A,17 • TRANSITION ON AIRFOILS 



of sides of 2.43 to 1, 2800 for rectangular sections with ratio of sides 

 between 104 and 165, and 2400 for an annular section with ratio of radii 

 of 0.818 to 1, were obtained [49, p. 319]. The disturbances were relatively 

 large in these measurements, so that the values are comparable with 

 1900-2320 for a circular cross section. 



A,16. Transition on an Elliptic Cylinder. Studies of transition on 

 an elliptic cylinder of fineness ratio 2.96 have been made by Schubauer 

 [34] over a Reynolds number range of 21,000 to 160,000, the Reynolds 

 number being based on the minor axis of the elliptical section. The turbu- 

 lence of the stream was relatively high, from 0.85 to 4.0 per cent. At a 

 turbulence level of 0.85 per cent, and Reynolds numbers from 21,000 to 

 30,000, the boundary layer flow is laminar until separation occurs, and 

 transition occurs in the shear layer so far downstream that the flow around 

 the cylinder is not affected by it. From the observed pressure distribu- 

 tions, transition at higher Reynolds numbers occurs in the free layer and 

 the boundary layer reattaches. The pressure distribution, position of sepa- 

 ration, and reattachment change with Reynolds number until a value of 

 about 120,000 is reached. For still higher Reynolds numbers the flow and 

 pressure distribution were independent of Reynolds number up to the 

 maximum value reached of about 160,000. Here transition occurred soon 

 after separation, as described in Art. 8. 



The air stream turbulence was increased by the use of turbulence- 

 producing screens and the location of transition was measured as a func- 

 tion of the intensity and scale of the turbulence. The results have already 

 been given in Fig. A, lib for comparison with Taylor's theory. 



Unfortunately the gap between a turbulence of 0.85 per cent and the 

 lower end of the curve in Fig. A, lib was not covered, but the curve must 

 turn sharply to the right at a smaller value of the turbulence parameter 

 to reach values of x/D of about 2.7 as experimentally observed at a 

 turbulence of 0.85 per cent. The relatively stationary position of tran- 

 sition at x/D of about 1.53 corresponds approximately to the point of 

 minimum pressure where the pressure gradient changes from favorable 

 to adverse. 



A,17. Transition on Airfoils. The measurements of transition on 

 airfoils as reported in the literature constitute a record of the develop- 

 ment of wind tunnels of lower and lower turbulence and of improved 

 techniques of producing smooth surfaces free from waviness. At first no 

 detailed boundary layer measurements were made and the only data 

 given were drag curves vs. Reynolds number based on the air stream 

 velocity and airfoil chord. The critical Reynolds number of the airfoil 

 was taken as that at which the minimum drag coefficient was reached 

 or at which a perceptible rise occurred. Later the location of transition 



(41 > 



