Fabula 



The absence of non-Newtonian, viscoelastic effects is attributed to the 

 largest viscoelastic relaxation times (according to molecular theory) 

 being necessarily much smaller than the smallest inertial spectral 

 transfer time scales of the turbulence. The raggedness phenonnenon 

 prevented operation under conditions for which a significant viscoelas- 

 tic effect on the energy spectrum would have been likely. 



INTRODUCTION 



The mechanism of high-polymer-additive friction reduction in turbulent 

 pipe and boundary-layer liquid flows is currently of great research interest. It 

 is clear that turbulence measurements in the shear flows with friction reduction 

 are of interest. However, in such work it would be desirable to be able to re- 

 solve the smallest turbulence scales which are known to be important. This is 

 difficult to do in liquid-flow facilities of ordinary size with available turbulence 

 sensors. Thus other types of turbulent flow were also considered in planning 

 this investigation. 



In choosing the type of flow for study, the following requirements were con- 

 sidered essential: 



1. The experiment should contribute to eventual understanding of the 

 friction-reduction phenomenon in wall -turbulence shear flows. 



2. The available turbulence sensors for use in liquids should have adequate 

 spatial resolution and frequency response for the flow to be studied. 



3. The experiment should be free as far as possible from bias due to ef- 

 fects of degradation of solution properties by mechanical cleavage of polymer 

 chains. 



With these considerations in mind, it was decided to study a turbulent flow 

 without mean shear, specifically a grid-turbulence flow, which offers a useful 

 approximation to isotropic turbulence. This hypothetical type of turbulence is 

 more amenable to theoretical treatment and still is of practical value in vinder- 

 standing actual turbulence, in which the fine -scale structure is known to be 

 nearly isotropic. 



Requirements 2 and 3 led to the selection of a towing-tank experiment in 

 which the scale of the turbulence could be made large enough relative to the 

 available sensors, and in which the test fluid is sheared only by the moving test 

 grid. If the same experiment were performed in a water tunnel, the test fluid 

 would also be sheared by the pump and in the tunnel -wall boundary layers. 



Many investigators of the friction-reduction phenomenon have suggested 

 that the mechanism of friction reduction involves the viscoelastic properties of 

 the dilute solutions. The specific aim of this work was to determine the differ- 

 ences, between solution and solvent, in the longitudinal one -dimensional grid- 

 turbulence energy spectra, and to relate the differences to the viscoelastic 

 properties of the solutions. A concurrent theoretical investigation was made by 



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