title: Drag reduction by a linear viscosity profile.
abstract: Drag reduction by polymers in turbulent flows raises an apparent contradiction: the stretching of the polymers must increase the viscosity, so why is the drag reduced? A recent theory proposed that drag reduction, in agreement with experiments, is consistent with the effective viscosity growing linearly with the distance from the wall. With this self-consistent solution the reduction in the Reynolds stress overwhelms the increase in viscous drag. In this Rapid Communication we show, using direct numerical simulations, that a linear viscosity profile indeed reduces the drag in agreement with the theory and in close correspondence with direct simulations of the FENE-P model at the same flow conditions.
authors.affiliation: Dipartimento di Meccarica e Aeronautica, Università di Roma La Sapienza, Via Eudossiana 18, 00184, Roma, Italy.
authors.forename: Elisabetta
authors.initials: E
authors.lastname: De Angelis
authors.forename: Carlo M
authors.initials: CM
authors.lastname: Casciola
authors.forename: Victor S
authors.initials: VS
authors.lastname: L'vov
authors.forename: Anna
authors.initials: A
authors.lastname: Pomyalov
authors.forename: Itamar
authors.initials: I
authors.lastname: Procaccia
authors.forename: Vasil
authors.initials: V
authors.lastname: Tiberkevich