LARGE-EDDY SIMULATION FOR CANOPY TURBULENT FLOW
植被层湍流的大涡模拟

The boundary layer theory proposed by L. Prandtl in 1904 has played a tremendously important role for aeronautical and aerospace engineering during the last 100 years or so. Nowadays it has found wide applications both in industry and geophysical flows. The present paper mainly address to the turbulence in Atmospheric boundary layer (ABL) or its surface layer, which is closely related to climate and ecological environment prediction. We have reviewed on its principal numerical approaches: Reynolds average N-S equation method (RANS), large eddy simulation (LES) and direct numerical simulation (DNS) and indicated that LES has become a potential approach for practical engineering applications in the near future as more powerful supercomputers are accessible to ordinary researchers. However, a most crucial task for the success of LES is to work out an appropriate subgid scale model reflecting real physics implied in it and exhibiting correlation with DNS a priori test. As an example, we have investigated the turbulence within and over forest canopy in the current paper. Considering the characteristics of flows, we have proposed a new subgrid scale model, which is the combination of Deardorf model and structure function model. This new model is capable of dealing with both strong shear and buoyancy appropriately.At the same time, distributed momentum and energy source (or sink), the strength of which is dependent upon leaf area index (LAI), models the vegetation layer reasonably. By case study, we have established the database for a few typical flows including strong and moderate convective ABL and analyzed their statistical features such as mean velocityt Reynolds stress, turbulence energy etc. The results are proved in good agreement with observations. The model is also justified by evaluating subgrid scale vortex energy within 5% of total energy. So do the contributions of respective Reynolds stress components. The results also reveal the observed phenomena such as adverse gradient transport and organized pattern of temperature ramp in strongly convective ABL,the mechanism of which can been attributed to the emergence of coherent structure in turbulent flows.