 and

1.2 x 105. A double fairing (hair longitudinally 180 degrees apart)was supplied by BRAINCON for testing. Drag coefficient was determined

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from the length and diameter of the sample, measured depth of the out-board end, weight, design lift-drag ratio, lift coefficient of thedepressor, and drag of the depth recorder. Tow speed versus towingdepth graphs were plotted for the fairings tested. Visual observationof strumming amplitude indicated that no reduction was obtained usingthe cloth hair fairing; no strumming suppression data were reportedfor the BRAINCON fairing.

During the summer of 1974 a cable strumming suppression study wasconducted at the Naval Undersea Center (NUC) [12], following workdone previously utilizing a helical ridge suppression device. TheBRAINCON Haired Fairing was the only haired fairing tested. The testmodel was a 20-foot (6.10 m) length of polyvinyl chloride pipe with aBRAINCON faired cable attached longitudinally along the trailing edgeof the pipe. Vertical and horizontal accelerations were measured atthe midpoint of the pipe, and simultaneous tension readings were takenat each end of the pipe using two matched load cells. Tests wereconducted at angles of 15, 10, and 5 degrees. Both constant accelera-tion - from 4 to 16 ft/s (1.2 to 4.9 m/s) - and constant speed -

6, 10, and 14 ft/s (1.8, 3.1, 4.3 m/s) - tests were made.The data from the acceleration runs were used to give a qualitative

indication of the strumming reduction properties of the variousfairings. The constant-speed data were reduced to give peak linelevels of spectra of summation tension and vertical accelerometerreadings at each angle of inclination for each speed run. Axial andtangential drag coefficients were determined for each angle of in-clination as a function of Reynolds number. The BRAINCON fairing wabfound to be quite effective in suppressing strumming and exhibited adrag coefficient of from 0.5 to 0.9.

The drag behavior and strum reduction of several devices is dis-cussed briefly in a report by Dale, McCandless and Holler [13]. Ahaired fairing consisting of no. 50 grade cotton thread was used withthe hairs oriented spirally on a 9-inch (228-mm) pitch. Figures 12 and13 illustrate the drag coefficient and strum force data as a functionof Reynolds number.

As part of the NUSC cable development program discussed in theprevious section a contract was issued to Prodesco, Inc. to develop afabric-backed fairing which could be attached helically around acable. The resulting material has a polyester tape body 5/8 inches(0.016m) wide with 3-inch (0.076-m) polypropylene fibers protrudingfrom each side. NUSC elected to use the Wall Rope Works fringe fairingdiscussed previously based on its success and did not utilize theProdesco fairing.

WHOI [6] tested the Prodesco fairing during the strumming suppres-sion study discussed previously. The results of this study aresummarized in Figure 14 which shows the strumming suppression ofProdesco fairing compared with three ribbon fairings. The largescatter in the data necessarily makes interpretation of the data

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qualitative in nature. Essentially, it can be stated that the f airingdoes reduce strumming, but that the amount of reduction is not clearlydiscernible.

Philadelphia Resin Corporation is developing a haired (brush) fair-ing (Figure 15) for the Naval Research Laboratory. Previously,Philadelphia Resin utilized chenille to make a fuzzy fairing; thestrumming suppression characteristics of both fairings have yet to bereported.

Ribbon Fairing

Ribbons attached to a cable either helically or longitudinally havebeen used to suppress strumming. Ribbons attached longitudinally wereused in the initial work at NUSC [5] for the NAVFAC Cable DevelopmentProgram mentioned previously. South Bay Cable Company investigatedmethods to attach 0.010-inch (0.25 mm) thick, 2-inch (50.8 mm) wide,6-inch (152.4 mm) long polyurethane ribbons to a 0.455-inch (11.6 mm)diameter polyurethane jacketed cable. Thirty-foot-long samples weretested from a pier at NUSC in currents up to 1 knot (0.52mm/s), withfavorable strumming suppression results. Subsequently 16,000 feet(4877 m) of the faired cable was tested at sea and found to provide"good" acoustic and mechanical performance; handling was satisfactoryand ribbon loss was minimal. No attempt was made to reconfigure theribbon fairing to obtain the same suppression with less ribbon or tosee if better suppression could be obtained. During the developmentof Kevlar cables it became evident that a fringe fairing could beattached without the plastic jacket required by the ribbon fairing;therefore, a Kevlar fringed fairing was developed in lieu of a ribbonfairing.

In work conducted at the David Taylor Naval Research and Develop-ment Center in Washington, D.C. (DTNSRDC) in 1971 [14] both bare andribbon faired cables were studied to determine cable vibrationfrequency and amplitude. A 0. l15-inch-(0.38-mni-) thick polyurethanesheet was used to fabricate the ribbon in all the experiments; theribbons were attached by inserting them under two outer strands of adouble armored steel cable. The lay of the cable resulted inlongitudinally attached ribbons spiraling along the cable length; thceffect of the spiral was not studied. Accelerometers and a force gage 1jwere placed on the cable, as shown in Figure 16.

The results of the tests are shown in Tebles 7 chrough 12. Thenotation for ribbon configuration gives ribbon length by width byspacing - all in terms of cable diameters, The peak in the powerspectrum of the transvw:se acceleration of the 0.35 inch (8.9 mm) or0.5 inc' (12.7 mm) diameter cable, tensioned to 1,200 pounds (5338 N)and subjected to a flow of 6 knots (3.1 m/s) was used as the norn for

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all subsequent tests with the faired cables. Data in the last column ofTables 7 through 12 represent the percentage of the bare cable accelera-tion for each ribbon configuration value and not the percent of reduc-tion in strumming acceleration.

The report concludes that:1. The ribbon-faired cable peak transverse acceleration is gener-

ally lower than the peak transverse acceleration of a bare cable.2. The level of vibration is independent of ribbon length,

provided the ribbon is between 6 and 10 diameters long.3. The level of vibration is indepenident of ribbon spacing for

spacing up to 2 or 3 diameters.4. Ribbon 2 diameters wide more effectively reduces vibration than

ribbon one diameter wide for an angle of inclination to the flow of45 degrees. At an angle of inclination of 90 degrees, both ribbonwidths were equally effective.

5. The configuration with maximum strumming suppression was 6diameters long and 2 diameters wide with a spacing of 1 to 2 diameters.

During fiscal year 1974 DTNSRDC undertook a comprehensive programto develop the Hydromechanics Technology of Towed Arrays [15, 16]. Twoof the program goals were (1) development of strum-suppressed towlinesand (2) prediction of drag for high speed arrays and tow cables.

The strum suppression study was conducted with an 18-1/3-foot-(5.6-m-) long, 0.528-inch-(13.4-mm-) diameter, 24 x 24, double-armoredcable. The cable was suspended between a pair of struts at a constantangle of 15 degrees to the horizontal (and thus to the flow velocityvector) and was tensioned to 500 pounds (2224-N). Accelerometerswere placed at the midpoint and the quarter point. Table 13 gives therange of parameters tested.

