Drag reduction, elastic theory

However, Sreenivasan and White (89) point out that the connection between fluctuating strain rates and large extensional viscosity is circumstantial. Further polymer coils can only be partially stretched in a random field of strain rate. Sreenivasan and White (89) point out that the elastic theory proposed by de Gennes (84) is compatible with at least two experimental observations ie, the dependence of drag reduction onset on polymer concentration and maximum drag reduction asymptote. 

The various theories explaining drag reduction may be divided into three categories first, an explanation in which the increase in extensional viscosity for the polymer solution is the main ingredient second, a theory that stresses the importance of anisotropic effects introduced by the extended polymer molecules and third, a proposed explanation in which elastic effects are responsible for drag reduction. 

Sreenivasan and White (89) recently invoked the theory based on the elastic behavior of stretched polymers of de Gennes (84), who states that coil-stretch transition does not occur in turbulent flows in randomly fluctuating strain rates and that, if moderately stretched, the polymers produce no measurable change in viscosity. Sreenivasan and White (89) were able to explain the onset of drag reduction and maximum drag reduction asymptote on the basis of the elastic theory, and their results qualitatively explain the existing experiments. 

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