Johns son and Srfntvedt 



fluctuations in non-ca vita ting flow is demonstrated in connection with 

 the results in cavitating flow, see section 5. 6. 



Cavitating Flow : 



V. 4. Measurement of Pressure Fluctuations in Cavitation Tunnel . 

 Variation of Test Parameters, Wall Effects 



Apart from the wake distribution the two most important test 

 parameters in the cavitation tunnel are water speed and air content. 

 Both parameters are known to affect the extent of the cavitation. With 

 regard to the air content a /a s it is plausible that it might affect the 

 damping of the water and thereby the level of the pressure fluctuations 

 registered by the transducers. 



In order to investigate this influence, four different combina- 

 tions of water speed and air content (4 and 5 m/sec, <*/ct = 0. 13 and 

 0.40) were investigated for one propeller - hull configuration (six- 

 bladed propeller, model without fins). From the results, some of 

 which are given in Figure 13, it can be concluded that the influence of 

 these parameters on the amplitudes was considerable for the filtered 

 signals but rather small for the maximum values of the non-filtered 

 signals. 



A problem, which should not be neglected in connection with 

 this kind of measurements, is that of wall effect. This has been dis- 

 cussed by Huse in Q 0J , from whom we quote : 



"The pressure signal recorded by the pressure transducers 

 may be split into two parts, 



1) the "direct pressure wave" induced by propeller and 

 cavity, propagating directly to the field point (pressure 

 transducer), and 



2) the reflected pressure wave induced by propeller and ca- 

 vity, propagating to the field point by one or more reflections 

 at the tunnel walls. " 



On the basis of some approximate calculations Huse concludes 

 that for the combination of test section and propeller size used at his 

 experiments 



"wall effects are of minor importance in the case of a non- 

 cavitating propeller and also in the case of cavities of constant 



594 



