Large Eddy Simulation Of Forcing Homogeneous Isotropic Turbulence In Viscoelastic Fluid

Since the phenomenon that the addition of a minute amount of drag-reducingpolymer or surfactant additives into turbulent liquid fluid flow may cause a dramaticfrictional drag reduction was found by Toms, domestic and foreign scholars have beenfocusing on the drag-reducing phenomenon. Some scholars used numerical simulationmethod to study turbulent drag reduction by additives. Large eddy simulation (LES) cannot only reduce the computation burden but also can get more information ofmicro-scaled turbulent flow parameters. It can be used both for theoretical research andto solve practical engineering problems. In this article, LES method will be used tostudy the characteristics of forcing homogeneous isotropic turbulence in viscoelasticfluid.Firstly, two kinds of LES subgrid scale model are introduced in detail, that areADM model and TADM model, and then two kinds of model are compared. Two kindsof model are used to simulate Newtonian fluid case. In order to validate whether twomodels are correct, the results obtained by two models are compared with that ofNewtonian fluid DNS database. ADM model and TADM model represent spatial filtermodel and temporal filter model, respectively, namely two categories of LES subgridscale model. By comparing ADM model and TADM model, the differences betweenspatial filter model and temporal filter model are obtained. And the better method isused to simulate viscoelastic fluid case.Secondly, TADM model is chosen to simulate forcing homogeneous isotropicturbulence in viscoelastic fluid, and the characteristics are analyzed. In order to validatewhether TADM model is correct for viscoelastic fluid, the results obtained by TADMmodel are compared with that of viscoelastic fluid DNS database. According to theresults for viscoelastic fluid by TADM model, turbulence parameters, enstrophy andstrain are analyzed. Drag reduction in viscoelastic fluid cases enhances with increase ofconcentration of polymer solution. By extracting vortex structures in Newtonian fluidand viscoelastic fluid cases, the phenomenon that the small-scale structures are inhibiteddue to viscoelasticity is found. This phenomenon also shows that the small-scaleintermittency is inhibited. According to the study of high order correlation function ofvelocity partial derivative, it more directly shows that the small-scale intermittency inturbulence is inhibited. By researching the trace of molecule’s deformation rate inviscoelastic fluid, it is found that the Weissenberg number deeply influences stretchingdegree of viscoelastic fluid molecules, but concentration of polymer solution has nomuch influence on stretching degree.Finally，according to the analysis of velocity field by the proper orthogonaldecomposition, it is obtained thatthe number of eigenfunctions needed is relatively small for viscoelastic fluid when the same turbulence kinetic energy are captured inNewtonian fluid and viscoelastic fluid cases. It shows that coherent structures areinhibited and flow is relatively regular in viscoelastic fluid.