Large Eddy Simulation Of Turbulent Drag-Reducing Flows Of Viscoelastic Fluids

The addition of small amount of flexible long-chain polymers or some kinds of surfactants into a turbulent flow of water or organic solvents can significantly reduce the flow resistance. This phenomenon is called turbulent drag-reducing effect by additives. In order to better apply this effect in the real industrial systems, it is necessary to investigate its flow characteristics and turbulent drag-reducing mechanism. Since the conformation field of microstructures and the elastic stress field in turbulent drag-reducing flow of viscoelastic fluid are unavailable via experiments, many researchers focused on numerical simulation. Large eddy simulation (LES) attracts extensive attention becuase its computational cost is less compared with direct numerical simualtion (DNS) at the same conditions and it can obtain more information than Reynolds average numerical simulation. When LES is used for simulating a specific turbulent flow, the chosen subgrid-scale (SGS) model is of particular important. Within the published SGS models, almost all are for the Newtonian fluid turbulent flows. LES study on turbulent drag-reducing flow of viscoelastic fluid is still on the beginning stage. So far only temporal approximate deconvolution model (TADM) is attempted in LES of viscoelastic fluid. This greatly limits the development of LES study on turbulent drag-reducing flow. To better use LES to investigate turbulent drag-reducing flow of viscoelastic fluid, it is needed to develop more effective SGS models. From this viewpoint, the present thesis combines turbulent drag-reducing mechanism of viscoelastic fluid, aiming at establishing a new SGS model for turbulent drag-reducing flow of viscoelastic fluid. For LES study on turbulent drag-reducing flow of viscoelastic fluid, it has important theoretic meaning and scientific value.From the perspective of turbulent drag-reducing mechanism of viscoelastic fluid and based on the idea of reflecting the effect of viscoelasticity on coherent structures, the newly established SGS model creatively combines the idea of spatial filter and temporal filter, which is called MCT (Mixed subgrid-scale model based on Coherent structures and Temporal approximate deconvolution). LESs of forced homogeneous isotropic turbulence (FHIT) of viscoelastic fluid and channel flow of viscoelastic fluid are performed in order to verify the established MCT. The LES results at relatively low Reynolds number are compared with the corresponding DNS results for this two kinds of turbulent flows, and the LES results of turbulent channel flow at relatively high Reynolds number are also compared with the experimental results. The comparative results show that MCT can describe turbulent drag-reducing flows with/without wall effect, and the intended purposes of the present study are achieved. By using MCT and TADM to simulate FHIT of viscoelastic fluid at relatively high Reynolds number, it is discovered that TADM performs excessive dissipation for turbulent drag-reducing flow at relatively high Reynolds number. But, MCT does not, indicating that MCT is superior in simulating turbulent drag-reducing flows at relatively high Reynolds number. This is the basis of LES study on turbulent drag-reducing flow at high Reynolds number.MCT is employed to simulate FHIT in viscoelastic fluid at relatively high Reynolds number and its flow characteristics are analyzed. It is obtained that the flow features are obviously changed in turbulent drag-reducing flow of viscoelastic fluid, in which vorticity and enstrphy are decreased and the amount of small-scale coherent structures is distinctly reduced. With the enhancement of viscoelastic effect, turbulent drag reduction rate increased and the stretch of viscoelastic fluid molecules becomes severer. Moreover, one-dimensinal and two-dimensinal wavelet tranform are performed to research on the influence of viscoelasticity on turbulent multi-scale features.It is shown that the intermittency in turbulent flow enhances with the decrease of the scale. Coherent structures and the intermittency in the whole flow region of viscoelastic fluid are inhibited, and the contributions of local coherent structures to the intermittency are reduced.MCT is also applied to simulate turbulent channel flow of viscoelastic fluid at relatively high Reynolds number. By combining the numerical and experimental data in turbulent channel flow of viscoelastic fluid, the flow characteristics and turbulent drag-reducing mechanism are investigated. It is manifested that the mean velocity field, turbulent fluctuation intensities, Reynolds shear stress, low-speed streak structures and so on are all changed in viscoelastic fluid flow, and the buffer layer in viscoelastic fluid flow trends to extend to the main flow region. Then turbulent drag-reducing mechanism of turbulent channel flow of visoealstic fluid is explained well based on the results of the contributions of flow resistence and turbulent kinetic energy, scalling law and the intermittency based on wavelet transform. From an overall perspective, the decrease of turbulent contributions is larger than the additional viscoelastic contributions in viscoelatic fluid flow, leading to the reduction of frictional coefficient in turbulent channel flow of viscoelastic fluid and then producing turbulent drag-reducing effect. From the viewpoint of the influences of viscoelastic effect on the different regions at the wall-normal direction, it is found that viscoelastic effect mainly inhibits the coherent structures in the buffer layer, which are ejected from the linear substrate layer.