High-Speed Water Tunnel 



There were several reasons for the choice of water as the 

 fluid medium for the experimental measurements of the dynamic 

 forces on projectiles „ In the first place, at the time of the 

 initiation of the work, the projectiles under consideration 

 were principally those having only underwater paths In the 

 second place , it was known that th 3 phenomenon of cavitation 

 was involved frequently in the underwater behavior of projec- 

 tiles,, Since cavitation is a phenomenon that is peculiar to 

 motion in a liquid^ there was nc satisfactory way of studying 

 it in anything other than a water tunnel „ In the third place, 

 the equipment available as a nucleus for the work was adapt- 

 able to use with water and not with air » 



Finally j, it can be shown that over a wide range of con- 

 ditions the results obtained by a study of projectiles in a 

 water tunnel are applicable to the projectile whether in air 

 travel or in water travels The only serious limitation is that 

 when the results are applied to air travel they must be re- 

 stricted to cases in which the projectile velocities are suf- 

 ficiently low so that the air can be treated as an incompressible 

 fluido This means in general that the results obtained in the 

 water tunnel can be applied with acceptable accuracy to the air 

 flight of projectiles having velocities cf less than 700 feet 

 per second. 



The high-speed water tunnel was a vertical, closed-circuit 

 tunnel in which water was circulated continuously by means of 

 a propeller pump driven by a variable speed motor „ The working 

 section., which was 14 inches in diameter and six feet long, was 

 located in the upper horizontal run of the circuits It had the 

 smallest cross-section and consequently the highest velocity 

 and the lowest pressure in the circuits The velocity in this 

 section could be maintained at any desired value up to 70 feet 

 per secondo The models of the projectiles or other devices to 

 be tested were mounted on the spindle cf a three-component 

 balance of the National Physical Laboratory type. 



Measurements of the drag force, the cross force, and the 

 moment could be made with a projectile aligned parallel to the 

 direction of flow or rotated in the horizontal plane to any 

 desired yaw angle between /20 degrees of the direction of the 

 flow. The absolute pressure in the working section was con- 

 trollable independent of the velocity and could be held at 

 any desired value from five atmospheres down to the vapor 

 pressure of the water The working section was provided with 



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