A Numerical Method For Large-eddy Simulation Of The Wind Field Around A Bluff Body With Moving Boudaries

The fluid-structure interaction (FSI) between membrane structures and atmospheric wind is a very important factor for the wind-induced response of membrane. Numerical simulation is an effective research tool for the FSI. Accurately simulating the time-dependent feature of flow field is one precondition to figure out the formation mechanism and development pattern of the FSI. The flow field in the FSI between wind and membrane is a near ground wind field around a bluff body involving moving boundaries. There are some problems to be solved in the Large-eddy Simulation (LES) of the wind field using the Computational Fluid Dynamics (CFD) technique.Firstly, Sub-grid Scale (SGS) model that suit for large project is one of the hotspot problems in the theoretical research of LES. Secondly, the projection method is an effective algorithm for decoupling the velocities and pressure in governing equations of the near-ground wind which is treated as incompressible flow. The time accuracy of pressure is lower than that of velocity in most existing projection algorithms. The time accuracy of pressure is a direct influence factor in FSI simulation. Therefore, pressure accuracy of projection method should be improved. Furthermore, an appropriate grid system for simulating the flow field involving moving boundaries is the Arbitrary Lagrangian Eulerian (ALE) dynamic grid. Mesh updating is quite frequent in the simulation. Geometric quality of grid and the grid conformance during update are two key factors for the accuracy of the simulation. Grid quality is the only concern in most existing mesh updating strategies, and their grid conformance is not tested. Likewise, the computational time of mesh updating strategies needs to be considered.To respond to the above problems, theoretical investigations and numerical researches about the SGS model, a projection method and its implement in ALE dynamic grid system, dynamic mesh updating strategy are conducted. The corresponding solution and main works of this thesis could be summarized as follows:1. Test and comparison of SGS models. Non-dimensional LES governing equations that describe near ground wind field and the construction processes of some SGS models were elaborated. A CFD code that solves the equations above was compiled on Cartesian static grid system using finite differencing method. The applicability, accuracy and efficiency of various SGS models were tested. An efficient and relatively accurate SGS model was selected for the LES of the wind field around a bluff body and was compiled as a subprogram module.2. Establishment of numerical method in dynamic grid framework. LES governing equations in ALE dynamic grid system (ALE-LES equations) were expounded. By decoupling the grid motion and the physical parameters of the wind field, a staggered ALE (SALE) strategy was established. Meanwhile, a Fully high-order accurate continuous Projection algorithm (FP) by which the time-accuracy of pressure could reach the same order of velocity was constructed. By combination with a second-order accurate predictor-corrector scheme for grid velocity, a fully second-order accurate projection method for solving ALE-LES equations was proposed. Then, a coordinates transformation from Cartesian grid system to Body-Fitted curvilinear Structured (BFS) grid system was performed on the proposed numerical method, and the main program module based on the transformed method was compiled.3. Comparison of analyses for mesh updating strategies. A grid quality evaluation model was concluded and used for testing the grid quality improvement of various mesh updating strategies in two skewed meshes. The grid conformance during updating and the efficiency are also tested and compared. The comparison of results showed that Nine-point rezone strategy was an optimum from comprehensive performance. Then, Nine-point rezone strategy was extended from two-dimension into three-dimension, and was renamed to be a distance weighting strategy according to the basic idea. A dynamic mesh updating subprogram module was compiled.4. Development and validation of the numerical simulation program. A CFD program called (BFS\SALE-D\FP)LES for simulating the near-ground wind field around a bluff body involving moving boundaries was developed by combining the chosen SGS model and the chosen mesh updating strategy with the main program module described above. The two dimensional lid-driven flow and the Taylor vortex streets were used to validate the stability and accuracy of this CFD program respectively.5. Application of the numerical simulation program. The near-ground wind field around an enclosed building of which the flat roof vibrating in second-order harmonic mode was simulated by (BFS\SALE-D\FP)LES program. By comparison with the wind field around the same building with a rigid roof, the effect of roof vibration on the pressure coefficient at the building surface and on the flow phenomenon nearby building was investigated. The comparison shows that the roof vibration is a key factor that influences the mean and rms pressure coefficient on the building. The roof vibration complicates the vortex structure, and increases the characteristic turbulence intensity of the wind field around the building. The roof vibration has notable influence on the instantaneous pressure distribution and value. The second-order harmonic vibration mode may induce resonance of fluctuating wind and roof, and is not conducive for the wind-resistant of building.The successful application of the (BFS\SALE-D\FP)LES program indicated that this program could used as an effective fluid solver for the FSI numerical simulation platform of membrane structures.