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

The flow field resulting from the DNS of the turbulent channel flow of a dilute polymer solution has been analysed to investigate the influence of polymer addition on coherent vortical structures near the wall. The structures have been detected by a method based on a generalization of Jeong and Hussain's coherent structure definition. The average vortex in the polymer flow is weaker, less tilted and less inclinated than in the Newtonian case, although more elongated in the streamwise direction. While cross-flow velocity fluctuations near the vortex are damped by polymer addition, the intensity of low-speed and high-speed regions is increased. The orientation and stretching of the polymer chains induced by the vortex causes high additional polymer stresses in the flow, which modify the stress and pressure distribution around the vortex itself. The results suggest that the polymer may influence the energy transfer in flow, opposing to the redistribution of turbulent kinetic energy from the streamwise to cross-flow directions.