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Since the discovery of strongly reduced pressure drop in pipes by Mysels and Toms and due to a significant practical benefit in many applications, the problem of drag reduction in turbulent flows with additives has been studied extensively. For instance, the review of Nadolink and Haigh (1995) listed above 2500 entries. Since then the number of contribution keeps growing continuously with a couple of noticeable trend changes such as the discovery of coherent structures or direct numerical simulations with viscoelasticity entering the range of Reynolds numbers relevant for drag reduction. More recent review of White and Mungal (2008) summarized the progress in the field of drag reduction.
We want to address only certain aspects of turbulent flows which are strongly modified by very dilute solutions of flexible polymers. Unfortunately, in spite of extensive research, the physical mechanisms underlying the effect of dilute polymers on turbulence remain poorly understood. Most of our knowledge is based on the Eulerian data and inertial range effects, on the level of turbulent velocity fluctuations and the mean flow properties. There is a consensus (with some exceptions) that the direct action of the polymers is on the small scales of turbulent flows. There is still an ongoing dispute on whether these are time or length scales, or whether the effects are due to extensional viscosity or elasticity. As our main interest is the understanding the turbulent flow per se, we would like to see the dilute polymers as active Lagrangian material “sensors” in a turbulent bulk. We devise physical and numerical experiments in which a) the effects at large and small scales could be separated, b) the effects associated with the solid-liquid boundaries could be minimized or isolated from the effect in the turbulent bulk, and c) Lagrangian framework can be used. There are several examples of turbulent flows affected by the presence of dilute polymers without significant mean shear, that will be presented and discussed.
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