C,24 • SOME AERODYNAMIC PROBLEMS 



to moderate and small wavelengths, the former opinion is not yet ruled 

 out. For a more detailed discussion of the arguments for and against the 

 two standpoints, see [88, pp. 93-98]. Work decisively distinguishing be- 

 tween them is clearly needed. In this connection, it would perhaps be 

 worthwhile to obtain some special solutions in magneto-hydrodynamics 

 analogous to those obtained by Taylor and Green [36] in the ordinary 

 case to get an idea of the validity of the existing arguments. 



C,24. Some Aerodynamic Problems. There are a number of aero- 

 dynamical problems associated with turbulent motion in which its dif- 

 fusive nature takes on a secondary role. The random nature of turbulent 

 motion still makes it necessary to use statistical treatments. In this cate- 

 gory of problems, we briefly discuss (1) the dynamical effects of turbulent 

 motion, (2) the effect of contraction on wind tunnel turbulence, and (3) 

 the effect of damping screens. 



Dynamical effects of turbulent motion. Dynamical effects caused by 

 turbulent motion are often treated by statistical methods. For example, 

 in the case of a pendulum suspended in a turbulent wind, the spectrum 

 of the motion of the pendulum can be calculated in terms of that of the 

 turbulent motion and the dynamical characteristics of the pendulum [89]. 

 Recently, Liepmann [90] tried to apply these methods to the buffeting of 

 airplanes moving through a turbulent stream. 



Effect of wind tunnel contraction. The effect of wind tunnel contrac- 

 tion on the intensity of turbulence has been studied by Prandtl and 

 Taylor [91, p. 201]. Recently, Ribner and Tucker [92] apphed Taylor's 

 ideas to the study of the influence of the contraction on the spectrum. 

 The combined effect of damping screens and stream convergence have 

 also been studied by Tucker [93]. The reader is referred to the original 

 papers. An experimental investigation of the detailed behavior of the tur- 

 bulent fluctuations during contraction has been made by Uberoi [94\- 



Effect of damping screens on homogeneous turbulence. Damping screens 

 have long been used for the reduction of turbulence level in the wind 

 tunnel. While these screens no doubt act also as a grid in producing turbu- 

 lence, the scale of such turbulent motion is usually so small that it damps 

 out at a comparatively small distance behind the screen. The resistance 

 of the screen to the flow, on the other hand, tends to reduce the large 

 scale turbulent motion already existing in front of it. 



The characteristics of a damping screen are usually described in terms 

 of two force coefficients Ke and Fe. If the screen is placed with its normal 

 at an angle Q relative to a stream of speed U, there is a drop of pressure 

 across it, given by 



V2-Vi = Ko-kpU' (24-1) 



where pi and p^ are the static pressure upstream and downstream of the 



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