188 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. -16.3 



tlic roviTsed (low uf fully tk>vi'l<>i)eil scpaiatioii, 

 respectively. 



The fish-oye view of Fin. It'U, taken in the 

 TMH eireiilating-water eluiiinel, siiows partially 

 rcversetl tvifts in a separation zone at about the 

 designed waterline (marked by the heavy black 

 stripe) and abreast the propeller. The disturbed 

 under surface of the water in this region also 

 indicates tiie presence of eddying flow in the 

 separation zone. 



Strings or tufts are attached to thin wands like 

 fishing rods, and colored dyes are ejected from 

 long, thin tubes, both moved bj' hand to any 

 position desired around the model. Fig. 46. E 

 pictures the trail of india ink flowing from such 

 a tube (visible at the extreme right). Part of the 

 flow from the orifice position passes outboard of 

 the offset rudder while a small part of it un- 

 e.xpectedlj- swings inboard of the rudder. 



The predominant characteristic of the flow 

 revealed by the ink in Fig. 40.F is its slowness and 

 what might be called it.s uncertainty. The tufts 

 reveal the presence of a very large longitudinal- 



axis vortex, giving tiic How alongside the hull a 

 definite upward component of velocity to the 

 left of the ink tube and a marked downward 

 component to the right of it. The flow picture in 

 Fig. 4G.F is not clear and complete because the 

 forward ends of some of the tufts shown are 

 attached directly to the model and others are 

 fixed to the outer ends of pins projecting from its 

 surface. 



High-speed flash (and motion-picture) photo- 

 graphs, of which Figs. 78. E, 78. F, and those 

 reproduced in TMB Report 810 are further 

 examples, provide a permanent record of the 

 steady continuous flow hoped for over all parts 

 of a ship or of the reversed- and varying-flow 

 characteristics of separation zones. If minute air 

 bubbles are injected into the water, it is possible 

 to detect sw'irling bubbles in a separation zone 

 formed by a projecting strut-arm pad as small 

 as 0.3 inch wide and projecting only about O.OG 

 inch from the fair surface of a model. 



Vortexes which are not steady or stationary 

 are detected by dye injected in the vicinity, are 



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