377 



FIGURE 14. Progressive shed- 

 ding of vapor from sheet cavity 

 trailing edge, K = 0.26, 

 V^ = 11.5 m/s, P_^ = 76.2 kPa, 

 a°°= 3.25° + 1.55°°sin oit. 



FIGURE 15. Cavity surface de- 

 pression producing pressure 

 "SPIKE" Pi gage location, 

 K = 0.36, V^ = 11.5 m/s, 

 P = 76.2 kPa, a = 3.25° + 

 1.55 sin tot. 



vicinity of a pressure gage. Since numerous pressure 

 "spikes" can occur during the life of the sheet 

 cavity it appears improbable that they are due to 

 the interaction of a postulated reentrant jet with 

 the sheet cavity surfaces [Knapp et al. (1970)]. 

 These "spikes" frequently have amplitudes which are 

 comparable to the dynamic pressure and certainly 

 exceed the estimated static pressure in the free 

 shear layer over the pressure gage location. Quite 

 possibly, these pressure "spikes" are due to the 

 free shear layer itself since they only occur when 

 the cavity surface indicates a turbulent shear 

 layer is present. When the reduced frequency is 

 high, for example at K = 1.55, the fully wetted 

 pressure variation leads the foil angle by 68° and 

 then no pressure "spikes" are produced at the pressure 

 gage location as can be seen in Figure 16. At 

 these high reduced frequencies the periodic shedding 



from the sheet cavity trailing edge downstream of 

 the pressure gage is still observed. 



The last aspect of the leading edge sheet cavity 

 instability to be described in this paper is that 

 which will be called cloud cavitation. The three 

 principle features of cloud cavitation for K > 

 are as follows : 



(1) A large surface area of the sheet cavity 

 becomes highly distorted and undergoes a 

 significant increase of overall cavity 

 height in the distorted region, (Figure 17). 



(2) Once this distorted region begins to 

 separate from the main part of the sheet 

 cavity, the upstream portion of the sheet 

 cavity develops a smooth surface and a 

 reduced thickness (Figures 18 and 19) . 



(3) The trailing edge of the smooth surfaced 



