Large Eddy Simulation of turbulent backward-facing step flow
| Autor: | Yu-ChiehChen, 陳宇杰 |
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| Rok vydání: | 2013 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 101 Turbulent flow over backward-facing step was simulated by large eddy simulation (LES) model with the dynamic Smagorinsky sub-grid model (DSM). The computational case is conducted at a Reynolds number of 18400 based on the step height (h) and the inlet free-stream velocity (U_c) with the expansion ratio of 1.66. The the predictions are compared with available experimental data performed by Fessler and Eaton (1999). The turbulent characteristics the characteristic the backward-facing step flow are also discussed in this study. For lack of w-direction fluctuation component of experimental data, the predicted partial turbulent kinetic energy, which is defined as two fluctuation components (u_rms^, v_rms^ ) plus two-third of sub-grid turbulent kinetic energy, is adapted for the comparison with available measured data. It is found that both the predicted mean stream-wise velocities as well as partial turbulent kinetic energy are in good agreement with experimental data. The temporal simulation results indicate that the reattachment point is not fixed but oscillates forth and back over a range of 1 step-height distance, which is in consistent with the reported observation in Eaton and Johnston’s experiment (1980). Moreover, the fact that the moving frequency of “flapping motion” is approximately one order of magnitude less than that of “vortex shedding” can be easily observed in energy spectral distributions at the respective locations near the mean reattachment point and the separation point. It is also concluded that, according to the suggestion Pope (2004), at most 20% of total turbulent kinetic energy should be resolved in the sub-grid scale of eddies in the calculation with the sub-grid turbulence model to meet the isotropic characteristics of sub-grid eddies. The shear layer is a critical mechanism for development of the backward facing step flow. Therefore, the highly grid resolution is essential not only for recirculation zone but also for shear layer extending to the downstream region far from step. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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