The study concluded that the most effective ribbon configurationwas 6 diameters long and 1 to 2 diameters wide. A 25% coverage wasadequate to reduce the strum level to less than one-tenth of that of thebare cable. The shorter fairing produced a similar strum reduction butrequired a higher percentage of cable coverage.

The objective of the second phase of the DTNSRDC strummingreduction study was the determination of the hydrodynamic drag of thefaired cable both in the tow tank and during sea tests [15]. Theresults were reported graphically for tow angle versus speed, kitingangle versus speed, and normal and tangential drag versus Reynoldsnumber. The hydrodynamic force coefficient data are shown in Figure 17and J8. The drag coefficient data shown in Figure 17 indicate that theribbon configurations tested generally lead to a higher drag coeffici-ent markedly so at higher Reynolds numbers (> 10 ). Compared withhelical ridges discussed later in this report, the ribbon fairing dragis higher than the ridge which tends to be equal to or lower than barecable drag.

In conjunction with the testing of other suppression devices atNUC, ribbon faired models supplies by the Zipper-tubing Company weretested. This particular fairing is designed so that it can be applied

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to existing cables. The test arrangement is described in the previoussection. The model characteristics are given in Table 14 and theresults shown in Table 15. Table 15 gives the value of the normal dragcoefficient and reduction in cable acceleration (expressed in decibels)for each Reynolds number and cable angle tested; the reduction inacceleration at 25 degrees is plotted in Figure 19. The bare pipe dragdata are given in Table 15. Using the bare pipe hydrodynamic drag asreference, flags were found to generally reduce the normal drag co-efficient while increasing the tangential drag coefficient. Thischange in hydrodynamic drag would have a large effect on tow cable

angle and on tow cable tension which need to be considered whenselecting a strum reduction method.

The WHOI series of experiments discussed in the two previoussections also included ribbons. The test results shown in Figures 14and 20 are for the configurations shown in Figure 21. The ribbon

fairings tested are as described below.FSW (Fringe-Spiral-Wrap) - This fairing consisted of a strtp of

0.006-inch (0.15 mm) polyurethane 8 inches (203 mm) wide, cut transverselyto within 1.5 inches (38.1 mm) of one edge in strips 1.0 inch (25.4 mm)wide. This was wound and glued in a spiral wrap around the cable. Thefairing length was reduced to 4.0 inches (101.6 mm) after testing theoriginal.

NUSC - This was made of 2.0-inch (50.0 mm) wide polyurethanestrips folded over and bonded to a polyurethane jacket which had beenextended on to the cable. The flags had a length of about 5.0 inches(12.7 mm).

Rochester - 0.5-inch-(12.7mm) wide ribbons of polyurethane 9 inches(228.6mm) long were threaded under one strand of the outer layer ofthe steel cable. The ribbons were packed closely along the length.

Additional unpublished testing on ribbon and ribbon stubs for towedarrays was conducted at DTNSRDC in September 1974 and December 1975.The results of both of these studies are to be available during 1977.

The angle between the cable axis and the flow was found to be 20 degreesin both of these studies.

Based on the earlier work at DTNSRDC [14] the Naval Coastal SystemsLaboratory (NCSL) is utilizing a ribbon fairing, (Figure 22) on 150feet (45.7m) of 650 feet (198.1m) sweep wires for mine-sweepingoperations. Continental Wire Cable Corporation makes the faired cablefor use by NCSL. NCSL has reported favorable results with the system

as deployed.

Helical Ridge

One of the earlier studies of the suppression characteristics ofhelical ridges is that of P. Price [17]. His report primarily discusses

the use of shrouds; however, five models with helical or longitudinalridges (as depicted in Figure 23) were tested. Price found the strakes

Sii'

to be ineffective suppressors; and, he states, the paralled wires and4 radial fins possessed only unidirectional effectiveness and hence would

not prove to be satisfactory suppressors in a stack application. Themost beneficial helical configuration was not sufficiently effectiveto merit further consideration.

The use of helical ridges for use on stacks and towers has receivedconsiderable attention at the National Physical Laboratory (NPL) inTeddington, England [18 through 24]. Beginning with the early work byScruton and Walshe [19] helical ridge studies have been made of manystacks and towers to provide aerodynamic damping.

Primarily, a three-start helical ridge device has been studied; fhowever, in work by Woodgate and Maybrey [24] 1-, 2-, 3-, and 6-starthelical ridge systems were tested. In all the reports cited, the draginduced by the use of the ridges is not considered, except in that byCowdrey and Lawes [181, Figures 24 and 25. The NPL work concluded thatsuppression of the vibration of a tower or stack can be achievedutilizing a three-start helical strake applied to the top one-third ofthe structure.

Weaver [25] investigated a four-start helical ridge system with apitch of 12 diameters and a height of 0.08 diameters. Figures 26, 27,and 28 show; the influence of the number of ridges, height of ridges,and the pitch of the ridges, where D is the cylinder diameter, Cks isthe maximum value of the fluctuating lift for a cylinder with

ridges, and Ckb is the maximum value of the fluctuating lift for a barecylinder. The tests indicated that an effective suppressor must havethe following characteristics:

1. four helical windings2. ridge diameter of D/16 to D/83. pitch of 8D to 16D

Weaver selected a height of 3D/32 and a pitch of 12D to reduce the lift

force to a minimum.Drag measurewents on a circular cylinder fitted with vortex

generators have been made [26]. The generators (ridges) were one-half

times the boundary layer height, and an optimum location of 50 degreeseither side of the front stagnation point was determined. Figure 29shows a comparison of the drag coefficient for a smooth cylinder withthat of a cylinder with vortex geterators.

Dale McCandless, and Holler [13] reported tests of twisted pairsof cables. Cables of 0.057-inch (145-mm) diameter were used with apitch of 15 diameters. Figures 12 and 13 show the strum reductioneffectiveness and the drag coefficient as a function of Reynolds number.

The first series of NUC experiments [26] tested models of 1.31-inch (33.3-mm) OD PVC pipe (to simulate a taut cable) fitted withvari3us combinations of helical ridges, both round and rectangular.Table 16 lists the model characteristics; Figure 30 shows the typesof ridges used; and Table 17 lists the ridge cross-section parameters.

12

Measurements of the effectivcness of the ridges was done by comparing

the highest amplitude of the accelerometer trace during accelerationS~runs.rsFigure 31 shows the dependence of the amplitude of the accelero-

meter trace on ridge height to cable diameter ratio d/D for a fixedpitch; Figure 32 shows the dependence on ridge pitch to cable diameterratio for a fixed ridge height; and Figure 33 indicates the dependenceon ridge removal. For the one case tested with multiple ridges, nohydrodynamic benefit was noted. As would be expected, it was found thatfor equal effectiveness a rectangular ridge does not have to be ashigh as a roun" ridge.

