Direct Numerical Simulation Of Turbulent Drag Reduction And Flow Instability In Polymer Solution

Turbulence is an internationally recognized problem in physics,mathematics and mechanics,and "the last major unsolved problem in classical physics." In the process of understanding the traditional Newtonian fluid turbulence,new turbulence phenomena have been discovered,such as elastic turbulence,elasto-inertial turbulence and turbulence drag reduction in non-Newtonian fluids with viscoelastic rheological physics.The discovery and understanding of these phenomena,the revelation of their mechanism and the establishment of theoretical system are the only way to comprehensively understand turbulence and finally solve turbulence.In this paper,turbulence drag reduction and flow instability of polymer solution were studied based on the open source CFD software Open FOAM.The main research contents and conclusions are as follows.Due to the dissipative nature of turbulent energy,it is necessary to supply energy continuously from the outside world to maintain its motion state.In numerical simulation,an isotropic turbulence maintenance method is developed based on the transient solver piso Foam.In the studies of homogeneity turbulence,grid convergence is verified.In order to capture the smallest vortex structure in the flow field,large grid([512] ^3] is used for direct numerical simulation.The dissipative and compensating power,turbulence energy spectrum,power spectrum,structure tensor and Liutex vortex identification are analyzed.It is found that in Newtonian fluid,the dissipation of turbulent vortices accounts for most of the compensated power,while in viscoelastic fluid,part of the energy is stored due to the action of polymer chains.Through the analysis of turbulence energy spectrum and power spectrum,it is found that with the addition of polymer,the wave number range conforming to the-5/3 power law increases,and the energy attenuation rate slows down in the high frequency(small turbulence scale)range.In addition,Liutex analysis found that with the addition of polymer in the solution,the flow stability was significantly improved.The vortex structure of Newtonian fluids showed a fine tubular structure,while the vortex structure of the corresponding viscoelastic fluids changed to a larger and more regular sheet structure.In the study of drag reduction and flow instability around cylindrical turbulence,grid convergence is verified first,and drag reduction rate,power spectrum,structure tensor,streamline and vortex structure are analyzed.It is found that with the increase of polymer concentration,the viscoelastic resistance on the cylindrical wall decreases,and the energy decay rate in the wake slows down gradually.Through the analysis of power spectrum and visualization,it is found that the presence of polymer can significantly enhance the stability of the flow in the wake region of the cylinder,and because of the inhibition effect of viscoelasticity on the flow,the small-scale vortex structure on the cylinder wall is significantly reduced,and the chaos degree of the flow field is improved to some extent.