Grid Turbulence in Dilute High-Polymer Solutions 



and since concurrent degradation of elastic properties can be expected to occur 

 (18), an increase in Uq/M is undesirable. If Uq/M is kept fixed, an increase in e 

 by a factor of 30 requires increases in u^ and M by a factor of n/^. This is 

 completely impracticable in the present towing tank. Thus it is clear that Uq/M 

 must be increased to increase e , and therefore the signal raggedness problem 

 must be solved before the desired near match of characteristic times can be 

 achieved in the grid-turbulence experiment. 



Solution inhomogeneity, perhaps with polymer impurity as a contributing 

 factor, would seem to be the most likely cause of signal raggedness. For exam- 

 ple, if the solution ahead of the grid contains large clusters of macromolecules, 

 then a simple mechanism for the production of signal raggedness can be hypoth- 

 esized. During the short period of high shear rate which the fluid undergoes in 

 the grid-bar boundary layers, these clusters might be stretched out into long 

 "fibers." Such fibers may be expected to have a stiffness of form and a long 

 relaxation time, because a high degree of alignment has been given to the poly- 

 mer chains in the cluster and some solvent has been forced out of the interior 

 of the cluster. The difference between signal raggedness and solution noise 

 may be presumed to represent the greater disturbance to the shear flow over 

 the hot film by the passage of "fibers" as compared with the passage of soft 

 clusters. 



Another mechanism by which the bars of the grid might produce raggedness 

 is by accumulating smaller clusters on their stagnation lines. Presumably the 

 rate of accumulation would be balanced by a washoff of temporary agglomerates 

 of a much larger size than the constituent clusters, so that the sensor might de- 

 tect only the agglomerates. 



None of these hypotheses could be tested in this work. By working with a 

 smaller volume of liquid in a small flow facility or in a miniature tank, solution 

 purification techniques such as filtering and centrifugation could be used to see 

 if signal raggedness could be eliminated in that way. Also solutions with con- 

 trolled inhomogeneity could be tested to determine the dependence of signal 

 raggedness on existing clusters, and optical means might be used to determine 

 if washoff of coating material is involved. 



There is very little other work on hot-film flowmeter measurements with 

 which this work can be compared. However several concurrent investigations 

 should answer the natural question of whether or not there is any connection be- 

 tween the signal-raggedness phenomenon found in this grid-turbulence study and 

 the friction-reduction phenomenon in pipe and boundary -layer flows. 



ACKNOWLEDGMENT 



The author thanks his thesis advisor. Professor John L. Lumley, for com- 

 prehensive guidance during this work (1). This research was supported by the 

 Bureau of Naval Weapons as a part of a graduate study program in Hydrodynam- 

 ics of Submerged Bodies. The work of Professor Lumley was supported in part 

 by ONR. 



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