InternatioL. 1 Telephone and Telegraph (ITT), Cable-HydrospaceDivision was contacted during the course of the investigation concerningthe manufacture of a ridged cable. Three or more ridges were pre-ferred for die design reasons, but no unreasonable ridge height or widthdimension limitations seemed to exist for a direct extrusion processwith helix reversals at regular intervals.

The second series of tests at NUC [12], as discussed previously,extended the earlier work with helical ridges to include tests onflags, hair, and ribbons. Acceleration runs and constant velocityruns were made. Data were reported for tension, cable accelerationand drag. Table 18 gives the characteristics of the helical ridgemodels tested. The results are given in Table 19 as the normal dragcoefficient and acceleration reduction in decibels as a function ofcable angle and Reynolds number. The data are plotted in Figure 34.

The towed array tests at DTNSRDC discussed in Section 3.4 (6,7)also investigated the use of helical ridges. The helix was reversedat the midpoint of the cable model; the models tested for their strumreduction are summarized in Tables 20 and 21.

The results of the tests are given as the reduction in cableacceleration as a function of P/D (pitch/diameter) and d/D. Thecomponents of acceleration for the first through sixth harmonics wereconsidered. A pitch-to-diameter ratio of 15 to 20 was found toproduce maximum effectiveness.

Drag characteristics of the DTNSRDC cables with a helix wire wrap[15] were determined in basin tests and at-sea tests. In the basintests, a d/D = 0.24 was used with a P/D = 15; the helix was reversedevery 10 feet. In the sea tests, a d/D = 0.23 was used with P/D =15, 20, and 30; the helix was reversed every 14 feet. Figures 35 and17 give the normal and tangential drag of the cables tested.

The drag coefficient was determined from a triaxial force gauge;acceleration data were obtained through accelerometers placed at themidpoint, quarter-point and the three-quarter point on the cable.Acceleration components for the first through third harmonic werereduced from the data.

13

DISCUSSION

Criteria for Suppression Device Comparison

K By far the most common method used to determine the effectiveness

of a strumming suppression device is measurement of the accelerationat various points along a bare %able and comparison of theseacceleration readings to those obtained with a suppression deviceattached to the cable. Some investigators have measured the amplitudeof the cable displacement, both bare and faired, and used this as abasis for comparison. Obviously, if the acceleration component iszero for the even and odd harmonics, the cable displacement is alsozero, so either method provides a valid comparison. A problemdevelops, however, when a comparison of data from the various inves-tigators is attempted. No single parameter has been used by the various

investigators to determine the quality of a device other than "Does itreduce strumming?" Comparing several devices during a series ofexperiments will indicate which devices reduced the strummingsignificantly more than others but may give no indication of the rela--tive efficiency of the devices. The parameters listed in Table I canbe configured to yield a good suppression device regardless of the typeused; thus, two investigators may claim that two different devicesreduce strumming by 30 decibels, but which is the more efficient?

Many investigators use acceleration or amplituae data to express

the effectiveness of a suppression device. Although drag is not ameasure of strumming effectiveness, it does enter into the efficiencyof the device. That is, a device may eliminate strumming but inducea substantial drag; therefore, the efficiency of the device in reducingstrumming but not adding a drag problem is compromised. Both drag andacceleration/amplitude data are needed to classify a particularsuppression device.

Acceleration data are normally presented in decibels by

a = 20 log Ac 1 /Ac

where AcI is the acceleration of the faired cable and Ac is theacceleration of a bare cable under the same test conditions. Thisquantity will be negative for reduction of the vibrations. The percen-tage in strumming reduction with respect to a bare cable will also beused in this report to provide a linear scaling.

Table 1 li5ts geometric parameters which can be varied for eachtype of device. In addition tc these, there are three parameters

common to all the investigations:(1) Reynolds number of the flow based on the diameter of the bare

cable and the free stream flow velocity(2) Angle of attack of the flow relative to the longitudinal axis

of the cable (in this report, a cable normal to the flow has an angleof 90 degrees)

14

(3) f/f where f is the natural frequency of the bare cable in thefluid and f Sis the Strouhal frequency (in this report the comparisonsare based Son f/f = 1)The use of f/fs Stakes into account the tension, virtual mass of thecable, and length of the cable; therefore, a comparison between in-dependent studies with different cables, lengths, and tensions can beattempted.

Tables 22 through 25, Table 5, and Figures 36, 8, and 9 give theparameters considered in each study discussed and the values of thedrag coefficient CD and $. The value in parentheses in the 0 columnis the percentage in strumming reduction relative to the bare cable.

Suppression Effectiveness

Figure 37 utilizes the data in Tables 22 through 25 to show thepercentage in strumming reduction as a function of Reynolds number forfringe, hair, ribbon, and helical ridge fairings at all angles to theflow for which data are available (Re based on freestream velocity and

bare cable diameter). It is apparent that suppression effectivenessis not so much a function of Reynolds number as it is a function of thetype and configuration of the suppression device. Figure 38 considersonly those tests for which the cable is perpendicular to the flow.A Reynolds number dependence is not evident.

The usual basis for comparison of different devices in the sameseries of experiments is acceleration reduction. For example, if arelative difference of 30 decibels were found between devices "A" and"B", the device with the greater magnitude of reduction was consideredto be the better suppressor. In Figure 39 suppression devices arecompared on the basis of a logarithmic scale; i.e., 20 log (Ac /Ac).The use of a decibel scale can be misleading since it is not a linearscale. Because strumming can affect the operation of acoustic devices,a decibel scale is an obvious choice for comparison; however, it mustbe realized that 5 decibels represents a 44% reduction with respect tothe strumming of a bare cable. For each 5 decibels, an additional 44%reduction is achieved, and at a reduction of 40 decibels 99% of the barecable strumming has been eliminated.

It is reasonable to consider the level to which strumming shouldbe reduced in operational systems. For example, Griffin and Skop [27]specified a peak-to-peak displacement of 0.1 diameter as the thresholdof strumming. If a 90% reduction in displacement amplitude is taken, onthis basis, as an acceptable level of strumming, then only a 20-decibelsreduction is required. In effect, as shown in Figures 38 and 39,the majority of devices discussed in this report effectively suppressstrumming. Other aspects of the behavior of suppression devices(particularly the hydrodynamic drag) may govern the choice of fairingtype for a particular application. For systems with acoustic sensors,however, the acceleration amplitude may be the most important considera-tion.

15

The drag coefficeints for all devices can be plotted versusReynolds number (Figures 40 and 41). If each device tested wereevaluated within a range of Reynolds numbers, then modified, and thesequence repeated, a family of curves would result. A good exampleof this is seen in Figures 8 and 9 for the recent General Electricdata taken at DTNSRDC. Clearly, not enough drag data, within theReynolds number range for moored arrays, are available.

