376 



FIGURE 12. Sample oscillograph record for 

 the variation in foil surface pressure with 

 foil angle at K = 0. 26. V^ = 11. 5 m/s , 

 P = 76.2 kPa, a = 3.25° + 1.55 sin bit. 



PRESSURE GAGE P, 



PRESSURE GAGE P, 



PRESSURE GAGE P, 



FOIL ANGLE 



FOIL ANGLE MAX. 

 CAMERA PULSE TRACE 



millisecond duration, until the trailing edge of 

 the cavity recedes past the P^ gage (point D) . 



The absolute magnitude of the cavity pressure 

 could not be accurately determined from these 

 experiments since the in situ pressure gages were 

 not calibrated for the condition of a gas/liquid 

 interface at the entrance to Helmholtz-type chamber 

 over each gage. As shown in Figure 12, point B, 

 the growing cavity does not appear to produce large 

 foil surface pressure fluctuations at its downstream 

 edge. However, when the cavity recedes, (ie., point 

 D) then the foil surface pressure fluctuations can 

 be comparable to the magnitude of the' dynamic 

 pressure. 



Based on photographic records it appears that 

 when the cavity is expanding, its trailing edge is 

 disturbed as one would expect if the shear layer 

 were unstable at that location. Beginning at the 

 cavity trailing edge and then moving forward, the 

 cavity surface becomes highly disturbed, irregular, 

 and bubbles are introduced into the shear layer, 

 just as one would expect when transition in the 

 shear layer moves forward. The cavity pressure, 

 as measured by the gages Pj, P2 and Pj, remains 

 constant throughout this change in the surface of 

 the cavity. 



During the early stages of sheet cavity growth, 

 when only the cavity trailing edge appears disturbed, 

 small regions of bubbles are shed from the sheet 

 cavity trailing edge. This shedding process becomes 

 more accentuated as the sheet cavity length increases 

 and more of its surface becomes disturbed. High 

 speed movies taken at 9,300 frames per second 

 clearly show the highly turbulent characteristics 

 originating at the trailing edge of the sheet 

 cavity and progressively moving upstream. 



The sequence of vapor shedding from the cavity 

 trailing edge, as determined by high speed movies 

 taken at 9,300 frames per second, is as in the 

 sketches of Figure 13. The photographs of Figure 

 14 demonstrate a phase in the vapor bubble shedding 

 process from the sheet cavity as sketched in 13c 

 with two regions of shed vapor downstream. It 

 should be noted that since the foil, surface is 

 very smooth, a reflection of the shed vapor is 

 seen in the side views. Therefore a dashed line 



has been added to Figure 14 to indicate the 

 separation of the vapor and its reflected image. 

 This shedding process is periodic and for the 

 example shown in Figure 14 the shedding frequency 

 at a given spanwise location is nominally 700 hertz. 

 The view shown in Figure 14 covers nominally the 

 center third of the foil span. Visual observations 

 with strobescopic lighting indicate that the leading 

 edge sheet cavitation, for nonzero values of K, 

 typically consists of a series of 3 dimensional 

 cavities across the span. 



In Figure 15 the top view shows a depression in 

 the cavity surface (a) just above Pj and a rise in 

 cavity height (b) just downstream of the depression. 

 at this instant a pressure "spike" is detected by 

 Pj (see for example C in Figure 13) . This condition 

 precedes the shedding of a small region of vapor 

 bubbles upstream of the sheet cavity trailing edge 

 and significantly deforms the cavity trailing edge 

 shape. It is the forerunner of the condition that 

 will be referred to in this paper as "cloud" cavita- 

 tion. It is interesting to note that after 

 correlation- of over 600 photographs of the leading 

 edge sheet cavitation with the pressure gage signal, 

 the pressure "spike" always occurs when a depression 

 in the cavity surface exists over the pressure gage. 

 The converse, however, was not observed, ie., the 

 "spike" can occur when no depression was discernable 

 in the photographs. These "spikes" can occur without 

 any significant gross change in the observed 

 character of the sheet cavity surface in the general 



CAVITY 



(a) 



(c) 



FIGURE 13. Sequence of vapor shedding from the cavity 

 trailing edge. 



