A • TRANSITION FROM LAMINAR TO TURBULENT FLOW 



Moreover actual airplanes have unavoidable local roughness at access 

 doors and elsewhere which generates local turbulence. Turbulence so gen- 

 erated spreads laterally, and this process has been studied by several 

 authors [65,66]. In practical testing, using surface films for detecting 

 transition [67], dust particles produce the typical wedges of turbulence 

 behind them. The observed angle of spread (one half the vertex angle of 

 the wedge) is 8.8°, the various determinations scattering over a range 

 from about 8.5° to 11°. If there are a sufficient number of sources of 

 disturbance and the wing chord is sufficiently large, the turbulence will 

 cover the entire wing span. 



A propeller generates turbulence in its wake and hence transition on 

 that part of the wing lying within the slipstream will occur at a tran- 

 sition Reynolds number corresponding to a stream of large turbulence 

 [68]. A tractor propeller produces a large effect; in the case studied its 

 operation moved transition from midchord to less than 10 per cent of the 

 chord from the leading edge. A pusher propeller was observed to have no 

 measurable effect on transition on the wing ahead of it. Likewise the 

 vibration due to an operating power plant appears to have little effect; 

 wind tunnel measurements for vibrations of frequency of 27 cycles per 

 second and amplitude of 0.1 inches gave no measurable change in the 

 transition point. 



The boundary layer on an airplane in flight is subjected to the noise 

 emanating from its power plant. Wind tunnel measurements in a low 

 turbulence wind tunnel [2] show that noise may affect transition under 

 certain circumstances. 



A,19. Transition on Bodies of Revolution. The simplest body of 

 revolution is a sphere and the effect of the occurrence of transition before 

 laminar separation in greatly reducing the drag coefficient has been known 

 for 37 years. That transition on a sphere is greatly dependent on the tur- 

 bulence of the air stream has been known for the same period and for 

 many years the critical Reynolds number of a sphere was used as a meas- 

 ure of wind tunnel turbulence. The relationship is plotted in Fig. A, 11a. 



For reasons not fully understood the sphere is not a good indicator of 

 turbulence when the turbulence level is less than a few tenths per cent. 

 One hypothesis is that, because of the blunt shape, disturbances are set 

 up at the forward stagnation point which mask the effects of low turbu- 

 lence levels. 



Gortler [69] has suggested that the concavity of the streamlines in the 

 neighborhood of a stagnation point in two-dimensional flow leads to an 

 instability of the type discussed in Art. 4, resulting in vortices with axes 

 along the flow lines. Calculations for the two-dimensional case have been 

 made by Hammerlin [70]. Presumably a similar instability would be found 

 near a stagnation point in three-dimensional flow. 



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