Table 2.4. The Excitation Force Coefficients on Vibrating BluflF Cylinders; 

 Description of the Data in Figure 2.7 



Symbol 



Type of cylinder 



Medium 



Cylinder material 



Investigator(s) 



A 



D 



Flexible 

 cantilever 



Water 



PVC 

 PVC 



Aluminum 

 Stainless steel 



King (1977) 



• 



Pivoted 

 rigid cylinder 



Water 

 & Air 



Brass 



Vickery and 

 Watkins (1964) 



+ 



Spring-mounted 

 rigid cylinder 



Air 



Aluminum tubing 



Griffin and 

 Koopmann (1977) 



D 



Rigid cylinder, 



forced 



oscillations 



Water 



Stainless steel 



Mercier (1973) 



O 



Rigid cylinder, 



forced 



oscillations 



Water 



Aluminum tubing 



Sarpkaya (1978) 



A 



Flexible 

 cantilever 



Air 



Aluminum 



Hartlen, Raines 

 and Currie (1968) 



The excitation component Qf of the total hydrodynamic force is important because it is this 

 component of the fluid force system that transfers energy to the structure or cable and drives the 

 strumming vibration. Several important characteristics of the unsteady lift and pressure forces that 

 accompany vortex-excited oscillations are clear from the results. First there is a maximum of the excit- 

 ing force coefficient at a peak-to-peak displacement between 0.6 and 1 diameters for all the cases shown 

 in the figure. Second, the maximum of the force coefficient is approximately Q^ = 0.5 to 0.6 for all 

 but one case, the sole exception being the result at Q^ = 0.75. Qf then decreases toward zero in all 

 cases and results in a limiting eff"ective displacement of 2 to 3 diameters (peak-to-peak). This limit is 

 clearly shown by the displacement amplitudes measured at low reduced damping in Fig. 2.2. 



The decomposition of the fluid dynamic forces described here is based upon the supposition that 

 both flow-induced lift (excitation) and flow-induced reaction (damping) forces act on a resonantly 

 vibrating cable or cylinder. The "wake-oscillator" models developed by a number of investigators (see 

 Appendix D) are also based upon the hypothesis of a fluid damping or reaction force that is exactly 



12 



