The strumming behavior of the cables tested during these experiments has been classified into 

 three general categories: resonant lock-on, non-resonant lock-on and non-lock-on. The first of these 

 categories, resonant lock-on, is characterized by very stable motion of the cable in one of its natural 

 modes where the displacement is sinusoidal and the displacement amplitude is essentially constant. 

 Non-resonant lock-on is characterized by small modulations in the cable vibration displacement and fre- 

 quency. These two lock-on regimes are shown in Fig. 3.13 where two data records from the Castine 

 Bay experiments are plotted. Non-lock-on occurs when the natural vortex shedding frequency is just 

 outside of the synchronization range. The results obtained in this latter regime are discussed in a 

 recent paper by Kennedy and Vandiver (55). 



The results from the Castine Bay experiments that pertain to the case of resonant lock-on are now 

 discussed. The actual vibration frequencies measured at currents between 0.2 and 0.6 m/sec (0.4 and 

 1.2 kt) with an unfaired polyester (Uniline) cable and an unfaired Kevlar cable positioned in the tidal 

 flow are plotted in Figs. 3.14 and 3.15. The tension changed slightly (less than 5 percent) during the 

 run time, so that the measured frequencies are scaled here by [Tension] '^^ in order to account for the 

 slight variations in the natural frequencies of the cable. When this adjustment is made, the natural 

 modes of the Uniline cable are clearly highlighted as shown in Fig. 3.14. Five natural modes of the 

 cable in = 2, 4-7) appear over the range of tidal currents shown and all are characterized by resonant 

 lock-on. In the case of the Kevlar cable, five natural modes (n = 3-7) also are shown in Fig. 3.15. 

 The mode numbers were estimated from the taut cable equation 



" ^ 2L/„ -^/^ (3.2a) 



after taking into account the added mass of the cable. The frequency response of these relatively long 

 cables (23 m or 76 ft) is similar to the strumming response of a meter-long cable employed by Dale, 

 Menzel and McCandless (45) in their small scale experiments discussed in Section 3.1. The strumming 

 response of the short sample of cable is shown in Fig. 3.1; the similarities between the field and the 

 laboratory are evident. 



61 



