FfG 3 



is obtained in the free streamline, in effect a method of analytical continuation of the 

 flow. Apart from the insight-giving character of this method, one of the integrations 

 is already performed by geometrical considerations, thus greatly simplifying the tech- 

 nique. The method also affords a direct geometrical interpretation of the drag coeffi- 

 cient. 



To give the simplest possible illustration, it is a provable theorem that the image 

 of flow in an angle is flow outside an equal angle, Fig. 4. Thus for a jet running along 

 a wall ABC we reduce the problem to flow in a channel formed by the original wall 

 and a parallel wall A'co B' Coo got by translation along the bisector of the angle Aoo B 

 Coo. 



A 



: - : .: : . : ."-- ; : j channel"'' •v : - : ".'-' 



FIG 4 



7 



