Bernd 



considerably after approximately a 1000:1 contraction in the surface area of the 

 nuclei, as measured by the change in tensile strength, ultimately reaching a 

 maximum tensile strength of about 45 to 60 psi, at which dissolving ceased. 

 This gives one an idea of the maximum tensile strength likely to be achieved by 

 these waters in practice, since the attendant tensile strength gain due to the 

 rise of nuclei to the surface is little. 



No dissolving, typified by an extremely slow tensile strength gain. This 

 occurred as a consequence of natural films that had been produced by algae 

 allowed to grow in tap water (4). 



These tests were done in still water; turbulent flow should considerably 

 increase the rate of dissolving. 



Nuclei Dissolving in Water from a Water Tunnel 

 (David Taylor Model Basin*) 



Now, having an appreciation of the effects of surface films, it was desirable 

 to determine the behavior of water taken from a cavitation test facility. This 

 was important to evaluate the first part of the following argument: if fast dis- 

 solving waters exist, and if in addition small nuclei are produced during cavi- 

 tation, then tensile strength might be likely to occur in the test section of a 

 water tunnel, provided sufficient time occurred during transit of the water in 

 the tunnel to appreciably dissolve nuclei. 



Water Tunnels - The 36-in.-tunnelt water, the 24-in. -tunnel water, and 

 drinking water from the David Taylor Model Basin were tested, and were found 

 to be identical in surface film behavior (Fig. 6). There was a rapid, high gain 

 of tensile strength after cavitation, much higher in fact than any natural, "proto- 

 type" water that had been tested. This is directly opposite to what one would 

 prefer to see: little or no tensile strength gain. The rate of dissolving was 

 similar to distilled water (Fig. 7). 



The water supply for the David Taylor Model Basin is principally Potomac 

 River water that has been treated. Chlorination is one of the treatments used. 

 The implication is that chlorination has destroyed the organic contaminants that 

 would normally be responsible for surface film formation. The tunnels are 

 closed systems using well water for makeup, so that there is little likelihood of 

 subsequent contamination. 



A peculiarity of the test results is that the tensile strength versus time data 

 randomly fell on two curves. It was as if two types of surface films were capa- 

 ble of being formed, but the start of one — a random event — excluded the other. J 

 Both of the curves obtained represent rapid dissolving. 



♦After March 31, 1967, renamed Naval Ship Research and Development Center. 

 tThroat size of nozzle in test section. See Ref. 10 for a description of the David 



Taylor Model Basin tunnels and the towing basins. 

 tThis effect would not be noticeable at a low tensile strength since many nuclei 



are present. Water possessing appreciable tensile strength generally has only 



a few nuclei of a given strength in the volume of water (approximately 1 cc) 



stressed to rupture. 



92 



