The drag and strumming force (fluctuating tension at the attachment) are plotted in Figs. 3.3 and 

 3.4. The tests were conducted with a smooth cable 2.72 mm (0.107 in) in diameter. The third in = 3) 

 and fourth (n = 4) mode resonances of the cable are clearly shown as the relative flow velocity (Rey- 

 nolds number) is increased. A portion of the second mode (n = 2) is visible at the left-hand side of 

 both figures. The drag coefficient Co is amplified for all of the modes from the stationary cable refer- 

 ence value plotted in the figure. The peak value of the tension fluctuation for each cable mode 

 corresponds to the peak strumming displacement amplitude and to the peak drag coefficient for that 

 particular cable mode. In other experiments conducted as part of the same program. Dale and 

 McCandless measured strumming drag coefficients as large as C'o = 2. This corresponds to an 

 amplification of the steady drag by a factor of about two, and this finding is in good agreement with the 

 results discussed in Section 2 of this report. 



Some indication of the changes that occur in the flow pattern in the wake of a strumming cable 

 can be obtained from the photographs in Fig. 3.5. The- photographs were taken at NRL in a wind tun- 

 nel equipped with an aerosol flow visualization system (48). Two views of each of the cable wakes are 

 shown with the flexible cable held stationary and with the cable vibrating with the sinusoidal displace- 

 ment amplitude pattern along its length that is typical of strumming. In the photographs the maximum 

 displacement amplitude is about 30 percent of a diameter, and the wake is highly coherent with Pab~ 

 0.9 (see Section 2.3) over a distance of 10 diameters along the cable's length. One photograph from 

 the wake of the vibrating cable represents the wake of a node in the vibration pattern; in this case the 

 wake is irregular and similar to the pattern downstream from the stationary cylinder. When the dis- 

 placement amplitude is 0.3 D the wake flow is highly regular and coherent, and the vortex pattern is 

 typical of the wake of a resonantly vibrating bluff" body. 



3.2 Towing and Flow Channel Experiments. A program of cable strumming experiments was carried 

 out in the towing basin at the David W. Taylor Naval Ship R&D Center (DTNSRDC) (29,49,50). 

 These experiments added more details concerning the behavior of flexible cables to the fundamental 

 results that are described earlier in this report. 



53 



