The Reduction of Skin Friction Drag 



Finally, it should be mentioned that, even if the boundary layer can be sta- 

 bilized in the absence of large disturbances, the wake cannot. The turbulent 

 wake is known (Townsend (1956)) to be subject to large-scale, unsteady organized 

 motions of the character of instabilities, which on a sphere interact strongly 

 with the boundary layer and are responsible for the wandering of a rising free 

 balloon of small size (Scoggins -private communication). It is possible that 

 these motions, which are present in the wake of a streamlined body also, can 

 disturb the supposedly stabilized boundary layer there. 



Change of Profile 



Of the various stabilization techniques* (see Fig. 1), the first method we will 

 discuss is the alteration of the velocity profile to a more stable one. Roughly 

 speaking, the stability of a profile is increased by an increase of the curvature 

 of the profile, since the lower critical Reynolds number above which small dis- 

 turbances will grow is monotonic with curvature of the profile at the critical 

 layer. The critical layer is that layer at which the wave velocity and fluid ve- 

 locities are equal (Lin (1955)). A more exact way of correlating this change in 

 profile is throi^h a shape parameter. 



DRAG REDUCTION 

 IN LIQUIDS 



TO THE TURBULENT LAYER 



STABILIZATION OF 

 LAMINAR LAYER TO 

 SMALL DISTURBANCES 



COMPLIANT 

 BOUNDARY 



VISCO-ELASTIC 



NON-NEWTONIAN 



ADDITIVE 



INCREASING 



CURVATURE OF 



PROFILE 



CONSTANT 



FLUID 



PROPERTIES 



VARIABLE 



FLUID 



PROPERTIES 



PRESSURE 

 GRADIENT 



HEATING 

 WALL 



DISTRIBUTED 



NON- 



NEWTONIAN 



ADDITIVE 



WALL LAYER 

 OF 



LOW^ FLUID 



FILM 

 BOILING 



SUBLIMATION 



CHEMICAL 

 REACTION 



Fig. 1 



Techniques for the stabilization of the laminar layer 

 to small disturbances 



='-Specific citations will not in general be given; reference should be made to 

 the appropriate section of the bibliography. 



917 



