472 



5 LVVl 



HI, H2, H3, A, D, E,F,G 

 are pressure transducers 



1/2 



FIGURE 3. Arrangements for noise measurements on complete ship model. (Tunnel No. 2) 



Arrangement 1 is intended to be the standard 

 measurement procedure at SSPA and results are easily 

 compared with full scale measurements using the 

 same equipment. This arrangement gives essentially 

 the near field noise from the propeller. 



If it is of interest to know the radiated noise 

 into the farfield, arrangement 2 can be used. 

 Arrangement 4 also gives the farfield noise, but 

 has its problems, as discussed above. Arrangement 

 2 has less problems with reflected acoustic waves 

 and vibrations than arrangement 4 . The main reason 

 why arrangement 4 is still used is to compare results 

 directly with older measurements. Arrangement 3 

 (Figure 3) has been especially developed for explor- 

 ing the influence of variation in cavitation and 

 the effect on the near field noise. Other arrange- 

 ments of hydrophones have also been used for special 

 purposes. 



Since the main concern in the noise measurements 

 is cavitation noise, the effect of flow noise due 

 to the turbulent boundary is of minor importance . 

 Usually the increase in noise level due to cavita- 

 tion is quite substantial, as can be seen in Figure 

 4, which shows a typical example for a propeller in 

 non-cavitating and cavitating condition. 



2. EXPERIMENTS WITH AN OSCILLATING HYDROFOIL 



Background to Experiments with Oscillating Hydrofoil 



A typical example of the pressure signal from a 

 cavitating propeller model is shown in Figure 5. 

 The pressure was measured by a hydrophone near the 



• Non-cavitoting propeller 

 o A Cavitating propeller 



Sound pressure level re 1 [iPa in 1/3 octave band 

 190 



180 



170 



30 50 100 200 500 Ik 2k 5k 10k 20k iOk 



Hz 



FIGURE 4. Noise measurements on propeller-model. 

 (Tunnel No. 2) 



