Study On The Drag Reduction Mechanism Of Viscoelastic Turbulent Flows

The addition of drag reduction additives(polymers or surfactants,etc.)to oil pipelines is an important technology for energy saving and emission reduction.It can be used to increase the throughput of existing pipelines or reduce the power costs of pipelines.The viscoelastic turbulent flow drag with additives is often lower than that of the original Newtonian turbulent flow.The turbulent drag-reduction phenomenon has been reported as early as the 1940 s,but the mechanism of drag reduction and the accompanying heat transfer reduction are currently uncertain,which seriously hinders the application of turbulent drag reduction technology with additives.In order to promote and guide the application of the technology in oil pipelines,direct numerical simulation(DNS)method is used to investigate the viscoelastic isotropic turbulent flows and turbulent channel flows in this study.For numerical simulation,DNS methods are developed for the viscoelastic isotropic turbulent flow and the viscoelastic turbulent channel flow.The second-order bounded MINMOD scheme is used to discretize the convection term of viscoelastic fluid constitutive equation to improve the calculation accuracy on the premise of ensuring calculation stability.For the viscoelastic turbulent channel flow,the flow rate is controlled to be constant by correcting the mean pressure gradient for each time marching.The comparison between the MINMOD scheme and the Kurganov-Tadamor scheme shows that the MINMOD scheme is more stable and computationally efficient.For the viscoelastic isotropic turbulent flow,the turbulent heat flux transport equation for the flow is deduced and analyzed,to uncover the heat transfer reduction mechanism.Drag reduction additives on the one hand suppress turbulent fluctuations and weaken convection to drive turbulent fluctuations to transfer heat from high temperature to low temperature,and on the other hand,introduce elastic stress that drives turbulent fluctuations to transfer heat in the opposite direction.These two effects result in the decrease of large-scale turbulent heat transfer,inverse heat transfer in partial medium scales,and finally the heat transfer reduction phenomenon in the viscoelastic isotropic turbulent flow.From the perspective of energy dissipation,the drag reduction rate of viscoelastic isotropic turbulence is redefined,the viscoelastic isotropic turbulent drag reduction mechanism is further studied.For the drag reduction mechanism of viscoelastic turbulent channel flow,the friction decomposition is re-derived from the perspective of energy transport and it is found that each shear stress is weighted by shear stress to contribute to the friction.It is found that the turbulence not only introduces the contribution of Reynolds shear stress to the flow drag,but also enhances the contribution of the viscous shear stress to the flow drag.Drag reduction additives,on the one hand,introduce elastic stress,and on the other hand,suppress turbulent fluctuations and thus suppress the contribution of Reynolds shear stress and viscous shear stress.Turbulence drag reduction phenomenon occurs when the latter drag reduction effect is higher than the former drag increase effect.The friction coefficient contribution expression is further deduced for analyzing laminarization effect on the friction coefficient.It is found that drag reduction additives not only laminarize the mean velocity distribution and reduce the flow drag,but also introduce an amplification factor on the flow drag.When the former effect exceeds the latter effect,turbulent drag reduction phenomenon occurs.Finally,the phenomena of the drag reduction onset and maximum drag reduction asymptote are further studied.