519 



noise measurements made at the University of Tokyo's 

 and the Ship Research Institute's (SRI) cavitation 

 tunnel. 



Figure 1 shows the measurement apparatus. The 

 hydrophone was set in a 50 mm acrylic cup mounted 

 on the tunnel's observation window and filled with 

 water. The measurements were made in uniform flow 

 at constant speed with the section pressure lowered, 

 using propellers and foils. The propeller was SRI 

 No. 121 (D = 250 mm, z = 6, area ratio = 0.8, 

 constant P/D = 0.75). The foil (1/4 SRI Foil) was 

 a scaled version of Dr. Ukon's (SRI) design using 

 NACA 4412 wing section and a planform of c(n) = 

 Cg(l-ri^)''. The 1/4 SRI Foil had an aspect ratio 

 of 3, semi-span = 50 mm, and base chord, c. = 40 mm. 



The measurements are briefly illustrated in 

 Figures 2, 3, and 4. In Figure 2 the noise spec- 

 trum and envelope of tip vortex cavitation noise 

 is shown for SRI and Tokyo University tests. The 

 intermittant tip vortex noise appears as spikes in 

 the spectrum between 2 and 6.3 kHz, as denoted by 

 "2" in this figure. Using the complete test 

 record it is possible to construct the envelope 

 shown in Figure 2D. The shifts in the frequency 



appear to be a function of both the low pressure 

 vortex core and the condition of the water. 

 In an attempt to gain an understanding of the noise 

 mechanism, additional experiments were performed. 

 In Figure 3, the intermittant tip vortex noise 

 signal at 6.3 kHz was used to trigger the camera 

 shutter to photograph the intermittant tip vortex 

 cavitation. It appeared that the noise mechanism 

 is due to the pressure wave caused by the filling 

 of the low pressure vortex core by dissolved gases. 



To test this hypothesis of the tip vortex cav- 

 itation noise mechanism, air was injected from the 

 1/4 SRI Foil tip and the noise spectrum measured. 

 Figure 4 shows the results of the initial tests 

 illustrating a qualitative agreement in the actual 

 tip vortex cavitation noise spectrum and the sim- 

 ulated tip vortex using air injection. At the time 

 of writing, it has been possible to improve this 

 technique and duplicate the intermittant "spikes" 

 in the noise spectrum. 



Thus by the experimental results a basis for 

 understanding the low frequency aspects of tip vor- 

 tex cavitation noise has become possible. 



10 dB 



TRIGGER SIGNAL : 6,3 KHz BAND CENTER FREQUENCY 



• • • 



10 SEC. 



1/4 SRI FOIL, 10 M/S, 10°, (5y = 3,35 

 AIR CONTENT: 257., 1,9 PPM 



• PHOTO 



FIGURE 3. Intermittent tip vortex cavitation 

 noise signal and photo. 



10 



30 



20 



10 







0,5 1 2 5 10 KHz 

 1/3 OCTAVE BAND CENTER FREQUENCY 



KEY: 



1 0^= 5'8 



NO CAVITATION 



2 (5v= 3,8 



AIR INJECTION 

 FROM FOIL TIP 



3 6^ 3.0 



STEADY TIP 

 VORTEX 

 CAVITATION 



1/4 SRI FOIL 

 V = 12 M/S, 10° 



AIR CONTENT: 

 24 %, 2 PPM 



FIGURE 4. Comparison of tip vortex cavitation noise 

 spectrum trace and simulated tip vortex using air 

 injection. 



