Pvopellev Excitation and Response of 230000 TDW Tankers 



field due to cavity motion. The main problem in measuring wall effect 

 in this case is to design a transmitter to produce the required oscil- 

 lating pressure pole field. Figure 2 shows the principle of the pres- 

 sure wave transmitter that we have developed in Trondheim. A signal 

 generator feeds an AC current to the coil winding. This makes the 

 rubber membranes oscillate, producing an effective volume variation. 

 The amplitude of the volume variation itself is measured by measur- 

 ing the pressure fluctuation in the closed air volume inside the trans- 

 mitter by means of a pressure transducer. 



Now we first fit the afterbody model in the cavitation tunnel 

 with this pressure wave transmitter instead of the propeller as shown 

 in Figure 1. We adjust the volume variation to a certain level and 

 measure the pressure amplitude on the hull. This procedure is then 

 repeated in the towing basin with exactly the same volume variation. 

 The difference in pressure amplitude on the hull in the two cases re- 

 presents the wall effect. 



In our larger tunnel of 1200 mm diameter test section we have 

 in this way determined the wall effect for one particular afterbody 

 model with a 240 mm diameter propeller. On the hull directly above 

 the propeller the wall effect was found to be less than measurement 

 accuracy, i. e. less than 10 percent, in a frequency range up to 300 

 cps. Hence this tunnel seems to be acceptable for pressure measure- 

 ments on the afterbody model. In our second and much smaller cavi- 

 tation tunnel, however, where we have tried to measure pressure 

 fluctuations on a plate above the propeller, we have found the wall ef- 

 fect to be unacceptably high, amounting to a factor of up to 2. 



From investigations in our own tunnels, and by considering 

 the test section dimensions of the SSPA tunnel, I feel confident that 

 the wall effect has been negligible in the case of the tunnel measure- 

 ments described by the authors. 



Finally I have a direct question regarding the damping of 

 the acceleration potential mentioned by the authors. "I find it 

 hard to understand what physical phenomenon the authors are referr- 

 ing to, and I find it even harder to understand how they have been able 

 to make corrections for it. Could the authors please tell us the magni- 

 tude of the damping coefficient they have applied in their calculations, 

 and also explain the theoretical basis of this correction. 



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