TEMPERATURE STRUCTURE OF WAKES 



479 



the hull. The backward momentum of the slipstream 

 is nearly canceled out by the forward momentum 

 of the hull wake, except at surface ship speeds so 

 high that wave resistance becomes the most im- 

 portant retarding force on the ship. In the wake of a 

 submerged submarine this cancellation is exact. 



At moderately close distances astern, probably 

 much less than a ship length, these different streams 

 become intermingled and confused, giving rise to a 

 turbulent mass of water in which velocities in almost 

 any direction are equally likely. Over a small distance 

 called the patch size, the velocity at any one time is 

 reasonably constant, but the velocity at any point 

 fluctuates rapidly. Information on turbulent motion 

 is rather incomplete and no velocity measurements 

 are available in surface ship or submarine wakes. As 

 noted already in Section 27.2, not much is known 

 about the magnitude of the turbulent velocities, the 

 average patch size of the turbulent elements, or the 

 rate at which the turbulence gradually dies away. 



29.2 TEMPERATURE STRUCTURE OF 

 WAKES 



The water temperature at different points in a 

 wake has been the subject of more study than the 

 water velocity. This is partly because small tem- 

 perature differences can be measured much more 

 readily at sea than small fluid velocities. By the use 

 of sensitive thermopiles fastened to a surface vessel, 

 temperature fluctuations as small as 0.01 F may be 

 readily recorded. Data obtained with this technique 

 at the U. S. Navy Radio and Sound Laboratory^ and 

 elsewhere show that the presence or absence of ob- 

 servable temperature structure in wakes depends on 

 the presence of vertical temperature gradients in the 

 sea before the passage of the ship. 



29.2.1 Constant Temperature in 

 Surface Layer 



.When a ship is passing through water all of the 

 same temperature, such as is commonly found in the 

 top 50 ft of the ocean, especially during winter 

 months, no thermal structure in the wake can be 

 observed. Repeated wake crossings under these con- 

 ditions have failed to show any trace of temperature 

 structure. In such isothermal water, temperature 

 structure could be produced only by the heating ac- 

 tion resulting from the passage of the ship. Such 

 heating can readily be shown to be negligible. 



To consider an extreme case, suppose a ship at 

 30 knots is exerting 30,000 hp, and suppose that all 

 this energy goes into heating a wake with a cross sec- 

 tion 20 ft square. The increase of temperature result- 

 ing in this extreme case is 0.015 F. In most practical 

 cases, the temperature change will be very much 

 smaller. Although small patches of water might be 

 appreciably warmed by water discharged from cool- 

 ing systems, by dissipation of energy in intense 

 vortices, or by similar processes, most of the wake 

 behind a ship in isothermal water will have a tem- 

 perature which is practically the same as that of the 

 surrounding ocean. 



29.2.2 Temperature Gradient in 

 Surface Layer 



When a vertical temperature gradient is observed 

 in the top 20 ft of the ocean, the passage of a ship 

 disturbs the temperature structure and gives rise to a 

 measurable temperature structure in the wake. The 

 thermopiles used in research on this subject have had 

 slow response times, requiring 1 or 2 sec for 80 per 

 cent response; since the surface vessels used in the 

 work were under way at 3 knots or more, changes of 

 temperature over regions less than a few feet in length 

 could not be detected. 



The most detailed and quantitative work ^ was 

 carried out with four thermopiles attached to a long 

 pipe mounted vertically on the bow of a small cabin 

 cruiser; the thermopiles were at depths of 4, 6, 8, and 

 10 ft below the surface. In each thermopile, one set of 

 junctions was thermally exposed to the sea water; 

 the other set was thermally insulated and remained 

 at the average temperature of the surroimding water, 

 averaged over a period of minutes. The output of 

 each thermopile was measured with a self-balancing 

 potentiometer; since these instruments required some 

 7 sec to reduce an unbalance to zero, these quanti- 

 tative measurements recorded only the large-scale 

 features of the wake thermal structure. 



Results obtained with this technique are shown in 

 Figure 2, obtained in successive crossings of a 

 destroyer wake 8 and 15 minutes old. Accompanying 

 bathythermograph records are also shown. It is 

 evident that the fresh wake consists of warmer water 

 at the two sides, with cooler water in the middle. This 

 distribution probably results from descending cur- 

 rents at the sides, and rising currents in the center; 

 such currents could be produced by the rotation of 

 the slipstreams from the two propellers. 



