• Compute Strouhal frequencies and test for critical velocities, K„„ (in-line and cross flow), based 



upon the incident flow environment. 



• Test for reduced damping, k^, based upon the structural damping and mass characteristics of the 



structure or cable. 



If the cable system or structure is vulnerable to vortex-excited oscillations, then 



• Determine vortex-excited unsteady displacement amplitudes and corresponding steady-state 

 deflections based upon steady drag augmentation according to the methods of reference (59), if 

 applicable (see Fig. 4.2); 



• Determine new stress distributions based upon the new steady-state deflection and the superim- 



posed forced mode shape caused by the unsteady forces, displacements and accelerations due to 

 vortex shedding. 



• Assess the severity of the augmented stress levels relative to fatigue life, critical stresses, etc. 



4.3 Practical Design Data. Several dynamic models of varying levels of sophistication have been 

 developed to predict the displacement amplitudes that are excited by vortex shedding. One class of 

 models, the so-called nonlinear "wake-oscillator" type, has been described briefly here and in more 

 detail in Appendix D. None of the wake-oscillator formulations proposed thus far has been developed 

 to the stage where it truly represents a practical procedure for detailed design of structures in both air 

 and water, but, based upon a detailed study. Dean and Wootton (72) have suggested that the wake- 

 oscillator model of Skop and Griffin (see Appendix D) is perhaps the most promising for additional 

 development. At present the wake oscillator model of Appendix D has been used with considerable 

 success in the derivation of scale factors such as those in equations (2.3) and (4.3.1). 



Several empirical predictions of the dependence between the peak cross flow displacement ampH- 

 tude and the reduced damping have been developed over the past several years. The three most widely 



91 



