380 



FLOW 



FIGURE 20. Final stage in cloud 

 shedding process, K = 0.21, 

 V^ = 14.8 m/s, P^ = 124.1 kPa, 

 a"= 3.25 + 0.95 sin ut. 



TOP VIEW 



SIDE VIEW 



LOCATION OF DYE INJECTION 



FIGURE 21. Desinent condition 

 for leading edge sheet cavity; 

 K = 0.49, V_^ = 11.5 m/s, 

 P = 76.2 kPa, o = 3.25 + 1.55 

 sin ojt. 



SIDE VIEW 



was set to the maximum angle the oscillating foil 

 attained (4.2° for aj = 0.95 in Figure 25), the 

 maximum cavity length could be as much as a factor 

 of two larger than for finite values of K {eg. , 

 K = 1.2) . 



The data plotted in Figure 26 show that within 

 the accuracy of the experiments, a variation in 

 velocity from 11.5 to 15.4 m/s produced no signifi- 

 cant change in the results shown in Figure 25 other 

 than that expected for the small variation in a 

 that occurred between tests. It appears that the 

 parameters of K, a, and Oj , are sufficient to 

 correlate all of the present data with the presence 

 of cloud cavitation. 



6. CONCLUSIONS 



In order to improve the physical understanding of 

 the cavitation inception process and the formation 



of cloud cavitation on marine propellers, a large 

 two-dimensional hydrofoil was tested in the DTNSRDC 

 36-inch Water Tunnel under pitching motion. The 

 foil was instrumented with pressure transducers to 

 measure the unsteady surface pressure due to foil 

 oscillation, and photos were taken to correlate 

 cavitation inception and cavity patterns. 



Prior to the occurrence of cavitation on an 

 oscillating foil, the foil is in a fully wetted 

 condition. Knowledge of the pressure distribution 

 on a fully wetted foil can be expected to provide 

 useful information for prediction of unsteady cavi- 

 tation. Fully wetted, time dependent, experimental 

 pressure distributions were compared with results 

 from Giesing's method for calculating unsteady 

 potential flow. Good correlation between the 

 prediction and the experimental measurements was 

 obtained for both dynamic pressure amplitudes and 

 phase angles within the range of reduced frequencies 

 investigated (K = 0.23 to 2.30). This good corre- 



