812 



[SKCT. 6 



From a number of simultaneous records of u and w the stress - puw was 

 determined, the coefficient of correlation between u and w averaging —0.38. 

 In the case of the u fluctuations, the vertical scale appeared to be only about 

 one-third of the scale in the direction of the mean flow. 



Fig. 1 shows the average spectra of u'^, iv^ and uw, computed from six u,w 

 records at a height of 75 cm above the bottom. The u fluctuations are seen to 

 contain considerably more energy at the lower wave numbers than do the w 

 fluctuations, while most of the shearing stress appears to be due to wave 

 numbers k< IO-2 cm-i, with the peak at A: = 6 x IQ-^ cm-i approximately. 



0.3 



0.2- 



0.1 - 



0.1 



0.2 



0.5 



k (IQ-^ cm-') 



20 



Fig, 1. An example of turbulence spectra in a tidal current : from six u,w records, 75 cm 



above the bottom, off Anglesey : 



/2 . 7y,2 . 



A paper by Francis el al. (1953) is noteworthy as representing the first 

 attempt to measure directly the turbulent transports of both momentum and 

 heat. The observations were made at various depths in the Kennebec Estuary, 

 Maine, the horizontal velocity fluctuations being measured by a current meter 

 designed by Von Arx (1950) and the vertical fluctuations obtained from the 

 movements of a vane pivoted on a horizontal axis. The temperature fluctua- 

 tions were measured with a thermistor. The shearing stresses derived were very 

 variable and some values obtained near the surface were as large as those near 

 the bottom. Since the stresses were determined from readings at 3-sec intervals 

 over a record only 2 or 3 min long, it seems likely, in view of later experience, 

 that the sampling variations would be large. 



Four current meters arranged in a frame were used by Hamada and Okubo 

 (1952) to measure the u fluctuations in a tidal current at Suminoe Harbour. 

 The time response of the meters permitted fluctuations of periods down to 6 sec 

 to be recorded. Velocity measurements at a series of depths between the 

 surface and the bottom, in water 10 m deep, were made by Nan'niti (1956) in 



