High molecular weight polymers, drag reduction

Hydrodynamic drag reduction is an unusual effect involving macromolecules in a turbulent flow for very dilute solutions, the resistance to flow can be reduced compared to that of the pure solvent. Indeed, the presence of high-molecular-weight polymer chains can produce large levels of drag... 

Cowan, M.E., Garner, C., Hester R.D. and McCormick, C.L. (2001) Experimentally determined drag reduction efficiency and extensional viscosity of high molecular weight polymers in dilute aqueous solution. J. App. Poly. Sei., 82, 1222-1231. 

Drag reduction is found generally with dilute solutions of high-molecular-weight linear polymers and is believed to be related to the extension of these flexible molecules at high turbulent shear stress near the wall. The extended molecules increase the local viscosity, which damps the small eddies and leads to increased thickness of the viscous sublayer. With a thicker sublayer at the same... 

The mechanical stability of polymers was related to the polymer s conformation in some of the earlier drag-reduction studies. Above a critical stress, degradation was faster the more contracted and entangled the polymer s conformation (5-7). In petroleum applications the mechanical instability of synthetic relative to carbohydrate polymers was well-recognized. The relative stability problems (possibly related to DUEVs (8)) encountered in the use of high molecular weight hydrolyzed poly(acrylamide) (HPAM) led to the development of an inverse-emulsion polymerization technique (9). (Current research directions using this technique are discussed in Chapter 9.)... 

The polymers which give drag reduction and drag increase have all large unbranched or moderately branched macromolecules with extremely high molecular weights. 

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