Grid Turbulence in Dilute High-Polymer Solutions 



in vertical streamers, and the planes will tend to rotate first clockwise and then 

 counterclockwise in the tube. Another more unexpected observation, resulting 

 from the fact that our optical system was similar to that for a Tyndall beam 

 (which is very sensitive to small suspended material), was that very small col- 

 loidal suspended material was separated into regions of high and low concentra- 

 tion on passing through the fluidized bed. That is, as tracers they did not truly 

 follow the streamlines. This observation has led to the thought that inherent in 

 the absence of density differences larger particles of micron or submicron size 

 simply cannot follow the consorted movement of molecules precisely, and that 

 furthermore if such larger entities are present in appreciable concentration 

 they would tend to stabilize flow patterns such as eddies or vortices. Even in 

 the absence of elastic behaviour larger entities must move as a unit from, for 

 example, one eddy to a new eddy, while with eddies in collision, the fluid itself 

 can gradually be sheared off into, for example, a new smaller eddy. A macro- 

 molecule or larger entity would tend to be carried with the fluid unit in which 

 most of its mass resides, and any remnant in an opposing flow would be pulled 

 at least to the interface between the two. The shearing down of an eddy in size 

 would become more difficult, although loss of momentum would not necessarily 

 be prevented. 



With regard to the raggedness. Prof. Metzner in the previous paper has al- 

 ready suggested that the probe will have a thickened stabilized boundary layer 

 at the leading edge as well as further along the probe due to the viscoelastic be- 

 haviour of the fluid and that this layer is sensitive to factors other than strictly 

 the fluid velocity. It is also known for Newtonian fluids that a towed grid such 

 as used in this study will create not only general turbulence but also pronounced 

 vortices near the grid bars and particularly the crossings. These normally lose 

 their identity or die out a short distance behind the grid, but in a solution of 

 macromolecules these vortices might retain their identity for a considerably 

 longer distance behind the grid. The periodic interaction of these vortices with 

 the probe boundary layer would contribute to the raggedness observed. 



REPLY TO DISCUSSION 



A. G. Fabula 



Dr. Gadd's suggestion that the hypothesized macromolecular entanglements 

 might be absent in very weak concentrations seems reasonable in view of his 

 experimental evidence. In the grid turbulence tests of Polyox WSR-301 (pre- 

 sumably the same type as in the experiments reported by Dr. Gadd) the mini- 

 mum concentration was 18 ppm. Only much higher concentrations were of major 

 interest, so that the critical concentration range of about 10 and 20 ppm sug- 

 gested by Dr. Gadd's evidence was not checked in this work. It is not clear, 

 however, that only concentration determines whether or not entanglements occur. 

 It seems notable that 10 to 20 ppm is very much lower than the concentration for 



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