10 



SOME EXAMPLES OP THE EFFECTS OF CAVITATION 



The effects of cavitation on efficiency of hydraulic machines and 

 the damage associated with cavitation are well known. It will be sufficient 

 here to give only an example or two of these effects. The loss of efficiency 

 in pumps and turbines with cavitation onset is discussed in many publications 

 (see e.g., References l6 and 17)- Cavitation damage on turbine runners and 

 pump impellers, on baffle piers in the stilling basins below dams, on the face 

 of dams, and on large needle-valve installations are described in Reference 7. 

 Illustrations of the effects of cavitation on the efficiency of marine pro- 

 pellers will be found in References 18, 19, and 20. An example of data taken 

 with a propeller model during a characterization test in a TMB water tunnel 

 is shown in Figure 9» in which the thrust coefficient K™ and torque coeffi- 

 cient Kq are plotted as functions of 

 an "advance" coefficient J with the 

 cavitation number a as parameter. 

 Here, 



T 



*T 



pn 2 d- 



K A = 



pn 2 d- 



JL 

 nd 



w 1 



where T is thrust , 

 Q is torque , 

 n is rotation rate, 

 d is propeller diameter, and 

 p is vapor pressure. 



The cavitation number, which is a si- 

 militude parameter, will be discussed 

 somewhat further in connection with 

 modelling of cavitating systems. The 

 magnitude of the cavitation number is 



an indication of the degree of cavitation or of the tendency to cavitate. 



Thus, the lower the cavitation number, in general, the greater the volume of 



cavitation. It is clear from Figure 9, then, that the greater the "amount" 



of cavitation, the greater the loss in thrust and torque. 



Figure 9 - Propeller Thrust and Torque 



as Functions of Speed Coefficient with 



Cavitation Number as Parameter 



