A, S. Iberall 



>-^ 



(S>0 

 UJ I 



zo 



»- 



li-^lO-'^ 

 Oz 



^^ 10" 



U.Q: 



UJ 10"'* 

 Q. 



-80 ^ ^ ^ . 



10° lO' 10* lo' 10* lo' 10° 

 FREQUENCY - HZ 



Fig. 2 - Frequency spectrum 

 for velocity fluctuations par- ^ • 

 allel to mean flow 



JT__ _h_ 



0. 14 cm (= 0.06 in. ) 



A query by private communication to Laufer ("In your 1951-2 parallel plate 

 work, your spectral data (at R = 30,800) show significant observations at f = 

 3,000 Hz. In terms of the plate separation of 5 in., what is the size or scale of 

 'instantaneous' waves or eddies associated with this frequency level?") elicited 

 the response that the eddy size corresponding to 300 Hz was about 0.1 in. We 

 cannot help but feel that this minimal propagative "wavelength" (which appears 

 in all field components including its vorticity) and the minimal eddy size that 

 Laufer identifies by instrument inspection, are related, i.e., the field has a 

 limited graininess. We propose that it has a corollary limiting frequency 

 response. 



(ii) We may also attempt an approximate estimate of the low-frequency cut- 

 off for this experiment. 



when 



One may note that downstream propagation may vanish (e.g., § = 0, oj = oj^) 



726 



