Author(s): Donghong Ding; Shuqing Yang; Yu Han
Linked Author(s): Shuqing Yang
Keywords: Turbulence; Polymers; Maximum drag reduction; Pipe/channel flows; Non-Newtonian
Abstract: The addition of few parts per million of long chain polymers in turbulent flows can drastically reduce the friction drag, which is known as Drag Reduction phenomenon (DR). The polymer additives drive down the energy lost in pipelines and channel flows by suppressing the turbulence at the wall boundary layer. This technology has attracted intensively attention because of its relevance for applications. It has been successfully implemented to reduce pumping cost for oil and water pipelines, to increase the flow rate in fire fighting equipments and to help irrigation and drainage. Although it has a long history of DR, there exist only few available tools for engineers to predict DR, thus a reliable model is needed. To this end, according to the stochastic theory of turbulence, the current paper develops a theoretical model for DR in turbulent flows with polymer additives. In addition, the friction drag, in dilute polymeric solutions, firstly reduces with respect to the Newtonian case and it finally reaches a universal asymptote which is independent on the type of polymers or the concentration of the solution, and is known in literature as the Maximum Drag Reduction asymptote (MDR). Up to date, the clear mechanism for the MDR is poorly understood. This study builds a model by including the non-Newtonian effect of drag-reducing flows on turbulence near the MDR. The current model clearly reveals that the dynamic interaction between polymer molecular and turbulence leads to the reduction of Reynolds stress which should be mainly responsible for DR, it also indicate that the effect of non-Newtonian apparent shear viscosity of polymeric solutions on turbulence cannot be neglected near the MDR. The model provides a reasonable agreement with experimental data available in the literature. Quantitative agreement supports the validity of the model, and reveals the underlying mechanism of DR and MDR. Better understanding of DR and MDR would amplify its possible hydraulic applications for industry.
Year: 2013