Numerical Simulation Of Turbulent Drag Reduction In Viscoelastic Fluids

Since scientists discovered the turbulent drag reduction effect of polymer,the turbulent drag reduction effect has been widely used in mechanical engineering and other fields,but the action law of polymer turbulent drag reduction is not clear at present,which mainly depends on the experience of engineers,which hinders the further development of the turbulent drag reduction effect in the application.Therefore,turbulence drag reduction is of great significance in both academic and engineering research.In this paper,the drag reduction effects of isotropic turbulence and wall turbulence are studied by direct numerical simulation.The main results are as follows:Based on the direct numerical simulation of turbulence,a turbulence generation method is proposed.In the study of isotropic turbulence,turbulence is maintained and generated by calculating the dissipated energy of each time step,and then re-input the compensation amount to the system.In the study of wall turbulence,the pressure gradient is obtained under the desired conditions,and then the flow is generated and maintained by modifying the solver to use the pressure gradient as the driving source of turbulence,and then the parallel efficiency of the solver is tested.SIMPLEC algorithm was used to solve the governing equation,and logarithmic conformation tensor approach was used to improve the stability of viscoelastic flow solution.In the numerical simulation of drag reduction for isotropic turbulence,the convergence of grid is verified,and the results of grid used in this paper are in good agreement with those of larger scale.The results of simple shear flow field show that the elastic stress is up to 50 times of shear stress,and the results of uniaxial tensile flow field show that the tensile viscosity is up to 3335 times of shear viscosity,which proves that the parameters selected in this paper have enough elasticity.The changes of turbulence characteristic scale indicate that polymer inhibits the development of turbulence small-scale fluctuation.By analyzing the turbulent energy spectrum,it is found that the velocity distributions of viscoelastic flow and Newtonian fluid are different due to the addition of polymer,and the intersection of viscoelastic flow and Newtonian fluid in the range of large wave numbers shifts to the high wave number region with the increase of Reynolds number.The power spectrum shows that the addition of polymer slows down the energy decay rate of the flow field.The evolution of the conformation tensor shows that the tensile action of polymer fluctuates greatly in the flow field but tends to be stable.In the numerical simulation of turbulence drag reduction for wall turbulence,the convergence of computational grid is verified by comparing with previous results.Through the data statistics,the drag reduction rate was calculated,and it was found that the addition of polymer significantly improved the mean velocity profile and turbulent kinetic energy curve.The change of stress showed that the ratio of elastic force to viscous force reached the peak in the buffer layer,which revealed that the drag reduction control of the buffer layer was the most important in the whole flow field.The conformational tensor results show that the streamwise role is dominant.The vortex structure,streamline and band structure of viscoelastic fluid and Newtonian fluid were analyzed by visual means.It was found that the large scale structure of viscoelastic fluid increased significantly while the small scale vortex was suppressed by polymer,and the shape of vortex structure was different.