sensors were used to measure displacements at selected points along the cable. Each sensor was 

 comprised of two sets of electric dipoles, which were set 89 mm (3.5 in) apart; these were used to 

 sense the position of the cable in terms of two vector signals R\ and ^2- In later analysis of the data, 

 the horizontal and vertical displacements were derived from R^ and R2. The accuracies obtained in the 

 measurement and analysis processes were calculated and are discussed in detail together with equipment 

 and data analysis techniques in references 49 and 50. 



Table 3.1 



Cable Model Physical Characteristics: 



DTNSRDC Experiments 



Model 



Diameter 

 mm 



Length 

 m 



Weight, N/m 



In Air 



In Water 



Double Armored Steel (DAS) 



15.5 



4.38 



7.59 



5.71 



Uniline 



15.2 



4.35 



2.77 



1.01 



Small Diameter 



1.77 



4.34 



0.095 



0.070 



Experimental conditions were chosen for the three cables to vary the five dimensionless parame- 

 ters discussed above and to bracket each resonant condition with several runs at various constant tow- 

 ing speeds. To select these speeds for each static tension setting, the tow carriage was slowly 

 accelerated through a wide speed range. Strip chart recordings of the forces during a portion of a typi- 

 cal acceleration sweep are shown in Fig. 3.7 for the Double- Armored Steel cable oriented normal to the 

 flow and under 1650N (370 lb) of static tension. As the carriage speed was increased from zero, a reso- 

 nance appeared in the drag signal at about 0.1 m/s and then diminished at about 0.2 m/s (0.4 kt). This 

 represents an in-line mode where the cable was oscillating in the direction of the flow. As the speed 

 was increased from 0.4 to 0.8 m/s (0.7 to 1.5 kt) in Fig. 3.7, a fundamental-mode resonance appeared 

 in the midspan displacements and in the forces. Within this range, higher harmonics of the motion 

 begin to appear, especially in the flow direction. The displacement spectra for five speeds in this range, 

 shown in Fig. 3.8, clearly illustrate the build-up of these harmonics. Over a still higher range of speeds, 

 between 0.9 and 1.3 m/s (1.8 and 2.5 kts), a second harmonic resonance appeared in the displacements 

 at the quarterspan and in the flow-induced forces (49,50). 



55 



