The Reduction of Skin Friction Drag 



NON-NEWTONIAN (MILLING YELLOW: 1.608% CONCENTRATION) 



■— NEWTONIAN 



e 40 - 



VELOCITY PROFILES 

 Fig. 2 - Velocity profiles 



In our experiments we have made use of the fact that Milling Yellow sus- 

 pensions are doubly refractive when subjected to a shearing motion. Thus, the 

 flow field has been observed using standard birefringence techniques. 



So far we have measured three different quantities as functions of concen- 

 tration of the additive: 



1. "critical" or "neutral stability" speed for the primary (Couette) motion, 

 i.e., the rotation rate at which Taylor cells first appear; 



2. the rotation rate when the Taylor cells first become unstable. This in- 

 stability consists of a sinusoidal deformation of the cells, making the 

 heretofore steady flow time dependent; 



3. cell width of the primary cells, thus obtaining the wave number of the 

 perturbation that is most unstable. 



The results from these measurements appear in Figs. 3, 4 and 5. The crit- 

 ical velocity is given in terms of the Taylor number which is the significant non- 

 dimensional quantity for this problem: 



941 