For the discussion which follows, Tables 22 through 25 and Table 5are used, although no specific reference is made.

Fringe Fairing. The most comprehensive data for this type offairing are those of General Electric [8] and Cohen [7]. The initialwork by G.E. indicates that either a trailing fringe or a helical fringe(both manufactured by Wall Rope Works) will suppress from 87% to100% of the vibration, based on acceleration levels. The drag co-efficient data show considerable scatter, and the reliability of thedrag data may be doubtful. However, the drag data obtained in the Navyexperiments conducted in December 1975 (Figures 8 and 9) are good andindicate a CD of between 2.0 and 4.0 for a helical fringe. This iswithin the same range to be expected for a strumming bare cable. Ineither case, the total drag on the cable would be greater than that ona nonstrumming bare cable. Reconfiguration of the device for maximumsuppression with least fringe would reduce the drag. This wasattempted in ..e latest Navy experiment; however, test data have notbeen reduced at this time.

Cohen's [7] data indicate a trailing fringe will have about thesame drag characteristics as a bare cable with a maximum amplificationof 1.35 if the fringe becomes wrapped around the cable. Strumreductions, regardless of wrapping, range from 60% to 80%; this isconsistent with the G.E. data. In either case suppression is obtainedutilizing the Wall Rope Works fringe fairing.

Hair Fairing. Very few data exist for hair fairing except thosefor ENDECO's Haired Fairing. These data are for high Reynolds number(-Re = 105) and low angles of attack (-15 degrees). The drag dataappear consistent (with Cd from 0.5 to 0.9 and the strumming suppressionfrom 60% to 100% based on acceleration).

Philadelphia Resin Corporation brush fairing has not been experi-mentally tested and reported; however, experiments at MIT in June andJuly of 1976 utilized the brush fairing.

No data are available for ENDECO's Haired Fairing at 90 degreesto the flow.

Ribbon Fairing. DTNSRDC has made extensive tests of ribbon fairingsfor application to towed arrays. The ribbon and stub configurationsobtained by DTNSRDC [15, 16] provides excellent suppression with a CDin the range 2.0 to 6.0. Additional data were obtained in furthertests (as yet unpublished) and should provide a design with maximumsuppression with the least material. One test run wrs made with the

16

ribbon-faired cable normal to the flow, but excessive drag on the cablecaused a failure of the test rig. Apparently, the configuration ofribbons and stubs for a moored array would need to be modified fromthat for a towed array. The fact that ribbons can provide suppressionat 90 degrees is evident from the WHOI data [6]. The WHOI tests were:onducted in open water, and control over the flow parameters was notsufficient to provide consistent data.forThe zip-on ribbon fairing provided by the Zipper Tubing Companyfor NUC's testing [12] shows suppression characteristics equal to thatof the fairing developed at DTNSRDC; the drag coefficient range of 0.75to 1.3 is quite acceptable. The main problems with the zip-on fairingare handling and application on a long moorL'g cable.

Helical Ridge. Work has been conducted at NPL with three-starthelical ridges; however, the presentation of the data does not lenditself to comparison with data taken subsequently in the United States.The NPL parameters are consistent with those used by Skop and Griffin[27] and provide a measure of what the ridges do to the response ofthe system. The region of instability of a cantilevered beam withhelical ridges is reported as a function fo structural damping. Price'swork [17] offers little information. Weavers' data [25] althoughdifficult to compare to present work, does confirm the work done atNPL and indicates recommendation for a four-start helical strake witha height of 3/16 diameters and a pitch of 12 diameters.

Recent work was performed at DTNSRDC [16] and NUC [28, 12] comparinghelical ridges with other types of suppression devices. DTNSRDC foundthe drag coefficient for the helical ridges considerably less (1.1 to 1.9)than the ribbon fairing, but they had less strumming reduction. Theribbons were selected for further study, and work with helical ridges waspostponed. The NUC [28, 12] studies indicated a much higher drag thandid the DTNSRDC study (1.0 - 4.0). Both studies were conducted at lowangles of attack, and the NUC study found the helical ridge suppressioneffectiveness was reduced somewhat as the angle decreased from 25 to 5degrees. The NUC reports do not indicate superiority of a helical ridgeover ribbons or hair, nor do they indicate that a multistrake devicewould offer better suppression.

CONCLUSION

A review of Tables 22 through 25 indicate that all devices testedto date suppress strumming. The determination of which device to useis, therefore, usually based on user or investigator preference. Someinsights about the various devices, however, can be ubtained from theprevious investigative work and they are listed below:

0 Fabula's [12] data indicate that a single helical ridge isangle dependent; i.e., on the angle between the flow and the

17

longitudinal cable axis. This is to be expected since as theangle decreases less of Lhe ridge is "seen" by the flow.Walshe [22] noted that with a three-start helical ridge

system the strumming suppression characteristics axe inde-pendent of orientation angle.

"S The drag coefficient of helical fringe ranges from 2.0 to 6.0,whereas the trailing fritige tested by Cohea [7] was between1.0 and 2.0.

"" The drag coefficient for helical fringe fairing exhibits astrong dependence on Reynolds number (Figure 8). It isprobable that as the flow velocity increases the fringe tendsto lay down in the direction of flow thus reducing the appar-eent frontal area. Angle dependency is exhibited for helicalfringe only when the fringe is thinned; apparently, the sameeffect as with a single helical ridge occurs.

* Reliable and consistent drag data for strumming suppressiondevices are lacking (as seen in Figures 40 and 41),particularly in the region of concern for moored array.

0 Angle dependence of most devices has not been thoroughlyinvestigated. Most studies have been performed either at 90degrees to the flow or at a low angle simulating a towedconfiguration - but not both. Angle orientation needs to beconsidered in the design and application of the strummingsuppression de'rice.

* Helical ridges have been used successfully to reduce smokestack and tower vibrations; however, NUC and DTNSRDC haveshown the helical ridge to be less effective than ribbons,hair, or fringe. Helical ridges could, however, be easilyextruded on long cables [28].

* DTNSRDC ribbon experiments have indicated a substantial dragdifferential between towed and moored arrays for the sameribbon configuration.

0 Reynolds number does not appear to be a common parameter fordistinguishing the strumming suppression effectiveness ofvarious devices; however, for a single device CD is a functionof Reynolds number.

* A hierarchy for classifying devices by CD is not evident fromFigures 40 and 41. Strumming suppression device and vortexinteraction should be studied to determine how the deviceaffects the vortex formation, coherence, and strength.

The determination of a device's ability to suppress strumming cannot beachieved by simply placing the device on a cable and testing to see ifstrumming is suppressed. To pursue the problem economically and logic-ally, a basic understanding of the fluid dynamic damping obtained fromthe various devices needs to be achieved. This will, in turn, lead tothe efficient design of a strumming suppression system which meets theneeds of a particular cable configuration.

