A Determination of the Free Air Content 

 and Velocity in Front of the "Sydney-Express" 

 Propeller in Connection with Pressure 

 Fluctuation Measurements 



A. P. Keller 



Technical University Munich 



and 



E. A. Weitendorf 



University of Hamburg, 



Federal Republic of Germany 



ABSTRACT 



The Special Research Pool within the Institut fur 

 Schiffbau and the Hamburg Shipmodel Basin (HSVA) in 

 collaboration with the Technical University Munich 

 and Det norske Veritas executed extensive full-scale 

 measurements on the Single-Screw Container Ship 

 "Sydney-Express." The main task of the project was 

 the determination of the free air content of the 

 seawater in front of the propeller during the voyage 

 from Australia to Europe . 



Simultaneously the velocity was measured at the 

 control point within the Laser-beam, where the free 

 air content was measured by the scattered light 

 technique. Additional investigations were a deter- 

 mination of the water-quality, high speed films and 

 sterophotography of the cavitation at the blade, 

 and pressure fluctuation measurements above the 

 propeller. 



1. 



INTRODUCTION 



For several years the dynamic behaviour of small 

 gas bubbles or nuclei in hydrodynamic pressure 

 fields has been recognized as an important influence 

 on cavitation inception and its extent. Besides 

 other scale effects in the field of model propeller 

 testing, the importance of this influence of nuclei, 

 which also effects propeller excited pressure 

 fluctuation measurements, was often underestimated 

 and neglected. Thus, for instance, the results by 

 van Oossanen and van der Kooy (1973) have shown 

 that for equal non-dimensional flow conditions but 

 different absolute revolutions (i.e. n = 20 and 

 n = 30 Hz) the non-dimensional propeller excited 

 pressure amplitudes were different. After the 

 development by Keller (1973) of a practicable laser- 

 scattered-light (LSL) method for measuring the 

 undissolved air content, systematic cavitation and 

 pressure fluctuation measurements were carried out 



in the medium cavitation tunnel of the Hamburg Ship 

 Model Basin (HSVA) with the model propeller of the 

 "Sydney Express" [Keller and Weitendorf (1975) ] . 

 The results were similar to those by van Oossanen 

 and van der Kooy. Due to the additional application 

 of the (LSL) technique, the differences of the 

 nondimensional pressure amplitudes for different 

 revolutions could be clearly explained by the 

 influence of the free air content or nuclei on the 

 cavitation. A further finding was that the non- 

 dimensional pressure amplitudes and the cavitation 

 for a revolution of n = 15 Hz were increasing with 

 growing free air content, whereas the cavitation 

 and those amplitudes for n = 30 Hz remained more 

 or less constant. The different behaviour for 

 n = 15 Hz and n = 30 Hz were explained by Isay and 

 Lederer (1976, 1977). Using the theory of bubble 

 dynamics they found that the reactions of the 

 bubbles on the respective pressure gradient of the 

 propeller blades at n = 15 or n = 30 Hz were differ- 

 ent. Further, these investigations led to criteria 

 of cavitation similarity of such a kind that the 

 number of nuclei per unit volume of the model flow 

 had to be increased compared with the number of 

 nuclei of the full scale flow. By geosim tests 

 with hydrofoils or propellers it should be determined 

 to what extent these additional criteria for cavi- 

 tation similarity are applicable. 



Keeping in mind these physical connections , the 

 full scale trials on the container ship "Sydney 

 Express" were planned. These investigations were 

 the first attempt to measure the nuclei distribution 

 in seawater around a ship by means of the LSL 

 technique . The nuclei distribution could serve as 

 a basic value for the geosim tests and perhaps as a 

 comparative standard value of the water quality for 

 model cavitation investigations. Furthermore, the 

 experiences, made during the almost adventurous 

 measurements on the "Sydney Express" with the LSL 

 technique in front of a full scale propeller, could 

 be of common interest because the introduction of 



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