Uq J (u^-u 



D.o„ = - P U„ (u,-u) dS (75) 



and hence, from the asymptotic form of (73) , 



"0/ ^ 



Dgco = - 4TTP Uq j M dT (76) 



BLW 



Comparison with the asymptotic form of (72) now yields 



dT = (77) 



J "^"i 



BLW 



From the asymptotic form of (73) , we can also show that 



/ 



4iT y dx = Q (78) 



BLW 



which implies, by (74) and (76), that 



D300 = - ° ^^^^ 



This is derived by neglecting u in comparison with u in (6) , since the 



Betz sources are concentrated near the body, so that u^ diminishes as the 



inverse square of the distance, while, for a turbulent wake, |ul diminishes 



as the inverse 2/3-power of the distance. 



Put V + ¥' = ■V„TtT ^^^ define a mean value u of u^ by 

 BLW 1 i 



u^ y dT = y u^ 



dT (80) 



J. 



30 