18

REFERENCES

1. Civil Engineering Laboratory. Technical Memorandum TM No. M-44-76-5:Strumming suppression - An annotated bibliography, by B. E. Hafen,D. J. Meggitt, and F. C. Liu. Port Hueneme, Calif., Apr 1976.

2. Civil Engineering Laboratory. Research plan for dynamic response ofcables suspended in the ocean, by D. J. Meggitt, B. D. Taylor, andH. S. Zwibel. Port Hueneme, Calif., Dec 1974.

3. National Advisory Committee for Aeronautics. NACA Technical Note3169: On the drag and shedding frequency of two-dimensional bluff bodies,smby A. Roshkc.

4. Navy Mine Defense Laboratory. Report i-132: Drag and vibration of• some wire ropes and fairings, by R. E. Kelly and C. N. Goff.

Sep 1967.

5. Naval Underwater Systems Center, New London Laboratory. TechnicalReport 4915: The cable development program for suspended sensorapplications, by R. C. Swenson. New London, Conn., Sep 1975.

6. Woods Hole Oceanographic Institution. WHOI-75-47: Strumming testson two faired cable, by E. E. Hays, R. T. Nowak and P. R. Boutin. WoodsHole, Mass., Oct 1975.

7. S. H. Cohen. The hydrodynamic behavior of a streamer type fairedcable in various flows, M. S. thesis, Massachusetts Institute ofTechnology, Cambridge, Mass., 1975.

8. General Electric Company. TM 990-2014: Strum and drag measurementof underwater cables, by Y.H. Chey. , Jun 1975.

9. Massachusetts Insitute of Technology. Cable strumming experiments,by I. Kan. Cambridge, Mass., Jul 1975.

10. J. K. Vandiver and C. H. Mazel. "A field study of vortex-excitedvibrations of marine cables," paper presented at Offshore TechnologyConference, 1976. (Paper No. OTC 2491)

11. Environmeatal Development Corporation. Data Sheet No. 23: Hairfairing. Marion, Mass., May 1975.

12. Naval Undersea Center. NUC TN-1379: Tow basin tests of cable strum

reduction, second series, by A. G. Fabula and R. L Bedore. SanDiego, Calif., Aug 1974.

19

7713. American Society of Mechanical Engineers. 68-WA/FE-47: Water drageffects of flow induced cable vibrations, by J. R. Dale, J. M. FicCandless,and R. A. Holler. Dec 1968.

14. Naval Ships Research and Development Center. Vortex indiced vibra-tion of bare and ribbon cables, by R. Blevins. Carderock, Md., Sep 1971.

15. MAR Inc. Technical Report No. 128: Hydrodynamic characterizationof various towed array tow-cables, by J. S. Diggs. Rockville, Md.,

16. MAR Inc. Unpublished Report: NSRDC basin cable strum tests, byR. D. Doolittle. Rockville, Md., Jan 1974. i17. P. Price and R. W. Thompson. "Suppression of the fluid-inducedvibration of circular cylinders," Journal of the engineering MechanicsDivision, American Society of Civil Engineers, vol. 82, no. EM3,Jul 1956, pp 1030-1 to 1030-22.

18. National Physical Laboratory. NPL/Aero/384: Drag measurentents athigh Reynolds numbers of a circular cylinder fitted with three helicalstrakes, by C. F. Cowdrey and J. A. Lawes. Jul 1959.

19. National Physical Laboratory. Report Aero 334: A means for avoidingwind excited oscillations of structures circular or nearly circularcross section, by C. Scruton and D. E. J. Walshe. Teddington, England,Oct 1957.

20. National Physical Laboratory. NPL Aero Note 1012: Note on a devicefor the suppression of the vortex-excited oscillations of flexiblestructures of circular or near-circular section, with special referenceto its application to tall stacks, by C. Scruton. Teddington, England,Apr 1963.

21. C. Scruton. "On the wind excited oscillations of stacks, towers andmasts,: in Proceedings of Wind Effects of Buildings and StructuresConference, June 1963. Teddington, England, National Physical Labora-tory,

22. National Physical Laboratory. NPL/Aerol/1007: The influence ofwind inclination on the effectiveness of strakes in suppressing windexcited oscillations of cylinders of circular section, by D. E. Walshe.Teddington, England, Jan 1963.

23. National Physical Laboratory. NPL Aero Report 1310: The use ofaerodynamic stabilizing strakes on chimney stacks supporting pipes,by D. E. Walshe and C. F. Cowdery. Teddington, England, 1970.

20

24. National Physical Laboratory. NPL/Aero/381: Further experimentson the use of helical strakes for avoiding wind excited oscillationsof structures of circular or near circular section, by L. Woodgate

A and J. F. M. Maybrey. Teddington, England, 1959.

25. W. Weaver. "Wind-induced vibrations in antenna members," Journalof Engineering Mechanics, American Society of Civil Engineers, vol 87,no. EMl, Feb 1961, p. 131.

26. P.N. Joubert and F. R. Hoffman. "Drag of a circular cylinder with

vortex generators," Royal Aeronautical Society. vol. 66, Jul 1962,S~pp 456-457.

27. Naval Research Laboratory. Quarterly Progress Report: Flow-inducedvibration of mooring arrays, by 0. M. Griffin, R. A. Skop and S. E.Ramberg. Washington, D. C., Oct 1975.

28. Naval Undersea Center. NUC TN-1188: Towing basin tests of cablei strumming suppression, by A. G. Fabula and R. L. Bedore. San Diego,

Calif., Oct 1973.

02

II I lit

(a) 3 in (76-mm) ii)lon with 7/8-in (22-mmi) ipacing. cable dulmlr 1/2 in 12.7 mim)

(b) 3 5-inch (89 mim) polypropykn( with 7/8-inch (22-mim)spacing cable diamecter = 5/8 inch (16 mm)

? Figure 1. Wall Rope Works fringe fairing.

4Figure 2. Thonged fairing, 8 inches (204 num) long, 6/in.I(from Reference 4).

22

-n-1

1.000 I I I I I I I II

500 -

100 , El

0

•E)

50

s-o

06

10

Kevlar5 A Unihne

0 All fairingsL 01 1 out of 2 tufts removed/ 1 out of 4 tufts removed

1 I I I I I i I I I I I I

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 08 0.9 0.10 0.11 0.12

Current (kn)

Figure 3. Effectiveness of fairing density on Kevlar line.

(from Reference 6).

