Sec. 46.5 



DATA ON SEPARATION, EDDYING, AND VORTEXES 



139 



Fig. 46.F Elevation of Ship Model Afterbody in Cikculating-Water Channel with Ink Trail, Tufts on 

 Surface, and Tufts on Pins Projecting from Surface 



observed by eye, or are recorded by high-speed 

 motion photographs. Aggravated air-leakage situ- 

 ations reveal themselves in the channel by air 

 bubbles drawn into the — Ap regions around a 

 model, its propulsion devices, or its appendages. 



The circulating-water channel lends itself to 

 the simultaneous recording of pressures at many 

 small orifices in a model surface, so that pressure 

 contours may be plotted for any given conditions 

 and the areas of — Ap may be definitely traced. 



46.4 Predicting Apparent Flow Deflection 

 Around Separation Zones. The detection meth- 

 ods described in the preceding section are most 

 useful in studying the apparent deflection or 

 diversion of flow around separation zones, dis- 

 cussed in Sec. 7.10 on page 134 of Volume I. 

 Prediction of this deflection is, in the present state 

 of the art, based largely upon a background of 

 experience, built up by watching flow tests, 

 studying and analyzing the photographic records, 

 and thinking about the problem. 



Since pressures from external regions are not, 

 as a rule, transmitted through separation zones 

 to the ship hull, there may be little information 

 of direct value to resistance studies in a knowledge 

 of the potential-flow pattern, with its changing 

 velocities and pressures, outside the separation 

 zones. However, for the prediction of flow into 

 propulsion devices, and for special appendages 

 and attachments to be carried by a ship at some 

 distance from the side, knowledge of these 

 "outside" flow patterns is a must if the special 

 design is to be logical and the service performance 

 is to be predicted. 



For example, in the case of the ten tanker 



models for which resistance and propulsion data 

 were presented by R. B. Couch and M. St. Denis 

 [SNAME, 1948, pp. 360-379], the flow close to the 

 hull and into the propeller disc was in many cases 

 very nearly horizontal, if not actually downward 

 in some regions. Fig. 46. F shows exactly this 

 type of flow, above the ink "trail." The downward 

 flow was unexpected because the general buttock 

 slope in these regions was distinctly upward, at 

 an appreciable angle to the horizontal. The 

 phenomenon may be explained, at least in one 

 way, by the presence of separation zones abaft 

 the DWL and the near-surface WL's, and by 

 downward deflection of the water passing under 

 the stern. Normally, this water keeps on rising, 

 all the way to the stern, but with a separation 

 zone extending nearly to the top of the propeller 

 aperture, as in Fig. 46. F, the under-the-bottom 

 water and the eddying water obviously can not 

 both occupy the same space. 



Another explanation of the downward flow is 

 that it is due to the presence of a large vortex 

 rotating about a longitudinal axis, streaming off 

 the region of the bilge at about the after quarter- 

 point in the manner outlined by the sketch of 

 Fig. 25.F in Sec. 25.6 of Volume I. 



46.5 Estimate of Separation Drag Around a 

 Ship. Sec. 7.8, supplemented by Fig. 7.H on 

 page 130 of Volume I, explains how the specific 

 separation-drag coefficient may be approximated 

 by noting the sensibly constant specific pressure- 

 resistance coefficient derived from model tests at 

 low Froude numbers, below the hmit at which 

 wavemaking resistance manifests itself [Davidson, 

 K. S. M., PNA, 1939, Vol. II, p. 76; SNAME, 



