341 



TABLE 340B.— FORCES ON NONROTATINQ CIRCULAR CYLINDERS (FIG. 7) 



(concluded) 



The variation of Cn with aspect ratio for Reynolds number of 80,000 is as follows. 



If the axis of the cylinder is inclined to the wind direction, the force remains approxi- 

 mately at right angles to the axis of the cylinder, its magnitude falling off approximately 

 as the square of the sine of the angle of the axis to the wind. 



TABLE 340C— FORCES ON SPHERES (FIGS. 8-10) 133 

 For spheres, the linear dimension / is taken as the diameter of the sphere d and the area 



A as —7-- For values of Reynolds number between 



),000 and 400,000 at low values of 



Mach number the value of the drag coefficient Ci> depends in large measure on the tur- 

 bulence of the air stream. As the Reynolds number is increased in this range the drag 

 coefficient of the sphere and the pressure coefficient at the rear of the sphere decrease- 

 rapidly. The pressure coefficient is equal to the ratio of the difference between free stream 

 stagnation pressure and local static pressure to the dynamic pressure q. The Reynolds 

 number at which the pressure coefficient at the rear of the sphere is 1.22 is defined as the 

 critical Reynolds number, Rcr. This value of pressure coefficient corresponds very nearly 

 to Cd = .3. The value of Rcr represented by point d in the figure is considered to be 

 typical of turbulence-free air. 



2.0 



1.5 



.5 



10 I0 2 I0 3 I0 4 I0 5 I0 6 



REYNOLDS NUMBER, R 



Fig. 8. — The drag coefficient Cd on spheres as a function of the Reynolds number. 



Drag, D = CoAq R = — £ 

 Sphere tests in wind tunnels indicate different values of Rcr for different sphere sizes. 

 Correlation of the data may be obtained if values of —p- (y ) = (K) are plotted as a 



function of R CT . The value Vit 2 is the root-mean-square of the fluctuation velocity in the 

 direction of the relative wind, T the velocity of the relative wind, d the sphere diameter, 

 and L is the scale of the turbulence as defined in the reference. The figure shows a cor- 

 relation (K) obtained with two sizes of spheres and several values of L. 



133 Allen, H. S., The motion of a sphere in a viscous fluid, Phil. Mag., vol. 50, p. 323, 1900. 

 Wieselberger, C, Further information on the laws of fluid resistance, NACA TN No. 121, December 

 1922. Millikan, C. B., and Klein, A. L., The effect of turbulence. Aircraft Eng., vol. 5, p. 169, 1933. 



Piatt. Robert C, Turbulence factors of NACA wind tunnels as determined by sphere tests, NACA Rep. 

 No. 558, 1936. Dryden, Hugh L., Schubauer, G. B., Mock, W. C, Jr., and Skramstad, H. K., 



Measurements of intensity and scale of wind-tunnel turbulence and their relation to the critical Rey- 

 nolds number of spheres, NACA Rep. No. 581, 1937. Ferri, Antonio, The influence of Reynolds 

 numbers at high Mach numbers, Atti di Guidonia, n. 67/69, Mar. 10, 1942. 



SMITHSONIAN PHYSICAL TABLES 



(continued) 



