Research And Application Of Filtering Grid Scale And Meshing Adaptive-control Strategy For Large Eddy Simulation

With the development of turbulence research, more and higher requirements are put forward for the flow analysis of natural environment and engineering turbulence. Under the present computational level, large eddy simulation (LES) is one of the most effective approachs to predict complex turbulent flow relatively accurately. LES equations of the resolved turbulent scale are established by the low-pass filtering of Navier-Stokes equations, so the filtering grid scale of the spatial filter plays a decisive role for LES. Based on turbulence theory, the proper filtering grid scale should be within or close to the inertial sub-range of turbulence. However, present filtering grid scale solutions for numerical computation and sub-grid stress model are mostly built up by the local computational mesh. Because of the improper LES mesh, the employed filtering grid scale solution will not match with the theoretical proper filtering grid scale. It may lead to lose some useful small-scale fluctuations, economical cost advantage and the rationality of LES results. Therefore, this work focus on the deep research on the proper filtering grid scale of LES and trys to establish a rational adaptive control strategy for proper LES mesh from the views of the physical significance.On the basis of turbulence theory and LES method, a new formula for proper filtering grid scale was derived from the energy spectrum distribution law of the inertial sub-range. By incorporating LES control equation with Karman-Howarth equation thoughts, anther equal formula for proper filtering grid scale was established as well. In this work, a new concept defined energy ratio coefficient in turbulent flow structure was proposed firstly to associate turbulent energy spectrum with the filtering grid scale and the key turbulent parameters was determined to reflect the proper filtering grid scale. Based on different turbulent characteristic scale and multi-scale wavelet method, the rational value range and commended equations of the energy ratio coefficient were set up firstly.In order to relize the above mentioned computational method, the relevant researches were carried out. Based on wavelet method and statistic theory, a new approach for proper sampled data scale was put forward to offer the support for the sample size in the analysis of the proper filtering grid scale. An improved wavelet threshold de-noising method was set up to remove the noise signals and get the relatively reliable data for the turbulent signal post-processing. According to wavelet and bi-spectrum analysis, a reasonable method to indentify the coherent structures and a new identification criterion of energy-containing scale were established. In terms of time-frequency characteristics and information entropy theory, an integrated method was put forward to determine the proper wavelet function in the analysis for the energy ratio coefficient.Based on the view that the computational mesh distribution should match with the proper filtering grid scale, a new physical strategy for LES mesh adaptive-control was put forward firstly to get optimized meshing size and distribution. The new index named grid ratio coefficient for mesh adjustment and the realizable method for near wall meshing treatment were offered respectively. Through the abovel strategy, the proper mesh of LES could be built directly from the adjustment of RANS mesh. This work applied the novel strategy to achieve LES mesh adaptive-control and data comparative analysis among LAM (LES of Adaptive-control Mesh), LFM (LES of Fine Mesh), DNS (Direct Numerical Simulation) and experiments for three typical flow cases, including Re=5147 backward facing step flow, Re=37500 backward facing step flow and Re=5147 turbulent channel flow. The results showed that the grid numbers of LAM were less than those of LFM evidently and the data of LAM were in a good agreement with those of DNS and experiments. It was revealed that results of LAM were very close to those of LFM. The conclusions provided positive evidences for the rationality, reliability and economy of the novel strategy.This work used the novel strategy to carry out LES grid index analysis, LES mesh adative control and data comparative analysis for the centrigugal blower with the equal width blade. The visual centrigugal blower experiments were set up to get the inner velocity distribution and performance data of three different blades by PIV and performance tests. It was shown that the adjusted LES mesh distribution could well reflect the inner flow structure characteristics and local mesh size requirements. The results indicated that the data of adaptive control LES mesh were in a good agreement with those of PIV experiment and performance test. The conclusions offered further confidence in the rationality, realizability and good prospect for complex turbulent flow of the novel strategy.