Cup Anemometer 



DISCUSSION OF THE PARAMETERS INFLUENCING h AND k 



It is immediately apparent from figures 3 and 4 that the parameter d/D, which was chosen 

 first of all because it seemed the most likely, is not the only controlling parameter but that both 

 in the values of A and k there enters an independent "scale effect. " 



The scale effect on the factor k is easily accounted for by a combination of the separate effect 

 of Vtj and the inherent turbulence of the air stream in which the anemometers were tested. Thus 

 the effect of F would be described in the form of a Reynolds Number dV^/v while the turbulence 

 effect not accounted for by this Reynolds number could be expressed as the ratio \/d, where X is 

 some length introduced to characterize the magnitude of the inherent air stream turbulence. 



Table II 

 Harrington's Tests at Various Frictional Values 

 Instrument Constants ^=.115; £ = .254; d/D^.^y, Beaded semiconical Cups 



F — X io-" h k 



■~1~ - 2I 6 3 . 16 1.535 



.904 .72 3.15 0.465 



1. 1 2 .89 3. 11 0.625 



Thus k is a function of the three parameters d/D, dV^/v and \/d. The variation of k with 

 dFn/v for d/Da.nd \/d constant is indicated by Harrington's tests 6 (Table II) on the same anemom- 



Table III 

 Tests of Bureau of Standards with Different Turbulence Values 



d d/D F h k Turbulence % 



eter at various frictional torques, k appears to decrease with increase of the Reynolds Number. 

 Also for d/D and dV^/v constant, tests made at the Bureau of Standards (Table III) indicate 

 that k also decreases with increase of inherent air stream turbulence.* The tests available for 

 the discussion of k are, however, insufficient for the separation of these two distinct scale effects. 

 The factor h may now be considered, and has been replotted in Fig. 5 against D alone. From 

 this figure it is apparent that h is independent of the dissimilarity ratio d/D but in general, has 

 only a pure scale effect. 



* In their study on "The Effect of Turbulence," 7 Millikan and Klein conclude that the lowest possible 

 tunnel turbulence corresponds most nearly to conditions in the free atmosphere. 



