A Holographic Study of the Influence of 

 Boundary Layer and Surface Characteristics 

 on Incipient and Developed Cavitation on 

 Axisymmetric Bodies 



J. H. J. van der Meulen 

 Netherlands Ship Model Basin 

 Wageningen, The Netherlands 



ABSTRACT 



This paper describes an experimental investigation 

 of boundary layer flow and cavitation phenomena on 

 three axisymmetric bodies . The bodies possess 

 different boundary layer or surface characteristics. 

 The importance of these features for incipient and 

 developed cavitation are studied by using in-line 

 holography. A good correlation is found between 

 observations and calculations of laminar flow 

 separation and siibsequent transition to turbulence 

 of the separated shear layer. The influence of 

 polymer additives on laminar flow separation is 

 studied in detail. The results of this study explain 

 the effect of cavitation suppression by polymer 

 additives on certain bodies. 



1 . INTRODUCTION 



Axisymmetric bodies have often been used to study 

 the inception of cavitation. These studies were 

 usually made by systematically varying the parameters 

 related to the liquid flow (velocity, turbulence, 

 air content, pressure history) or to the body (size, 

 surface roughness, wettability). Although a con- 

 siderable knowledge of cavitation was obtained in 

 this way, a complete understanding of many cavitation 

 phenomena was still lacking. A breakthrough was 

 achieved by Acosta (1974) who emphasized the need 

 for a thorough understanding of the basic fluid 

 mechanics of the liquid flow surrounding the bodies 

 in which cavitation takes place. This statement 

 was based on an earlier study by Arakeri and Acosta 

 (1973) in which the boundary layer flow was visual- 

 ized by the employment of the schlieren method. 

 Cavitation inception could be correlated with the 

 occurrence of laminar flow separation. Unawareness 

 of this important flow phenomenon had obscured the 

 results of comparative cavitation studies with 

 axisymmetric bodies, made in the past. 



In general , it can be stated that cavitation 



inception on a body is affected by nuclei, viscous, 

 and surface effects. The present study deals with 

 the two latter effects. The use of holography, a 

 three-dimensional imaging technique, enabled a new 

 approach. The employment of this method for the 

 observation of cavitation inception phenomena has 

 been reported before by Van der Meulen and Ooster- 

 veld (1974) . In the present study an extended 

 version of the method has been used by which boundary 

 layer flow phenomena also could be observed. Viscous 

 effects were studied by comparing two axisymmetric 

 bodies , a hemispherical nose having laminar flow 

 separation and a blunt nose not having it. Surface 

 effects were studied by comparing two hemispherical 

 noses, one made of stainless steel, the other made 

 of Teflon. 



The phenomenon of turbulent-flow friction reduc- 

 tion by polymer additives of high molecular weight 

 has been known for about thirty years . In recent 

 years an increased interest has been shown on the 

 effect of polymer additives on cavitation. In the 

 present work the influence of polymer additives on 

 the flow about the test bodies is studied and 

 related with the influence on cavitation. 



2. EXPERIMENTAL METHODS AND PROCEDURE 



Description of Test Facility 



The facility used is the high speed recirculating 

 water tunnel of the Netherlands Ship Model Basin. 

 Originally, the maximum speed in the 40 mm circular 

 test section was 65 m/s and the maximum allowable 

 tunnel pressure 35 kg/cm . A detailed description 

 of this tunnel and its air content regulation system 

 is given by Van der Meulen (1971, 1972). For the 

 present study a new test section was made . It has 

 a 50 mm square cross section with rounded corners 

 (radius 10 mm), to limit the influence of the walls. 

 The models, having a diameter of 10 mm, occupy 3.25 

 percent of the cross-sectional area of the test 



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