474 



Hydrofoil 



Hydrophones 



Hydrophone strut 



FIGURE 5. Experimental set up. 



Hydrofoil 



oscillation does not cause a time variation of the 

 angle of attack that is completely similar to that 

 of a propeller blade in a wake. The reason for 

 using this sytem was that, due to its strength, 

 high oscillation frequencies with large hydrofoils 

 could be obtained. If similarity with propellers 

 is most important it is probably better to use 

 oscillation systems of the types constructed by 

 Ito (1962) and Tanibayashi and Chiba (1977). 



Hydro foi 1 



In these introductory experiments an existing hydro- 

 foil, earlier used for studies in two-dimensional 

 flow, was used. The profile has NACA 16 thickness- 

 distribution and is typical of a relatively thick 

 propeller blade at about 0.7 of propeller radius. 

 The hydrofoil data are 



Mean line a = 0.8 



Camber ratio = fy/c = 0.0144 



Thickness ratio = s/c = 0.0681 



Chord length = c = 120 mm 



Span = 200 mm 



Profile shown in Figure 7 . 



Noise Measuring Equipment 



Two hydrophones (Bruel and Kjaer Type 8103 with 

 frequency response 0.1 Hz - 140 kHz ±2 dB) were 

 placed in notches in a tube supported by two hydro- 

 foils in such a way that photographing of cavitation 

 was permitted (Figure 6). The frequency response 

 of the hydrophones mounted in this manner was 

 checked by white noise. No significant change in 

 the frequency response was detected. 



The hydrophone signals were recorded on FM- 

 channels on a Honeywell 5600-C tape-recorder (0-40 

 kHz at 60 ips tape speed) . Recordings were also 

 made on direct channels (300 Hz - 300 kHz at 60 ips) . 

 It was then possible to write out the complete signal 

 (0-40 kHz) by use of tape speed reduction and UV- 

 recorder. 



Simultaneous with the hydrophone signals, a 



signal showing the events of maximum angle of attack 

 was also recorded. 



High-Speed Film Equipment 



The requirements set up for the filming were that 

 the film had to be synchronous with the noise 

 recordings and permit measurements of cavity size 

 as a function of time. The intention was not to 

 measure the detailed behavior of small or very fast 

 events. The minimum duration of the filming was 

 set to about one second. 



These requirements were met by a Stalex VS IC 

 camera capable of 3,000 frames/s. This is a 16 mm 

 rotating prism camera taking rolls of 30 m film. 

 Lenses with focus lengths of 9.8 and 50 mm were 

 used. For synchronization the camera could release 

 a flash at a preset time. The flash trigging 

 signal was recorded on tape together with hydrophone 

 signals and the flash was placed within the frame. 

 Only one flash was released during each filming. 

 The camera was also equipped with a crystal-controlled 

 time-marker, making one light marking every milli- 

 second on the edge of the film. This, together 

 with the synchronization flash, made it possible 

 to identify and follow cavitation behavior on the 

 film together with the corresponding pressure 



Geometric angle of Qttacl< =OC'«=ao*asin 2TC t fosc 

 FIGURE 7. Oscillating hydrofoil. 



