Johnsson and S^ntvedt 



the test section and the recess are filled completely with water. Up to 

 now flat plates have been used and no attempts have been made to si- 

 mulate the wave system around the hull. The maximum water speed 

 is 6.8 m/sec. An electric motor and a strain gauge dynamometer for 

 measuring thrust and torque for the propeller are placed in a water- 

 tight cylinder in the model. 



Measurement of Pressure Fluctuations on the Hull 



For the measurements of the pressure fluctuations differential 

 transducers were used, being of the strain gauge type, manufactured 

 by Statham. The maximum range for the transducers, used at the 

 tests in the cavitation tunnel is - 25 psi, the natural frequency being 

 about 9 kHz. The diameter of the membrane is l/4" (6. 35 mm). One 

 end of the transducer was connected to the atmospheric pressure. 



The signals were amplified and registered on an oscillographic 

 recorder, two channels being used for each transducer. One channel 

 was used for the original signals, on the other a filtered signal was 

 registered, the filter being tuned on the blade frequency. The natural 

 frequency of the galvanometers was 1 650 Hz (original signals) and 

 400 Hz (filtered signals). For obtaining higher harmonics a frequency 

 analyser (manufacturer Brtlel and Kjaer, type 2107) was used. The 

 range 63-2 000 Hz was used, the total sweeping time being 6 min 

 for this range. The band width is about 6% of the frequency registered. 



No accelerometer was fitted to the model during the tests as, 

 at earlier measurements of a similar kind, carried out at SSPA, only 

 low levels of the accelerometer signals were obtained. 



V. 2. Test Conditions in the Cavitation Tunnel 



The first condition to be fulfilled in order to obtain reasonable 

 results with regard to cavitation patterns, erosion patterns and pres- 

 sure fluctuations is to accomplish a realistic wake distribution behind 

 the ship model in the cavitation tunnel. This problem was thoroughly 

 discussed inQlJand it will only be stated here that, if the wake dis- 

 tribution in the towing tank is used as that to be aimed at, the agree- 

 ment obtained in this case was very good, in particular in fully load- 

 ed condition, as is evident from Figure 11. Thus it has been demons- 

 trated at these tests that a representative wake distribution can be 

 realised without incorporating a free water surface. The comparison 

 was made by using ordinary Prandtl -tubes. Recently, when using 

 five -hole spherical pitot tubes on another, similar project, the same 

 degree of agreement was obtained also for the tangential velocities 

 and flow angles. 



592 



