Large Eddy Simulation Of Atmospheric Boundary Layer Flow Based On FLUENT

With the development of the national economy, engineering structures are becoming to be more and more complex and sensitive to wind loads. Unreasonable magnitude of wind load applied on the structures may lead to structural damage or even cause huge losses. So obtaining the exact value of wind load is the key to the wind resistant design of structures. The characteristics, such as velocity profile, turbulence intensity, turbulence integral scale and power spectrum of the atmospheric boundary layer flow, influence the value of wind loads. So describing the flow around buildings accurately is the prerequisite of determination of wind loads.With the development of computer science, Computational Fluid Dynamics (CFD) technology is being paid more attention in science research and practical engineering. Compared with traditional methods, CFD is faster, cheaper and more comprehensive. It is becoming one of the most important methods of calculating wind loads and simulating the complex wind environment.Most engineering structures stand within atmospheric boundary layer, the near ground wind field and the flow around buildings could be recognized as incompressible turbulence. According to the treatment and calculation precision of eddies, the numerical simulation method of CFD can be divided into Direct Numerical Simulation (DNS), Reynolds Average Navier-Stokes(RANS), Large Eddy Simulation(LES). Considering the calculation accuracy and efficiency, large eddy simulation has become a reasonable choice of numerical wind resistant research. To get the inflow condition for wind engineering and determine wind loads, atmospheric boundary layer flows in different types of terrains will be simulated in this thesis by applying LES method.The work carried out in this paper could be summarized as:1,Large eddy simulation of atmospheric boundary layer flow is implemented by combining with the pseudo-periodic boundary condition, the roughness elements and random disturbance. In which the roughness elements and the random numbers satisfying the normal distribution are applied to increase the turbulence intensity of flows near ground and at the higher level, respectively. Approximate calculation formula of the standard deviation of random number is proposed. 2,Describing the pseudo-periodic boundary condition and the random numbers by C programs which is added to FLUENT as UDF. The parallel program is written which is aimed to increase the calculating speed.3,Settings of various parameters in CFD numerical calculation, including the computational domain selection, the roughness elements arrangement, the grid meshing and the time step, are discussed. Reasonable values of them for the atmospheric boundary layer flow are proposed.4,The numerical method is tested and proved practicable. The main parameters influencing the simulation results including the grid density, the measuring position, the roughness element height, the random number assignment are analyzed.5,Simulating the atmospheric boundary layer flow in different types of terrains using above methods. Needed flow is obtained through modifying the influencing parameters. The results can be adopted as the inflow condition for the large eddy simulation of flow around buildings.