23

I

I v I0

44

-ca)

-) u

sf-Ip

FZ4

24-

fii

01

00

'01

~~~14

< Ej -4

/ -~--

k (Ump) ip~ld/

N.25

44

0 00 0 04

- ý4

'1- "~ 'C X4

I Jnbwbi.

Ii26

3.0 I I I 32

--30

29

2.0

•,; -- 27

Amplitude

-26

S-- 25

Frequency

,L•- 24.1.0

- 22

S21

220

0 LT0105104

Figure 7. Amplitude and frequency for bare cylinder

vibration. (from Reference 7).

27

6.0

5.0

4.0

3.0

2.0

1.0

nonoscillating smooth cylinder

103 2 4 6 8 104

R,

Figure 8. Results of normal drag on G. E. helically fringedcable with tow angle of 60 degrees. See Table 5

for key to symbols.

28

6 -

{

5

4 -

CD

2 -

1 nonoscillating smooth cylinder

103 2 4 6 8 104i•, Re

Figure 9. Results of normal drag on G. E. helically fringedcable with tow angle of 90 degrees. See Table 5

for key to symbols.

29

_____ I

Figure 10. BRAINCON Haired Fairing.

(from Reference 11).

30

(1,012)4,000 0

10 '

*0

S- 20 U

300 3,000(984) 30

40

200 1,000(656) CdI-

Z

'. W.H.O.I. NAVI-THERM SystemP., V. Crawford

1 Typ- 167 (6 ft) V-Fin 2,000100 / Amergraph 7-14-1 Cable (0.30 OD)

(328) / Total Length - 1,650 ftla / lHair Faired - 500 ft

0- Depth0- Tension8- Surface cable angle

6-14-64

Braincon Co., Marion, Massachusetts

0 I - I -- I , 00 2.0 4.0 6.0 8.0 10.0 12.0

Speed (kn)Sm I I I • I I" I I I I I I I I

Hair Faired Cable- -- -Amergraph 3-H-1 (0.292 diam) 8 hairs per inchUnfaired Cable ,Amergraph 3-H-1 (0,292 diam)Calculated Depths * Drag Coeff. 0.7

10 IX Drag Coeff. 1.4

20 25 ft cables

So ft cables40 Jt ft cables5 05f

S60 _ . . . .

7- Speed vs. Depth Tow Test80 "Comparison With Type 317

2-ft V-Fin Haired & Unhaired90 ICables NRL Data Braincon Co.90 r Marion, Massachusetts

100 I ! !0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Speed (kn)

Figure 11. BRAINCON data sheet. (from Reference 11).

31

1.9 0

1. _T .8D -reference -

haired (smooth round /streamers cable)

1.5 reference 0.3 / antinode

0 (smooth round I splitters

1.3 \ cable) _sp i

twisted pair

0 oo 0 .

10.9/'-- -- x wite

antinode N/t0.1 /pair

0.7 weathervane-V - fairing noise level

0.5 D weathervatie0.5 20D 0.003 lb cant

'- 7D~j streamers

0.3L I a / I T o-S.500 700 900 1,000 1,300 1,500 0 400 600 800 1,000 1,200

Reynold's Number Reynold's Number.•,

Figure 12. Drag charac- Figure 13. Strummingteristics of special force characteristicscable designs. (from of special cableReference 13). designs. (from Ref-

ference 13).

32

1,000

500

100 'n f

xXxX XX G

.50 x Ao ®A

x

0 Prodesco ,R•: OO O0 4-in. FSW

0 • / NUSC-'• X Rochester

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4•i (0.05) (0.10) (0.15) (0.20) (0,25) (0.30) (0.35) (0.40) (0.45) (0.50) (0.55) (0.60) (0.65) (0.70)

• Current, Knots (m/s)

Figure 14. Acceleration ratios for four faired wire ropes.•" (from Reference 6),

33

,A 771

IN. 2 3 4' 51 6

CIVIL ENIGINEERIlNG LABORlATORlY

tC

Figure 15. Philadelphia resin corporation haired fairing: top,brush fairing applied helically on 0.75-in. (19.2-mm)diameter cable; bottom, cotton fuzz applied helicallyon 0.25-in. (6.4-mm) diameter Kevlar cable.

34 "

gk

4-1

44

4-)

~4-1

S E-4J

r4-

35O

..... .~ .~ . .

0.080

0.050 Ribboned Cable Sea Data

0 50% ribbons, 15 mil (0.38 mm)0 50% ribbons, 15 mil (0.38 mm)

S50% ribbons, 30 mil (0.76 mm)IA 25% ribbons x 25% stubs'•0.020 D 12-1/2% rbbons x 37-1/2% stubsSV 0% ribbons x 50% stubs

0.84-inch (21.3 mm) bare cable& 0.010

0.005

0.003 ___

2 x 104 5 105 3 x 105 0.100Re

10.0 0.050

5.05 14,0.020

2.0 ' 0

S1.0 0.003

z

0.5

0.002

0.2 -_ ----- 0 .001I2 x 10. 5 105 3 x 10 2 x 104 5 105 3x105

Re Re

Figure 17. Hydrodynamic drag coefficients for ribbon faired towcablescompared to bare cable, DTNSRDC. (from Peference 15).

3

- r -

1 0'

100

00 2 22 2 /0 U0 000 00 nMMc

0 44I ~In

N(gap) 1N2,-'..-s.-

V WV

4-4 -

'B --

0

E-4 -I

Wý-4

05 0C;1:)~ 1133 J4: sti

3ll'A t4

370 JJ-

'fn "n C11

ca w

u-

Li

4.5

f4-

0/ V o4))utapdwa

ANI 38

X XQ X x xx

- X x x x

< 0

"d100

xx

0

FSW

0 6 in. tails

S4 in. tails

110 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

(0.05) (0.10) (0.15) (0.20) (o.2S) (0.30) (0.35) (0.40) (0.45) (0.50) (0.55) (0.60)

Currcnt, Knots (mWs)

Figure 20. Comparison of accelerations for 4-in. (101.6 mm) and

6-in. (152.4 mm) FSW fairings, W1101. (from Reference 6).

39

NUSCFSW (SWENSON)

k. z~l

PRODESCO ROCHESTER

KEVLAR KEVLAR(ALL TUFTS) ,.:(TUFTS SPACED 4"

~~i ' i.• .

Figure 21. Fairings used in test, WHtOI. (from Reference 6).

\\~~ *~\40

1. Locate @ using tabulation block dimensioi.s.

2. Permanently tag identify "30003-3010068 -

Including applicable dwg rev letter, inaccordance with MIL-STD-130.

3. Mate @ with @ thus:(A) Separate the 12 outer strands of 0

wire from its core without permanent damage.

(B) Slide 2 und:r the open .trands of 'NVote - @ must pass under at least 3 strands

of 0 and outside the core of (D Position(C approximatrly ashown.

(C) Close thee strands of (,9 to hold 02 in place.

(D) Item @ to be installed central to item (Dwithin ±1 inch.

4. Material 0 - polyurethsne film having the

following physical prol'erties.t ASTM Test Method

Specitic gravity - - !---- 1.11 - - - D792-64T

