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



runs shown in Fig. 13, however. The lengths of the waves used in the runs 

 shown in these figures were all rather long compared with the water depth, 

 and were quite non-linear. The run shown in Figure 14 was chosen to show 

 the worst comparison that was obtained, which was for the case of the 

 mostnonlinear wave tested. 



" Lift" Force Distribution Function 



Referring again to Figure 6c, it appeared to the writers that 

 a "lift" force distribution function would be useful. Owing to this, 

 some "lift" force records were analyzed in detail. The method by 

 which this was done is shown in Figure 15. The "lift" forces for 30 

 consecutive force oscillations were measured for each of 10 runs. 

 Total "lift" forces (the sum of the outputs of the top and bottom strain 

 gages) were measured. The characteristics of the waves in these 

 runs are given in Table 2. The characteristics of representative 

 examples of the BEB data are given in Table 3. The distribution 

 functions are shown in Figure 16. 



Figure 15. Method of analysis of "lift" force records, UCB 1972 

 force measured from crest to following trough 



" Lift" Frequencies 



Chang (1964) stated the frequency of the "lift" forces was twice 

 the wave frequency. However, an examination of the sample records 

 reproduced in his report showed that for a pile diameter of 0. 083 

 foot, and a wave period of about two seconds, the "lift" force became 

 irregular as the wave height was increased. One record showed the 

 average "lift" frequency to be about three times the wave frequency. 

 The writer's data showed that "lift" frequency to become quite irregu- 

 lar, as was the case of Bidde's data for a number of runs (see Figure 

 6c, for example). The BEB data showed the "lift" force frequencies 

 varied from about 1.3 to 6 times the wave frequency. 



Irregular Waves 



Waves in the ocean are irregular. Irregular waves can be ge- 

 nerated in the laboratory using a wave piston type generator. An 

 example of such waves, together with the induced longitudinal and lift 



785 



