Johns son and Stfntvedt 



DISCUSSION 



Erling Huse 



Ship Research Institute of Norway 

 Trondheim 3 Norway 



First of all I would like to congratulate the authors on a very- 

 interesting and valuable contribution to our knowledge of cavitation as 

 a source to ship vibration. This is, I think, an aspect of vibration ex- 

 citation which deserves considerable attention. The work presented 

 by the authors is therefore of great value in our efforts towards better 

 methods of predicting and reducing propeller induced excitation forces. 



In connection with the oscillating pressure measurements in 

 the cavitation tunnel I would like to comment on the magnitude of the 

 wall effect mentioned by the authors. At the Ship Research Institute 

 of Norway we have recently determined experimentally this wall effect 

 in our two cavitation tunnels. I would like to show you how we do it 

 and some results. 



Let us first recall that the pressure field induced by the cavi- 

 ties on the propeller blades is mainly the sum of two components. 

 First we have the pressure field due to the cavity motion. This is in 

 principle a "dipole field" whose amplitude decays rapidly with increas- 

 ing distance from the propeller. Therefore one may expect the wall 

 effect for this pressure field to be relatively small. This we have also 

 confirmed experimentally. The experimental procedure is first to 

 measure in the cavitation tunnel the pressure amplitude on the hull 

 model surface with the propeller running with no cavitation and at 

 zero thrust. (The pressure field is in this case mainly due to blade 

 thickness and thus of the same type as that of cavity motion). The 

 same measurement is next carried out in the towing basin where there 

 are no tunnel walls in the vicinity of the propeller. The difference in 

 pressure amplitude measured on the hull in the cavitation tunnel and 

 in the towing basin is due to reflections from the tunnel walls and thus 

 represents the wall effect. 



The second and most important contribution to the total pres- 

 sure field is that due to the volume variation of the cavities. This 

 effect produces a pressure field which is in principle a "pole field". 

 Since its amplitude decays relatively slowly with increasing distance 

 it will be subject to a more pronounced wall effect that the pressure 



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