480 



VELOCITY AND TEMPERATURE STRUCTURE 



'I 



w 



o 



I- 10 



UJ 

 LJ 



1^20 



z 



130 



UJ 



□ 40 

 50 



BT AT 1216 



WAKE AGE 15 MINUTES 

 LAUNCH ENTERING FROM EAST 







>- 10 



UJ 

 tiJ 



z 

 f 30 



CL 

 UJ 



60 



-, 10 »- 

 HSECT 



EUAPSH) TIME OF CROSSING 



BT AT 1316 



EaSTSlDE 

 OP WAKE 



WEST SIDE 

 OF WAKE 



Figure 2. Horizontal temperature structure of a de- 

 stroyer wake. 



The thermal structure found for other types of ship 

 wakes is sometimes considerably different from that 

 shown in Figure 2, with single peaks sometimes re- 

 placing the double peaks. In general, however, when- 

 ever the thermopiles were at the depth of a marked 

 negative gradient — 0.5 degree in 10 ft — as shown 

 on a bathythermograph record outside the wake, the 

 wake near the surface was colder than the surround- 

 ing water at the same depth. When the gradient is 

 marked no such general rule may be made. It is 

 interesting to note, however, that thermal wake 

 signals have been readily detected when the gradient 

 outside the wake was almost too weak to be noticed 

 on a bathythermograph record — about 0.2 degree 

 in 20 ft. 



Measurements have also been made on the thermal 

 properties of the wake behind a submarine at peri- 

 scope depth, with a moderate negative gradient pres- 

 ent in the surface layer. It was found that effects ap- 

 peared even at the surface, where the water behind 

 the submarine was found to be a few tenths of a de- 

 gree cooler than the surrounding water outside the 



wake. The reason for this rise- of the submarine's 

 thermal wake to the surface is not known. 



The persistence of these thermal effects is some- 

 times quite marked. Identifiable thermal signals have 

 been obtained in crossing wakes an hour or more 

 after these were laid. Not all wakes exhibit identi- 

 fiable thermal effects for such a long period, even if 

 the gradient is marked. The limiting factors are the 

 decay of the thermal structure of the wake and the 

 background of thermal irregularities present outside 

 the wake. It is sometimes difficult to distinguish the 

 thermal change found in crossing a wake from those 

 frequently found in sailing through wake-free water. 

 The thermal irregularities in wake-free water also 

 tend to increase with increasing temperature gradi- 

 ents; thus a very strong gradient is not necessarily 

 the best for detecting a wake by its thermal proper- 

 ties. 



As shown in the next section, the acoustic effect of 

 thermal structure is greatest for temperature irregu- 

 larities whose size is about equal to the wavelength 

 of the sound being transmitted through the water. 

 Thus, to compute the scattering of supersonic sound 

 at 24 kc, information on the variation of temperature 

 over regions about 3 in. long would be required. No 

 such information is available, owing to the long time 

 constants of the measuring methods discussed above. 

 Temperature fluctuations over such small regions 

 might be expected in a relatively fresh wake. How- 

 ever, it would be surprising to find such a small-scale 

 temperature structure in a wake more than a few 

 minutes old. 



29.3 SCATTERING BY TEMPERATURE 

 AND VELOCITY STRUCTURES 



Any region in which the velocity of sound varies 

 with position will affect a sound wave passing through 

 it. For example, theory predicts appreciable reflection 

 from a surface separating two large bodies of water 

 differing considerably in temperature.^ If variations 

 of the microstructure of the ocean take place over 

 distances not too great compared with the wave- 

 length, an appreciable amount of sound will be scat- 

 tered in various directions. Although the exact anal- 

 ysis of these effects is complicated, certain results 

 may be derived relatively simply. These results, given 

 below, are sufficient to indicate the general magnitude 

 of the scattering of sound by the temperature and 

 velocity structure of wakes. 



Suppose that in some region S the velocity of sound 



