24 



THE HYSTERESIS REGION IN TRANSITION 

 THROUGH THE AIR -WATER INTERFACE 



Throughout the tests of the TMB planing float, It was noticed that 

 the float exhibited unstable characteristics as it passed from subsurface to 

 surface operation. Here the two conditions are defined in terms of the ap- 

 pearance of the flow about the float. If water continued to flow over the 

 top of the float, the float was considered to be in the subsurface condition; 

 when the flow changed so that spray was thrown forward and to the sides, the 

 float was considered to be in the surface condition. However, unlike the NRL 

 Mark 3 buoy (5)i which has a sharp discontinuity at the intersection of the 

 bottom surfaces and the deck and for which the speed of transition from sub- 

 surface to siu-face operation was sharply defined as based on the above crite- 

 ria,* the speed at which the change occurs for the TMB float is not clearly 

 defined. At the speed at which the TMB float passes through the air-water 

 interface, an unstable condition occurs in which the water is alternately 

 passing over the float and being thrown forward. During this condition, the 

 float was observed at times to yaw sharply to one side. The cause of the yaw 

 is attributable to small deviations from symmetry which produced slightly 

 different flow conditions around the two sides of the float. 



It was further observed, as can be seen in Figure 10, that under 

 some conditions the beginning of the hump in the drag curve has a sharper 

 curvature than under other conditions. Since the transition through the air- 

 water interface begins at approximately this position, it was thought that 

 there may be a hysteresis effect during the transition process. To test this 

 hypothesis, careful experiments were made during which drag measurements and 

 photographs of the flow were taken. 



These experiments were made under loads imposed by the 100-pound 

 weight towed from the forward towpoint. The measurements were made in two 

 ways. To obtain measurements with the flow over the top of the float, the 

 towing carriage was accelerated extremely slowly to the speed desired. If 

 the carriage operator accelerated beyond the originally desired speed, he was 

 instructed not to slow down to this speed, but to hold whatever speed he had 

 attained. To obtain the type of flow in which water was thrown forward and 

 to the sides, the carriage was run up to a speed well beyond the transition 

 region and then decelerated very slowly to the speed desired. The results of 

 these tests clearly showed the existence of the hysteresis region. 



Photographs of the flow about the float obtained at two different 

 speeds in this region are shown in Figure 21 . The resistance of the float in 



* The fact that this transition speed was so sharply defined for the NRL buoy made it possible to de- 

 fine its maximum load-carrying capacity on this basis. 



