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



bulb mounted at one end. The other end of the tube was heated and 

 drawn to make the tip opening the desired size. Stereophotographic 

 sets were taken of the trajectories of these tracer particles, and a 

 computer program (Glaser, 1966) was used to calculate the space 

 position of them. However, it was found to be too difficult and lengthy 

 a job to pursue. 



Owing to the difficulty described above, a description of the 

 wake regime was developed by Bidde which was based upon his ob- 

 servations of the water surface characteristics, using magnesium 

 powder sprinkled on the surface in the vicinity of the pile. An example 

 of the relationship between the wake characteristic and the wave 

 height, with the wave period being held constant is given in Table 1 

 together with the values of N R and Nj^q. Similar tables were con- 

 structed for a number of wave periods. The generalized results are 

 shown in Figure 5. It was found that N KC correlated reasonably 

 well with the different regimes of the surface wake characteristics. 

 When Nj£Q was about 3, one or two eddies formed, when its value 

 was about 4 several eddies formed and shed, having the appearance 

 of a von Karman vortex street, when it was 5-7 the wake started to 

 become turbulent, and when it was larger than 7, the wake became 

 quite turbulent, and the turbulent mass of water swept back and forth 

 past the pile. Using the concept described previously, H > about 

 3D/-7T , that is, the wave height should be about equal to one pile 

 diameter. As will be shown later, this was found to be the case for 

 two piles, one about four times the diameter of the other. 



The Reynolds number was between 4, 000 and 7, 000 for the 

 values of the Nj^q when the wake became quite turbulent with no de- 

 tectable von Karman vortex street. 



A similar phenomenon occurs in steady flow for N R greater 

 than 2,500, according to Rouse (1963). He states that for N R greater 

 than this value a trail as such can no longer be detected. Rouse fur- 

 ther states that the body continues to be subjected to alternating "lift" 

 forces, but that each vortex becomes progressively more unstable 

 during its formation with a resulting wake that consists of a hetero- 

 geneous series of eddies. 



" Lift" Forces (Bidde) 



When eddies form, in addition to their effect on the longitudi- 

 nal drag and inertial forces, "lift" forces are also exerted on the 

 cylinder. For a vertical cylinder these lift forces act horizontally, 

 but normal to the longitudinal forces (longitudinal being in the direc- 

 tion of wave motion), and should more properly be referred to as 



769 



