Cup Anemometer 



DETERMINATION OF V 



The influence of bearing friction on the operation of the anemometer is embodied in the 

 term V^ which may be regarded as the true wind velocity at which the cups turn so slowly 

 (o— »zero) that the relative velocity can simply be taken as ¥$. _ 



v is, however, still assumed to be finite so that the mean normal force coefficient C N on the 

 cups can be found by integration over 180°. 



Thus 



/■ * Force normal to cups face 



C^da. Cj? = 

 o A 1 



A = area of cup face 

 q = dynamic pressure = \pV' 1 . 



Brevoort and Joyner 5 have shown curves for C\ at various values of a trom o to tt and at various 

 Reynolds' Numbers. If C N is evaluated by mechanical integration from these curves (Fig. 6 

 of Ref. 5) it is found that 



C~n = A + BR 



(where A and B are constants and R = Reynolds' Number) represents the variation of C N with 

 R very well up to 7?=32Xio 4 (cf. Fig. 2.) 



20 30 



Fig. 2 



40 



