3. EXPERIMENTAL STRUMMING RESULTS 



3.1 Behavior of Cables in Fluid Flows. The measured frequency and displacement amplitude 

 responses for small-diameter taut cables undergoing cross flow strumming vibrations in water are plot- 

 ted in Figs. 3.1 and 3.2. These results are taken from laboratory-scale experiments reported by Dale, 

 Menzel and McCandless (45). In the first set of experiments (see Fig. 3.1) a 2.5 mm diameter cable, 

 0.9 m in length, was excited in three resonant strumming modes over the frequency range 14-28 Hz, 

 and in the second set of experiments (see Fig. 3.2) a cable of the same diameter, but 1.8 m in length, 

 was excited in six modes over the same frequency range. Predictions have been made for the response 

 frequency and the strumming displacement for the case shown in Fig. 3.1 and they are shown there as a 

 function of the flow speed V together with the experimental results obtained by Dale and his col- 

 leagues. The calculations of the cable response were made with the "wake-oscillator" model of Skop 

 and Griffin that is described in Appendix D. The agreement between the predication and the experi- 

 ments is generally satisfactory for the strumming displacement and frequency, the flow speed at the 

 maximum amplitude and the flow speeds at which the cable passes from one natural mode to the next. 

 The reduced velocities which correspond to the peak strumming displacements are noted on the figure. 



The higher modes and frequencies of the cable result in larger strumming displacements. This 

 follows from the dependence of the cable's damping ratio Cs on 1/f. which results in smaller values of 

 the reduced damping at the higher frequencies and, consequently, larger strumming amplitudes. A 

 similar result was obtained by King (46), who found that the higher normal modes of a flexible cantil- 

 ever corresponded to smaller values of ^Jfi or k^, so that the higher modes experience increasing peak 

 displacements. 



The steady drag and the tension fluctuations on a strumming small-diameter cable were measured 

 by Dale and McCandless (47). The cables employed in the experiments were between 1.45 mm (0.057 

 in) and 3.6 mm (0.140 in) in diameter and approximately 0.9 m (3 ft) long. A spherical mass of 0.23 

 kg (0.5 lb) was attached to the free end of the cables as they were towed through still water. The 

 details of the experimental set-up are discussed by Dale and McCandless. 



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