Yield, sq in./lb ------ 24,900

Tensile strength- - ---- 6,000 psi - - D882-61T MD3,000 psi---- ---- TD

Elongation at break, % - - 350 - - - - D882-61T MD650------ ---- TD

Impact strength, grams - - 390 - - - - D1709-62T (A) 0Tear strength, lb/in. - - - 250 - --- D1004 MD .ref ref

380 -------- TD

Moisture vapc nsmission,

GM/MI L/100 sq in./24 br- 70 - - - - E-96-E 4 .30 nnn----!Low temperature, deg F - 100- - - - D1790-62

surface X C-i

S~~~C-2 C --- -3--

Tab of a LocationsNo. "C" ft ± 0.5 ft

1 2

2 5

3 15

4 30

5 45

6 60

7 80

8 100

9 120

see note 1

14.00 i 0.50 -

4.75 1 0.25

ref

L 0.610 ±0.005

+0.003000 thick 2 swages 90 deg apart

4 Detail A (typ)S0.44 ± 0.06

Detail "D"

2 ref rotated out ofposition for clarityA

na -. . ...-.. nn ,, n ...- ,nn... .L LL- uu- uuu.--M u_~~

3 C4 CM- C-5 C-6 C-7 - C-8 C-9

145 ft ± 2 ft

all single arrow dimensionsoriginate from surface X

Figure 22. NCSL sweep wire ribbon fairing.

O .cc note 4 and detail "D

14.00 + 0.50

0.610 ± 0.005

2 swages 90 deg apart

Detail A (typ) 0. .

0.12• 0.12

C-7 C-81 C-9) V

41

1 p2Af CTl2.DC

2 CUL'O0R 3 2tf'ML PLRAIL= TO Ch. AXIS

~~Ww INE 0L00(16.)

M23

3 CMINDER & 3 WIRE M IS20

- -_ -•. 60 P" VEOY HLE fl fd•l.(l: D1W .UI..O: :

0.023 2

C=Zn20 6 WVUHE l.UEý(3e. Fig. B)P.ricIaE DIAJoErn(al.) PoTh(enge.)

0.023 0.5, 4,. p8, 16, 20

0.026 0.0.029 1

5 TLINDIR & DMUMal(fte Fig. B) 0.032 20

-U BEM. -U=4

FINm THIOxco - 0.0201n.

7 C= E .FMT FlU, It 20M O Fig e c) PCII2MT - 37%I

211112 URpT TO C11. AXIS

M 1. /10 PARIX,&= 10PM102C IMW

lIZ I222 7

Figure 23. Water channel models. (from Reference 17: "Suppression ofthe fluid-induced vibration of circular cylinders," by

P. Price and R. W. Thompson, in Journal of the EngineeringMechanics Division American Society of Civil Engineers, vol. 82,no. EM3, Jul 1956, pp 1030-6).

_ _ _ _ _- -,-- ----- 43 -

S0IU~ 0a' 00 103 1

-4-i co r

oc

0 0-4.0

00~ 0

0 )

W0)

0*0

P. 4-4

c 0 74 .01 No

44

m o 0

00

0 ;3

0

4J

C '.-.- ~~000

I44

0 00

0wu

Ito

Pn 4-

114:3~ .

Go rZ

x 45

7F W.•_

2.03D

3 in. diam cylinder; spoilers -3D @ 1413; v 40 mph.

0.5

00 2 4 8

Number of Windings

Figure 26. Influence of number of windings on

spoiler effectiveness. (from Reference 25):

(from "Wind-induced vibrations in antenna

members," by W. Weaver, in Journal of Engineer-

ing Mechanics, American Society of Civil Engineers,

vol 87, no. EMl, Feb 1961, p 158).

1.0

0.9

0.83 in. diam cylinder; spoilers 4 @ 8D

0.70.6

CKS 06D_K-_ 0.5 1.-- knee of curve at

CKB 1

0.4 v1 30 mph

0.3v 1 = 40 mphm

So.2 "-+7 -32-0.1 __ .. _ .__ "

00 1 1 3 9 3 9 3 (in. diam)

16 8 16 32 8 16 4

D 3D D 3D D

16 32 8 16 4

Winding Diameter

Figure 27. Influence of winding diameter on

4 spoiler effectiveness. (from Reference 25):

(from "Wind-induced vibrations in antenna

members," by W. Weaver, in Journal of Engineer-

ing Mechanics, American Society of Civil Engineers,

vol 87, no. E14, Feb 1961, p 159).

46

1.01

0.9 v, (mh)0.8 -6 in. diamn cylinder; spoilers 4-1/2 in. diarn2A 3

0.7k12) 0 40a050

0.6 6f

CKB 6

0.25 D lD 1D 1D 6 8 0

effctvees.-(fromtReferene 25): 16

(from ~~Ptc oWndidue viraionsding aten

members," by W. Weaver, in Journal of Engineer-ing Mechanics, American Society of C2ivil Engineers,

vol 87, no. EMi, Feb 1961, p 159).

clean c~ylinder

0.48 t05 e

6x 10 4 105 2 3 4 5 6 x106

Reynolds Number

* ~Figure 29. Drag coefficient for cylinder with andIwithout vortex generators. (from Reference 26).1

47

5z_ _ _ _

I I I

4 440

4.)

0)

)00

ý4.

Q)

Q) Pd~OIL)

4-

rX

480

4- -

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coI0i 10 4-J()

E~ .I-J

10: 06 41

- 0

('La)~e (pmIdw u ow~~

-6 6

N 06- 10

0.>

0. - - 4p r.)

bo :JCO 4

ýXI

0 41

ItI

a)0

4j 04

0 f

00 01

I-i $4.

r44-

00

0 Ij

C.-A 0 i/n 0 >

*$ 4 (A

(-u!) aprnildwvr 0nl uo$4ja

50

-1.0 Off

EA

r r

u 11

o.I I IU0.5 0

o IPA U A

00 5 10 15 20{

Length of Wrap Removed (ft)

Ridge Type 1; height/D = 0.153, P/D - 10

Figure 33. Dependence on ridge removal oftransverse vibration, as indicated byaccelerometer trace maximum peak-to-peakamplitude for V < 6 fps (1.8 m/s)(from Reference 26).

51

- --- - - - - --- 1

-I

to boUu

C'-I

CYO

00

O0. V o4-)upnajgiun3

52z

4 0.080

0.050 Helical Wrap Basin & Sea Data

d/D = 0.23

* P/D = 15, basindata

S0 P/D = 15, seadata'3 0.020 03 P/D = 20, ,eadarm

SP/D = 30, sea data

A 0.84-inch bare cable sea data

0.010

0.005

0.003 I2 x 10

4 5 105 3 x 105

0.100R-

10.0 0.050

5.0

0.020

C

2:: 0.010-1.0 0.005

z

S0.002-0.5

0.001 IV

2x104 5 105 3x10 5 2x104 5 105 3x 105Re

Figure j5. Hydrodynamic drag coefficients for helically wrapped cablescompared to bare cable, DTNSRDC. (from Reference 15).

53

3.0

(a) Orientation angle.

2.0

1.0 1.00

V 7.5 ft/s

00 900 1800 2700 360O

4. of rotation

1.3

(b) Velocity.

1.2'

1.01

1. = . x1'

k %

40 .81 1

0 2 4 0 12,VI1Veoct fI

00 dere

Figure~~~~~ 3.Dacofiien safnto fvlctand ~ oinaionage fo eeec )

-, 54

Vaeclne

% ITC P

0,

~0

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0 00.. .. '0 Vd

Cd

or tr. c

40

14 r-I

~2 .. rI4 ei

ca

00 00n

uoU13lPza %

55

I

0

-14

