NO. 4 WIND TUNNEL EXPERIMENTS IN AERODYNAMICS 21 



not exist there would be no eddies of finite size, no dissipation, and 

 at a constant speed no resistance. It means, rather, that the drag on 

 the body by the fluid is due to the continual drain of energy needed 

 to set up anew the turbulent eddying motion about the body ; and 

 that when these eddies have once been created it makes no difference 

 how fast they are dissipated by viscosity after the body has left them 

 behind. 



The Critical Speed 

 In the foregoing case of resistance proportional to S", the plot of 

 '~ri2C2 ^^ ordinate against " as abscissa gives, of course, a hori- 

 zontal straight line for bodies of a given series. But if the experi- 

 ments are carried down to smaller and smaller values of , a critical 



I' 



value may be reached where the relation ceases to hold and the 

 character of the fluid motion changes very rapidly, though apparently 

 not discontinuously, so that the function ip ceases to be a constant for 



low values of . For a given bodv in a given medium this critical 



V 



DS 

 value of — corresponds to a critical speed Sc which may be com- 

 j' 



puted a priori from the values of D and v, if the critical value ( — ) 



V " /c 



has once been determined for bodies of the given shape by varying 

 an)'^ one of the variables D, S, and v, or all together. Eiffel's observa- 

 tions on spheres ^ confirm the foregoing statements when we take into 

 consideration not merely a single speed for each diameter but the 

 whole critical range within which the rapid change in the form of if/ 

 occurs. 



Mr. Hunsaker's observations on sharp cornered disks of different 

 diameters, but the same thickness, are very interesting as showing the 

 possible importance of such sharp edges or corners. The disks were 

 not geometrically similar ; but the corners at the edges were not only 

 nearly similar but, inch for inch, very nearly identical. Accordingly, 

 that part of the total resistance which may be regarded as due to the 

 sharp corners at the edge reached its critical value always at about 

 the same speed, irrespective of the diameter of the disk which was 

 bounded by the edge. The rapid change in the total resistance near 

 this speed seems to indicate that the " corner resistance " formed a 

 considerable fraction of the whole. 



^ C. R. 155, p. 1597, December 30, 1912. 



