Research on LES for Flows with Generation of Turbulence at Small Scales

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The aim of the present study was to identify the structure which is most responsible for the transfer and dissipation of the turbulent energy, and conduct an assessment of the subgrid scale (SGS) models in large-eddy simulation (LES) on the accuracy for a prediction of energy transfer associated with these structures.We have developed a method to classify vortical structures into three categories, a flat sheet, a cylindrical sheet and a tube core. We generated direct numerical simulation (DNS) data of the homogeneous isotropic turbulence using 128ィイD13ィエD1 and 256ィイD13ィエD1 grid points for the cases with and without inputs of external energy at the large or small scales. It was shown using these databases that the proposed method yields an accurate classification of the data into three regions with the desired characteristics. By utilizing this method, we examined the contribution of these structures for the dissipation and cascade of turbulent energy in LES. It was shown that dissipati … More on and forward energy transfer primarily arises in the flat sheet region, whereas backward transfer primarily occurs in the compressed tube core region. In all of three regions, an intense generation of the azimuthal vorticity, placed perpendicular to the most stretched vorticity, was noticed.Then, we have conducted an assessment of the SGS models on its accuracy for prediction of the structures responsible for the transfer. The constant coefficient Smagorinsky model was an accurate model to represent the forward transfer. In the dynamic Smagorinsky model (DSM), the effect of the strain on the model was reduced, and an prediction accuracy for the structures responsible for energy transfer was significantly lowered.These SGS models were further assessed in actual LES. The Smagorinsky model yielded overestimate of forward transfer, and a poor prediction for an evolution of vortical structures. DSM yielded overestimate of contribution of the tube-core region, and a poor prediction of the dissipative structures. These drawbacks were circumvented in the SGS estimation model. Assessment of the results obtained in the present study at higher Reynolds numbers will be left to future work. Less