378 



PRESSURE GAGE P, 



PRESSURE GAGE P, 



PRESSURE GAGE P, 



FIGURE 16. Sample oscillo- 

 graph record for the variation 

 in foil surface pressure with 

 foil angle at K = 1.65, 

 V = 11.5 m/s, P = 76.2 kPa, 



a 



3.25 + 1.55 sin ost. 



FOIL ANGLE 



■ FOIL ANGLE MAX. 



CAMERA PULSE TRACE 



FIGURE 17. Initial stage in 

 the process of cloud cavitation 

 formation, K = 0.51, V^ = 14.8 

 m/s, P^ = 124.1 kPa, a"= 3.25 

 + 0.95 sin at. 



SIDE VIEW 



region then moves downstream, becomes 

 unstable at its trailing edge, and quickly 

 develops the characteristic appearance of 

 the leading edge sheet cavity elsewhere 

 along the span (see feature a in Figure 20) ; 

 or, the trailing edge of the smooth portion 

 of the sheet cavity moves upstream to the 

 foil leading edge and the cavity disappears 

 (Figure 21) . In Figure 21a dye trace 

 injected at the foil leading edge can be seen. 



When the foil is stationary (K=0) cloud cavita- 

 tion shedding can be very periodic as can be seen 

 in Figure 22 which shows the oscillograph trace of 

 the pressure gage response. The frequency of 

 shedding for the condition illustrated in Figure 



22 is 42 Hz based on the response of the pressure 

 gage Pi. Figure 23 shows a photograph of the type 

 of cavitation that produced the time pressure history 

 of Figure 22. In Figure 23, (a) is a cloud just in 

 the process of being shed, (b) is a cloud previously 

 shed at a nearby spanwise location, and (c) is a 

 cloud shed earlier at the same location as (a) . 

 The cavities did not shed in the manner of the two- 

 dimensional separation which typically occurs in 

 sharp leading edge foils [Song (1969) ,Besch (1969) , 

 Wade and Acosta (1965)]. Instead, cavity shedding 

 was highly three-dimensional and more or less 

 independent' of the sheet cavity instability occur- 

 ring several cavity lengths away along the foil 

 span. However, it appears that for the trailing 

 edge shedding and the cloud cavitation (at least 



