| Abstract: | Turbulent flow is one of the unsolved problems in physics and engineering even though most flows in every day life and engineering problems are turbulent flows. There have been more than ten decades of trials to solve turbulent phenomena since Osborne Reynolds's investigation of pipe flow. Development of current computer technology enables numerical approaches to turbulence. Large Eddy Simulation (LES), in which flow variables are subdivided into a 'mean' component and a 'fluctuating' component through space filtering, is one of the most important methods for study of turbulence. There has been, however, few applications of the LES to practical engineering problems. This study was performed to test the application of LES to practical problems such as flow induced vibration of steam generator tubes in nuclear power plants. Modeling of the Sub-Grid Scale (SGS) terms is a key to the accurate calculation of LES. In this study, existing SGS models were reviewed and a new SGS model was developed. This new SGS model includes cross terms, which are neglected by most SGS models. Taylor series expansion of double filtering operation and Bardina's transfer field concept were used in developing the new model. This model showed reflecting energy back scatter from small scale to mean motion. Results of calculations with this new SGS model showed better agreement with experimental data than Smagorinsky eddy viscosity model. A solution technique also needed to be found to cope with the huge memory and time requirement. The Incomplete Cholesky-decomposition Conjugate Gradient (ICCG) method was selected as a solution technique and a preconditioning technique was developed for problems which have an inner boundary, like bundle flows. With this technique, required memory can be reduced significantly while computational time is comparable to that of a direct solver. The developed SGS model and numerical technique were combined with GUST program and tested for single rod problems and bundle problems. Three dimensional simulations were performed and the results were compared with previous two dimensional calculations. The two results showed good agreement. |
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