the problem of water entry,, As the name Implies , the apparatus 

 differed from the majority of the existing launching tanks in 

 that provision was made to control the atmospheric pressure 

 above the water surface „ This control was necessary when work- 

 ing with scale models of projectiles , because simulation of 

 the conditions that exist when full-size projectiles are 

 launched from air into the ocean, requires that the air pres- 

 sure be reduced In the same ratio as the linear scale of the 

 model project lie o The controlled atmosphere launching tank 

 was a late development and was completed only a short time 

 before research under OSRD terminated,, 



Free -Surface Water Tu rmeL 



During the course of the research work numerous problems 

 arose in connection with the underwater travel of projectiles 

 which could not be investigated satisfactorily using the high- 

 speed water tunnel or other equipment available in the lab- 

 oratory o One of the problems concerned the study of the 

 behavior of projectiles 9 such as torpedoes , when they are 

 traveling so close to the surface that the flow of the sur- 

 rounding water loses its symmetry,, Another such problem in- 

 volved the effects of underwater jet propulsion, a study of 

 which necessitated the introduction of comparatively large 

 quantities of gas into the flowing water in the test section c 



A free-surface water tunnel was designed in order to 

 make it possible to study these and similar important problems „ 

 In principle it was quite similar to the high-speed water 

 tunnel but it had a larger working cross-section, 20 inches 

 square as compared to the 14-inch diameter of the high-speed 

 water tunnel „ The upper surface of the stream was in contact 

 with the air, the pressure of which could be controlled. 



Cavit ation Studies , 



More effort was devoted to studies of the various aspects 

 of the cavitation phenomenon than to any other phase of the 

 Hydrodynamics Laboratory's work The reason for this was that 

 basically the effects of cavitation constitute one of the most 

 serious limitations to the satisfactory performance of under- 

 water projectiles c Furthermore, less was known about cavita- 

 tion than about most other hydrodynamic phenomena 



The first general study in this field made by the Labora- 

 tory concerned the production of supersonic noi3e by cavitation 



98 



