Study on the Structure of Ship 

 Vortices Generated by Full Sterns 



Hiraku Tanaka and Takayasu Ueda 

 Ship Research Institute 

 Tokyo, Japan 



ABSTRACT 



Many attempts have been made to measure the vortic- 

 ity distribution of vessels tested at the Ship 

 Research Institute. This led to the successful 

 development of the rotor-type vortexmeter and a 

 method for its calibration. In order to investi- 

 gate the structure of the full ship stern vortices 

 and gain an understanding of interaction of the 

 vortices and propeller, the wake flow behind two 

 geosim models was studied experimentally. 



Using this vortexmeter, detailed diagrams of the 

 vorticity distribution are presented for the dis- 

 cussion of the structure and scale effects on the 

 stern vortices. The authors found the existence 

 of a separating vortex sheet in the vorticity dis- 

 tribution and indicated that, by using the vorticity 

 concentrated on the vortex sheet (Max. line) , it 

 was possible to simulate the original vorticity 

 distribution. With these experimental results the 

 relation between the vorticity distribution and 

 the propeller performance on the geosim models was 

 also analyzed. 



1 . INTRODUCTION 



In recent years, the knowledge of the wake structure 

 including stern vortices has made it essential for 

 the ship builder to obtain a better understanding 

 of the stern vibration with full stern forms. 

 Nevertheless, the stern vortex characteristics such 

 as its geometry and structure as well as the scale 

 effect remained obscure. This situation may be 

 partially due to the fact that the stern vortices 

 do not cause serious problems in the resistance 

 augmentation or in the self-propulsion factors. 

 To overcome this lack of detailed knowledge, 

 systematic investigations have been made concerning 

 the problems of full ship models with unstable 

 propulsive performance. This research was begun in 

 ,1975 under the Research Panel SR 159 of the Ship- 



building Research Association of Japan (Chairman, 

 Prof. H. Sasajima) which was mainly concerned with 

 the following areas: sources of the unstable 

 phenomenon, the unsymmetrical flows accompanying 

 this phenomenon, and the procedure for testing model 

 ships exhibiting this kind of phenomenon. 



Throughout the Panel discussion there was great 

 interest in the behavior of the stern vortices as 

 the basic approach to understanding this phenomenon 

 and this led to the request for quantitative data 

 regarding the stern vortices. The major part of 

 this paper was completed during the course of this 

 Panel's activities in which one of the authors was 

 placed in charge of developing a technique for 

 measuring the fluctuating stern vortices. As a 

 result of the discussions, a rotor- type vortex- 

 meter for obtaining a detailed description of the 

 structure of the stern vortices was adopted. 



Needless to say, by obtaining an illustrative 

 model of the stern vortices it will be possible to 

 develop a mathematical model which will be extremely 

 useful for understanding the flow around the full 

 ship stern. Various vortex models have been sug- 

 gested by Tagori (1966) , Sasajima (1973) , and 

 Hoekstra (1977). The structure of the stern 

 vortices can be roughly described by a stream line 

 which, flowing upward around the bottom of the 

 hull, separates at a separation line formed at the 

 bilge. This flow rolls up at the boundary layer 

 around the bilge forming a separated sheet with 

 vorticity. 



Sasajima has suggested a simplified model of 

 conical separating sheets as shown in Figure 1-1. 

 He assumed that the separating sheet could be 

 described by a triangular plane with which he 

 attempted to explain the basic character of the 

 stern vortices. This vortex model shown in Figure 

 1-1 has a core enclosed with a separating line 

 (S-S'), an attachment line (A-S') and the surface 

 of the separating sheet. In this model it was 

 assumed that the direction, velocity, and vorticity 

 of the flow along this developed vortex sheet would 



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