489 



U 1A 

 Hemispherical 



_2rrT 



N 39 (flat nose I 

 Elliptic 3:1 



One of the wings tested has cambered sections 

 and elliptical planform, and the other has symmetric 

 sections and trapezoidal planform, see Figure 35. 



Wing Angle Cavitation Type of 

 (SSPA ident- of number for cavitation 

 ification) attack, a cav inception 



Elliptic, 



cambered 



(16-12.12) 



+2° 

 172° 



2.5 

 3 



sheet 



vortex 

 vortex 



N 3 Iflot nose) 

 Elliptic 6 1 





T 



N 10 Iflot nose) 

 Elliptic 4:1 



^l 



FIGURE 33. Axisymmetric head forms. 



noise and made noise measurements almost impossible 

 at low cavitation numbers. There is also some 

 question whether such background noise from undesired 

 cavitation was obtained at higher cavitation numbers 

 than a = 0.4, when cavitation numbers are increased. 

 With regard to these findings the results given 

 here are limited to cavitation numbers a >. 0.6 and 

 only for decreasing pressure. 



In Figure No. 34 1/3 octave band noise spectra 

 for cavitation numbers a = 1 and a = 0.6 are given. 

 At a = 1.0 no visual cavitation was obtained and 

 the noise levels are almost the same as for the 

 empty tunnel (at the same velocity and cavitation 

 number). At a = 0.6 the cavitation is well developed 

 for the hemispherical nose , for the other head forms 

 no cavitation can be visually observed. There are, 

 however, rather large differences in noise spectra 

 for the three "non-cavi fating" head forms . Thus 

 head forms N3 and NIO have noise levels 10 to 2 

 dB above N39, for which the noise level is equal 

 to non-cavitating or empty tunnel conditions. These 

 differences cannot be attributed to unwanted cavita- 

 tion on the wing or tunnel walls. In that case the 

 noise levels for head form N39 should also have 

 increased. The conclusion is thus that head forms 

 N3 and NIO have audible but not visible cavitation. 



From the tests with axisymmetric head forms it 

 can be concluded that the cavitation numbers will 

 be low, which implies that effects of unwanted 

 cavitation will increase background noise levels 

 and violate results for the cavi fating head forms. 



Tests with Hydrofoils 



In order to obtain cavitation at higher cavitation 

 numbers tests with two wings have been carried out. 

 Using wings, vortex cavitation can also be obtained. 

 The problem is here rather to obtain other types of 

 cavitation without getting vortex cavitation. 



Trapezoidal , 

 symm rounded 

 tip (K7 Vbl*) 



Trapezoidal 

 symm with 

 end plate 

 (K7 Vp3*) 



0.5 



=1.5 



=1.2 



bubble 

 vortex 



sheet 



(*The wing K7 was tested with rounded tip, Vbl, 

 and a small end plate, Vp3, see also Figure 35). 



For the comparison of noise emitted from different 

 types of cavitation it is important that these 

 comparisons be made at the same cavitation number. 

 One inherent difficulty is that pure bubble cavita- 

 tion seems to be possible to obtain only at rather 

 low cavitation numbers compared with the other 

 cavitation types. 



dB re 10^° Pa 

 150 r 



UO 



130 



110 



100 



Cav number 0*= 1 



No cavitation 



40 f UHz) 



dB re 10 Pa 



150 



KG - 



130 



120 



40 f I kHz) 



FIGURE 34. Axisymmetric head forms, cavitation noise 

 (1/3 octave band). (Free stream velocity 9 m/s , 

 gas content 10%.) 



