41 



The width of the cavity b is 



fc= j + 2aVTT^ 



U i +VT 



The width of each flat plate is 



U 



i = 4^lJJ^Jl_ ei _ K , 



k* 



These equations fix k, a, a, and b in terms of a and I. 



In the above formulas K and E are the complete elliptic integrals of 

 the first and second kind, respectively, with modulus k, and K' and E' are 

 those with modulus k' = V~l - k 2 . The drag coefficient of a single plate is: 



Zoller gives the following approximate formulas for the Riabouchinsky model 

 (see Wehausen's commentary, Reference 68): 



-Ml 



where 



I 77+ 4 Iff' 4 



a - 4 1 + 3 , 1 -,_ 



<T' = 



2 + <7 



For the reentrant-jet model, Gilbarg and Rock give the following approximation 

 for the cavity length 



f- = 3.5 a- 1 " 85 



Here, the length a is taken to be the distance between the plate and the rear 

 stagnation point. Despite the extreme differences in the models, the lengths 

 and widths based on the definitions given here agree fairly well. 



The usefulness of the two-dimensional drag data for engineering prob- 

 lems has been demonstrated to some extent by the approximation to the three- 

 dimensional case made by Plesset and Shaffer. In connection with cavity width 

 the correspondence between two and three dimensions might be expected to be 

 in the area ratios rather than the width ratios, on the basis that the "wake" 

 is related to the drag. A comparison of the squares of the data of Figure 23 

 with the corresponding data for the two mathematical models is shown in Figure 



