| With the development of computer performance, numeric simulation method plays more and more important role in turbulence research. In all kinds of numeric simulation methods, large eddy simulation(LES) is the most effective way in accurately forecasting complex flow according to the present computer performance. The distribution of LES filter grid is related to calculation quality directly. Because of the limitation of current subgrid model, it is of great significance to improve grid distribution to accquire the best grid distribution with the least total grid number. However, according to the current principle of grid generation, we have some clues in the boundary layer only. Therefore, the main purpose of this paper is to research the principle of LES grid, and to set up a new concept of character frequency band turbulent index, so as to improve the calculation quality within the limits of total grids according to the computer performance. Morever, the new concept is applied in practical calculation such as within rotating turbine. The researching is described in detail as following.1. A new theory of LES filter grid improvement based on turbulent energy distribution over frequency was set forth, as well as its application method. The method is successfully applied in LES calculation. By comparison with several calculation examples, it's proved to be able to improve result precision by adding only a few grids.2. Wavelet analysis of speed signal was made. Taking local speed variation along time as research subject, we filtered out a particular frequency band, the character frequency band, with a entire flow field energy ratio. Taking advantage of time-space localization ability of wavelet analysis, we accquired the nominal frequency and speed scope of the character frequency band. In order to find a principle of fitting grid size, a concept of character frequency band turbulent index was made based on above analysis. This new concept is different from traditional index of turbulence description. It focuses on signal component within a character frequency band, and can describe flow energy distribution over time or spacial scale more detailedly.3. Using the new concept of character frequency band turbulent index, we analyzed the fitting size of LES filter grid in some flow examples. The results showes that this index can reasonably reflect the relationship between local turbulent speed variation and votex scale, and can be widely applied to LES calculation at different Renoylds number and physical boundary conditions.4. Based on the theory and method above, programs are made to analyze flow field character frequency band turbulent index and to adjust grid distribution. Using the program, we made LES calculation of two backward facing step flows with great parameter difference on expand ratio and renoyld number. Their grids are adjusted, and the flow fields are recalculated on the adjusted grids. The calculation result are compared to corresponding experiments, and have proved the new concept and method to be reliable.5. A low speed fan turbine flow tunnel was calculated with LES. The character frequency band turbulent indexs on curving grids are calculated, corresponding grid adjustment are made. By comparing the calculation result to parallel PDA experiment data, we proved that the precision of resulted total pressure increasement value at turbine outlet is improved by adding small number of grids inside the flow tunnel. This improved that this theory and its method can be applied to analyze fitting size of grid inside fan turbine flow tunnel. |
