Near-wall coherent structures in the turbulent channel flow of a dilute polymer solution

Near-wall turbulent coherent structures, obtained by applying Jeong and Hussain's vortex identification criterion to DNS flow fields of a dilute polymer channel flow, are compared with those observed in a Newtonian flow at the same Reynolds number. The mean vortex in the polymer flow is weaker, less tilted and inclined, while its length and radius are higher. Cross-flow velocity fluctuations near the wall drop consistently. The vortical structures orient and stretch the polymer chains near the wall, producing high additional stresses in the flow. The consistent result is a new pressure and stress distribution which acts in opposition to the redistribution of kinetic energy from the streamwise to the cross-flow direction. Although the basic features of near-wall turbulence are maintained, the whole turbulence cycle reaches an equilibrium state where Reynolds stresses and wall friction are lower.