ABSTRACT 



It has recently been shown that a jump in velocity potential 

 exists between infinite upstream and downstream directions when a 

 body translates unformly along a channel of finite cross section 

 such as a towing tank or wind tunnel. In this report a new block- 

 age correction formula for body speed is proposed. The speed 

 correction formula due to blockage is obtained by dividing com- 

 puted potential jump by body length, assuming that the body is 

 slender or flat in the direction of motion. The potential jump 

 is expressed explicitly in terms of the effective volume, i.e., 

 the sum of the displaced volume and added mass/density of the 

 submerged body, and the depth Froude number, if a free surface 

 is present. As a test of the present speed correction formula, 

 two cases are considered: (1) the Wigley parabolic ship model, 

 tested in both a small and a large towing tank, (2) a body of 

 revolution (prolate spheroid) tested in a circular wind tunnel. 

 In each case the mean-speed increment averaged over the entire 

 body surface is computed by a three-dimensional, finite-element 

 method applicable to free-surface flow problems. These are shown 

 to be in good agreement with those obtained by the approximate 

 speed correction formula. At high values of Froude numbers, the 

 main difference in the total resistance coefficients measured in 

 the two towing tanks by Tamura is due primarily to difference in 

 model wave resistance computed for the two tanks by a full- 

 fledged, three-dimensional, finite-element method. Results are 

 also compared to those obtained by using the speed correction 

 formula of Lock and Johansen. The present formula renders a 

 better approximation than that of Lock and Johansen when the 

 cross sectional area of a flow tunnel is not much larger than 

 the maximum cross section area of the body. 



ADMINISTRATIVE INFORMATION 

 This work was authorized and funded by the Independent Research Pro- 

 gram at the David W. Taylor Naval Ship Research and Development Center, 

 Task Area ZR Oil 0201, Element 61152N. 



INTRODUCTION 

 Many authors have investigated blockage effect and proposed approxi- 

 mate blockage formulas to account for towing tank or wind tunnel bound- 



1-9* 

 aries. The first approximation concerning towing tank blockage effects 



date back more than four decades. Owing to the difficulty encountered in 



computing flow separation, wake flow, free-surface effects, etc. , the exact 



*A complete listing of references is given on page 21, 



1 



