A,17 • TRANSITION ON AIRFOILS 



The effect of airfoil shape may be seen by comparing the curves for 

 the 0012 airfoil [58] and that for the 65215-114 low drag airfoil, although 

 data at the same Reynolds number are not available. The results for the 

 0012 airfoil [58] in the NACA 8-ft wind tunnel as compared with the 

 results on the same airfoil in the NACA low turbulence wind tunnel show 

 the effect of an increase in wind tunnel turbulence from a few hundredths 

 to several tenths per cent on a conventional airfoil. 



From the totality of information available on airfoils and other bodies, 

 we may reconstruct the qualitative picture of the influence of Reynolds 

 number on the location of transition. At extremely low Reynolds num- 

 bers the boundary layer separates from the surface as a laminar layer 

 and the separated shear layer remains laminar far downstream. As the 

 Reynolds number increases, transition occurs in the shear layer nearer 

 and nearer the point of separation. At some Reynolds number the flow 

 reattaches to form a separation bubble which decreases in size as the 

 Reynolds number increases. Over a certain range of Reynolds numbers, 

 transition remains fixed just beyond laminar separation. With further in- 

 crease in Reynolds number, transition moves forward, more rapidly while 

 in the region of adverse pressure gradient and more slowly as the pressure 

 minimum is passed to reach the region of favorable pressure gradient. At 

 very large Reynolds numbers, transition approaches closer and closer to 

 the forward stagnation point. Most of the available data are for the 

 Reynolds number range in which the transition lies between the laminar 

 separation point and the leading edge. 



When the airfoil is placed at a different angle of attack the pressure 

 distribution changes and aerodynamically we have to do with a different 

 body. A low drag airfoil experiences adverse pressure gradients at suf- 

 ficiently high angles of attack and leaves the low drag region. In general 

 terms transition moves forward on the upper surface and backward on 

 the lower surface as the angle of attack is increased. Typical experimental 

 data are found in the references previously cited. 



Fig. A, 17b shows the data plotted in Fig. A,17a in a slightly different 

 form, the ordinate now being {xt/c)Re which is a rough approximation to 

 the equivalent flat plate Reynolds number at transition. Exact data from 

 boundary layer computations are shown for the 652X5-114 airfoil for com- 

 parison; in this case the approximate values are too high by from 2 to 

 14 per cent. For most of the data plotted the equivalent flat plate Reyn- 

 olds number of transition increases with the airfoil Reynolds number. 

 It is believed that this increase is associated with the increased stability 

 of the boundary layer at the more forward positions of transition corre- 

 sponding to the higher airfoil Reynolds numbers, where the pressure 

 gradient is increasingly more favorable. 



The available wind tunnel data on roughness effects [13,15,63,63] are 

 plotted in Fig. A, 17c as transition Reynolds numbers based on airfoil 



<43 ) 