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N 4-1 0Yr co 4

w ___ _____ ____W

u 0J

Cflr

0 )

o. U. Li.)

Eww

)z.)

P4406 00

5- _ ____ ____ ____----'----OD

- ----.--- 4

CN

C14

100

0)

4-)

06)C/)

to

'.H

0

19)

57 0

toI

00I

4:1

r. 'Aza 0

Lr)

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_ _ _ _ _oa)

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00 N lC

N N VC

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'59

Table 1. Physical Parameter3 forStrumming Suppression Devizes.

Type of Device Geometric & Material Parameters

Type of MaterialDensity of CoverageLength

Fringe SpacingSGeometry

TrailingHelical

Type of MaterialHair Density of Coverage

Length

Type of MaterialLengthWidth

Ribbon Spacing• ~Geometry

TrailingHelical

GeometryRoundRectangular

Diameter or Height Relative to CableHelical Ridge Diameter Ratio (dID)

Pitch to Cable Diameter Ratio (P/D)Percent CoverageReversing HelixNo. of Ridges

60

__J,

Table 2. Structural and Fluid Dynamic Parameters

Notation Parametric Definition

f fundamental frequency of structure in test fluid

f Strouhal shedding frequencyS

U free stream flow velocity

angle between cable axis and flow (900 is cableaxis normal to flow)

M virtual mass (mass plus added mass)

D diameter of cylinder or cable

logarithmic decrement

p density of test fluid

CD drag coefficient

CL lift coefficient

Am vibration amplitude with suppression device

Am vibration amplitude without suppression deviceA c acceleration with suppression device

Ac accleration without suppression device

P pitch of helical spiral of device on cable

d diameter of ridge

C tangential drag coefficient

6 1

•;• 61

0H ) 0 4 0 4 0 0) b0H u -H H H "4H

V-4 4J 0- r4I 0r' p r4 0:OH 0 o 100 t0 00 0 00 0 00

r-4 09~ 4) ' A ) Wt

-. H 00 00 00 0

4..

4) A) 04.) 14 02.. 02) 02. 020

H r4 0 H H04V3 4 (0i T- - rI.

ALHA H 1) IAH LAO

0

l-H.0

Q. 41 C L '4L0 (d -. H Hq e4 H

to

CO

00 r4 r.- 0 4

-H 00 00 0

r40 0 f 040e 4.iH

H 0(0 1 4

w

.r4C1 r4 4

620

Table 4. Parameters for C.E.'s Faired Cable Tests [8]

Acceleration readings taken at midpoint, quarter point, andone-third point; tangential, normal, and lift forces obtainedfrom triaxial force gage.

Geometry Parameters

Helix wrap

IMaterial Polypropylene

Leacing 7 in. (120 mm)

Spacing 7/8 in. (22 mm)

P/D 10

Fringe applied to 0.25-in. (0.006 m) cable

T n spiraled around a 0.70-in. (0.018 m) cable

! Ti tiling

MaLerial Polypropylene and polyester monofilament

Length 4.5 in. (110 mm)

Spacing 7/8 in. (22 mm)

Fringe applied to 0.81-in. (21 mm) cable

63

Table 5. Summary of Parameters for Various Strum Suppressed Cables

Run Angle Statac."1Run Model (De Tension Symbol'

oe (lb)

4 Bare cable 90 45

3 j

5 I68 1389 134

11 136 Q14 13312131617 21218

24 Bare cable with helical wrap of 90 6225 fringe fairing 63 0

29 90 1363031283233

20 90 23521 234 I22 _ I

33 Bare cable with helical wrap of 90 138 A

35 fringe fairing V77

45 Bare cable with helical wrap of 90 13646 fringe fairing cut back to47 one- t hird lengt h fringe ___

aThese symbols are used in Figures 8 and 9.

- .64

Table 5. (Continued)

Run Angle StaticModel Tension SymbolNo. (Deg) (b

(lb)

40 Bare Cable with helical wrap of 90 13641 fringe fairing with one-third tufts I42 removed43

50 Bare cable with helical wrap of 90 13451 fringe fairing with two-thirds tufts 13352 removed53 134

64 Bare Cable 60 106656667 :

68 Bare cable with helical wrap of 60 9969 fringe fairing cut back to70 one-third length fairing71

72 Bare cable with helical wrap of 60 9973 fringe fairing with two-thirds tufts74 removed I75

65

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Table 16. Summary oi Model Characteristics (From Reference 28)

[Surface was smooth on all models except Model 10; helixreversal on all except Model 15.]

Model Ridge Pitch Ratio, Number ofType P/D Ridges

1 1 20 12 2 20 13 3 20 14 4 20 15 5 20 1

6 6 20 17 7 20 18 8 20 1

11 1 40 112* 3 40 113* 6 40 114* 7 40 115 1 20 1 {

16 1 10 117* 3 10 118* 1 20 219 1 20 320 9 20 1

21 1 5 122 1 15 123 4 10 124 4 40 125 (Plastic Film Fairing

26 (Splitter Plate Fairing)*Not tested

4 4

79

'It"

Table 17. Summary of Ridge Cross-Section Parameters(From Reference 2 8 )a

r idge ridgeridge ridge di / d"Type hegh width d wd height d/Dheight width .4D D

1 ---- 0.22 0.20 1.1 0.153 0.153

2 0.15 0.28 ---- 1.9 0.114

3 0.22 0.40 ---- 1.8 0.168 -

4 ---- 0.34 --- 0.260

5 0.23 0.23 ---- 1.0 0.176

6 0.12 0.25 ---- 2.1 0.092

7 0.12 0.50 ---- 4.2 0.092

8 0.23 0.52 ---- 2.3 0.176

9 ---- ---- 0.15 --- 0.114

aBlanks indicate that it is not applicable

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Wire Diameter/Cable diameter(d/D) Pitch Length/Cable Diameter (P/D)

0.24 20

0.12 20

0.36 20

0.24 5

0.24 10

0.24 15

0.24 30

0.24 40

83

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Table 21. G.E. Test Parameters

Tension lb, Pitch/Cable Wire Diameter/ Cable AxisRe to Flow,

(N) Diameter Cable Diameter dewdeg

3,217 61 (271.3) 10 0.25 90

4,817 138 (613.8) 10 0.25 90

6,435 246 (1094.3) 10 0.25 90

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09

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