B • TURBULENT FLOW 



turbulent boundary layer. Ashkenas and Riddell have noted this dis- 

 agreement and have pointed out possible causes of error in the experi- 

 ments of Young and Booth. 



Even without putting this case to actual test, it may be seen that 

 the independence principle would not be expected to apply in turbulent 

 flow. Let us imagine a wind tunnel experiment in which we have a flat 

 belt passing through slots in the tunnel walls and running diametrically 

 across the stream with the stream crossing it edgewise. If the boundary 

 layers on the two sides of the belt are laminar, running the belt has no 

 effect on the boundary layer associated with the action of the stream, 

 unless of course the belt is running so fast that heating effects change 

 the viscosity and density of the air. If, on the other hand, the boundary 

 layers are turbulent, then running the belt increases the turbulence be- 

 cause of the greater velocity relative to the surface. The eddy viscosity 

 is thereby increased, and this increase affects all motions. To the flow 

 component normal to the leading edge, the boundary layer now exhib- 

 its greater eddy viscosity. The friction to air flowing over the belt is 

 thereby increased and the thickness of the boundary layer is increased 

 correspondingly. 



CHAPTER 5. FREE TURBULENT FLOWS 



B,26. Types and General Features. The term "free turbulent 

 flows" refers to flows which are free of confining walls and exist in shear 

 motion relative to a surrounding fluid with which they mix freely. The 

 flows of common technical interest are jets, wakes, and mixing zones 

 between two uniform streams moving with different relative velocity. 

 Problems of technical interest are the rate of spreading with distance 

 from a source of the flow, velocity distributions, and the manner in which 

 other transported quantities such as heat and matter are distributed and 

 mixed with a surrounding medium. 



A characteristic common to this class of flows is a lack of viscous con- 

 straints on the mean motion in all parts of the field when the Reynolds 

 number is sufficiently high. This condition is practically always fulfilled 

 unless the Reynolds number is so low that the turbulent regime cannot 

 exist at all. In the case of mixing zones, jets, and two-dimensional wakes 

 this condition never degenerates; for no matter how feeble the relative mo- 

 tion may become with increasing distance from the source, the Reynolds 

 number either remains constant or increases due to the increase in size. 

 More specifically the Reynolds number increases with distance for mix- 

 ing zones and two-dimensional jets, and remains constant for axially 

 symmetric jets and two-dimensional wakes. The axially symmetric wake 

 is the one exception, for here the rate of decay of mean motion (and 



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