measured structural damping due to changes in the strut stiffness with yaw angle, which may account for 

 the response changes. The reduced damping ks of the cables is very low, and the peak displacement 

 changes only slightly with /Cj except for the anomaly just mentioned. The peak displacements are 

 slightly lower than one might expect from the limiting values at low damping in Fig. 2.2, but the meas- 

 ured strumming frequencies in Fig. 3.10 clearly show the constant frequency vs. flow velocity resonance 

 (lock-on) that characterizes vortex-excited oscillations. 



The second mode (n = 2) results for the three slack cables exhibited many of the strumming 

 characteristics that are evident from the fundamental mode results shown in Fig. 3.10. The peak dis- 

 placements and their corresponding reduced velocities are listed in Table 3.2. The resultant strumming 

 amplitudes of the DAS and Uniline cables are nearly the same as in the fundamental mode, but the 

 second-mode displacement for the small diameter cable is about half the fundamental mode response. 

 This anomalous behavior (compared to taut cables) is very likely due to the complex strumming 

 waveform in the n = 2 mode that is characterized by large values of the phase angle (ji^y (see Table 

 3.2) and the relatively large in-line components of the vibration at the strumming frequency. Based 

 upon the results from the preceding section, one would expect the cable strumming in the second mode 

 to exhibit slightly larger displacements than in the fundamental mode. The cable vibration frequencies 

 in the second mode again are typical of the frequency vs. velocity dependence that characterizes the 

 lock-on phenomenon as discussed in Sections 2 and 3.1 of this report. 



As part of the overall CEL/NCBC cable dynamics research program MAR Incorporated conducted 

 a program of experiments to investigate the effects of sensor housings (attached discrete masses) on the 

 overall cable response. A first report on the results obtained has been given recently by Kline, 

 Fitzgerald, Tyler and Brzoska (51). Some of these results are summarized briefly here and compared 

 with the previous findings from the cable dynamics program shown in Fig. 3.10. The tests were con- 

 ducted on the same "strumming rig" at the DTNSRDC that was employed in previous CEL-sponsored 

 strumming experiments. The recent MAR Incorporated experiments and the experimental layout are 

 described in detail in references (49) and (51). 



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