Hoyt and Fabula 



Dr. Hoyt further, if there is any upper limit with respect to the molecular weight 

 of the additives beyond which a film of a polymer solution is no longer obtained? 



DISCUSSION 



Marshall P. Tulin 



Hydronautics, Incorporated 



Laurel, Maryland 



The authors are very much to be congratulated for their fine experimental 

 studies. Their data should tend enormously toward a better understanding of 

 the unexpected and puzzling effects which small concentrations of macro- 

 molecules seem to have on turbulence. 



Our own experiments on a flat plate with leading edge injection confirm that 

 a maximum drag reduction results when as little as 10 parts per million of 

 Polyox WSR 301 is present in the boundary layer at the trailing edge. Unlike 

 the flows in pipes and on rotating plates, however, a rather rapid decrease in 

 effectiveness of the additive occurs when the concentration is increased only 

 slightly beyond its optimum value. Perhaps this has to do with the special cir- 

 cumstances which accompany injection of the fluid containing additive. 



We were curious whether macromolecules would affect "free" decaying 

 turbulence as distinct from maintained turbulence in a shear flow in close prox- 

 imity to a wall. Therefore we have studied the decay of a cylindrical cloud of 

 turbulence. Rather, we have measured the diffusive spread of the cloud. These 

 experiments show that additives do affect free turbulence and tend to increase 

 the rate at which it decays. 



I have been doing some theory on the effect on turbulence of weak solutions 

 of macromolecules. It seems to me that the shear stiffness of the resulting 

 viscoelastic solution is the crucial characteristic and that the generation of 

 elastic shear waves by turbulence offers a mechanism for significant "damping" 

 of turbulent motions. Figure 5 contains very clear evidence that the elastic 

 shear stiffness controls the turbulence damping effect; it may be shown using 

 certain results of the molecular theory for weak polymer solutions that this 

 stiffness is virtually constant on the lines of constant torque ratio in this Figure. 



960 



