C,15 • THE PROCESS OF DECAY 



can easily be seen that the Reynolds number of turbulence Ry, steadily 

 decreases in the case shown in the figure, contrary to the law of decay 

 for complete similarity. The earher experiments of Batchelor and Town- 

 send [33] also show a definite trend for the decrease of R^. It should be 

 noted that the curves for X^ and yr'^ versus time both have fairly large 

 slopes, and it is therefore more difln.cult to detect any shght deviation 



6 - 



I 

 O 



b 2 



50 



100 



X, cm 



150 



200 



Fig. C,15b. Decay of turbulence behind grids of differing 

 shapes. (After Stewart and Townsend {22\^ 



from a straight fine. On the other hand, R\ should remain constant 

 according to the assumption of complete similarity and is more sensitive 

 for detecting the departure from such laws. The comparison of the inter- 

 cepts of the straight fines in Fig. C, 15b is also a very sensitive method 

 for detecting the same effect. On the basis of the discussions of case (c) 

 of Art. 14, it may be expected that Rx should decrease finearly in f, if 

 u\ > 0, i.e. if the energy in the large eddies is smaller than that corre- 

 sponding to full similarity. The observed decrease of R\ indicates that 

 this is indeed the case. 



( 233 ) 



