Wave-Induced Eddies and "Lift" Forces on Circular Cylinders 



In the study by Bidde the ratio of "lift" to longitudinal force 

 was used as a basic parameter rather than Cj_, as this parameter is 

 comparatively less sensitive to any systematic errors in the instru- 

 mentation used to measure the forces, as similar errors would be 

 present in both "lift" and longitudinal force measurements, and these 

 errors would have a certain tendency to cancel out. Some of the data 

 are shown in Figure 7 of the relationship between the wave height and 

 the ratio of "lift" force to longitudinal force. 



Figure 7. Relationship between ratio of lift to longitudinal force and 

 wave height (Bidde, 1970, 1971) 



The relationships between Nj^-q and Np and the ratio of 

 "lift" force to longitudinal force are shown in Figure 8. This graph 



indicates that the "lift" forces start at N 



of "lift 1 

 ing. 



KC 



of about 15 the ratio 



to longitudinal force shows a slight tendency to stop increas- 



As can be seen in Figure 6c the amplitude and frequency of the 

 "lift" forces become irregular for larger values of Nt^q , and it is 

 necessary to specify what is measured. Bidde presented the ratios of 

 average maximum "lift" forces to average maximum longitudinal 

 forces. The longitudinal forces were uniform so that no problem exist- 

 ed in measuring and reporting them. Bidde drew a line by eye through 

 the crests of the larger "lift" forces and another line through the 

 throughs, and reported the "lift" force as the distance between the 

 two lines. Furthermore, Bidde reported the ratio of the "lift" to lon- 

 gitudinal force as measured only by the bottom strain gages. An ana- 

 lysis of a few records showed that this ratio was the same as the ratio 

 of the total forces. 



773 



