The Reduction of Skin Friction Drag 

 REFERENCES 



1. "The Reduction of Skin Friction Drag" 



2. "The Effect of Additives on Fluid Friction" 



3. "An Experimental Investigation of the Effect of Additives Injected into the 

 Boundary Layer of Underwater Bodies" 



4. A. C. Eringen, Nonlinear Theory of Continuous Media , McGraw-Hill, New 

 York, 1962. 



5. "Simple Micro -fluids," International Journal of Engineering Science , Vol. 2, 

 No. 2, 1964. 



6. A. C. Eringen, "Mechanics of Micromorphic Materials," ONR Technical Re- 

 port No. 26, April 1964, presented at the Xlth International Congress of Ap- 

 plied Mechanics, Munich, Germany, and scheduled for publication in the 

 Proceedings. 



DISCUSSION 



Alan Kistler 



Yale University 



New Haven, Connecticut 



Professor Lumley has given an apt summary of the various proposals for 

 reducing the skin friction on objects moving through a liquid. Since the motiva- 

 tion for studying these methods is to find a way to reduce the total drag of an 

 object, a few words about the rest of the drag problem for a submerged object 

 might be appropriate. The neglected component (pressure drag) is associated 

 with separation of the boundary layer. A technique that either increases or de- 

 creases the friction drag could have the opposite effect on the pressure drag. 

 The change of sphere drag with transition is the best known example. All of the 

 suggestions for affecting the friction could affect the separation either by chang- 

 ing the rate of momentum transport across the free shear layer or by changing 

 the location of the separation point. Sufficiently detailed measurements of the 

 pressure distribution about realistic shapes should be taken in order to evaluate 

 and understand what is occurring when a particular drag reduction technique is 

 being tested. 



Aeronautical experience has shown that most drag reduction schemes that 

 depend on the delay of transition, with the possible exception of boundary layer 

 suction, do not work well outside of the wind tunnel. Surface roughness, wake 

 interaction, and cross flow all work against laminar flow. For this reason, it 



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