Numerical Simulation Of Aerodynamic Effects Of Flow Around High-rise Buildings

Wind tunnel test is the main technology to analyze wind load on tall buildings.In recent years,since the development of computer technology and progress in Computational Wind Engineering,the research of numeral simulation of wind load on tall buildings has gotten great improvements.Computational fluid dynamics technology has been developed as a complementary tool to traditional methods(wind tunnel experiments and field measurements)for wind engineering applications.In order to study the wind load in the three-dimensional flow field of a high-rise building,the wind load on the rigid structure surface is simulated by RANS and LES.The main work is as follows:(1)The basic problem of CFD,the governing equations,is analyzed.Based on the finite volume method,the SIMPLE algorithm is used to solve the NS equations of incompressible fluid in FORTRAN language.Then introduce the half-staggered grid,improve the algorithm.Two different algorithms are used to calculate the NS equation.The classic case,the two-dimensional steady-state cavity,is applied to verify the feasibility of the two algorithms and the correctness of the programs.The diffusion equations are calculated for the four highly distorted grids by using the finite volume method(FVM)of the preserving positive format.Comparing to SIMPLE algorithm of FVM and FLUENT,this new method is proved to be stable and preserving positive.Based on the study above,when we use the Reynolds average method(RANS)and the large eddy simulation method(LES),we can get the choices of discrete method of the diffusion equations,the convection equations and the time equations,respectively.The calculation parameters are determined such as time step,variable relaxation factor,etc.The code of FLUENT UDF files are programmed.(2)Reliable results of numerical simulations requires stable inflow boundary conditions(atmospheric boundary layer wind speed fields that meet specific conditions)and good grids.In this paper,we get stable wind speed profile of the B and D landforms of the target boundary layer wind tunnel.The wind speed meet the demand of average wind speed,turbulence intensity,and the turbulent integral scale.Use MATLAB to implement the CDRFG method and generate wind speed fields that satisfy the Von-Karman spectrum.Update the CDRFG method,significantly improve computational efficiency and save computing resources.Good grid has a great influence on the numerical calculation results.This paper uses ANSYS ICEM to generate grid and study the influence of the grid’s first grid size y+ and grid elongation on the calculation results.After debugging,trial calculation,and finally generated a grid that meets the requirements of both RANS and LES.The grid’s self-retention is verified by monitoring the velocity field of the outflow surface.(3)The Reynolds average k-ε model is used to simulate the surface average wind pressure of super high-rise buildings with an aspect ratio of 9.The work includes geometric modeling,grid generation,setting boundary conditions,and calculating parameter settings.The wind speed file is loaded as the inflow velocity boundary through FLUENT UDF files.After setting initial conditions,boundary conditions,stepwise control calculation process,the results are saved.The results of the RANS is compared with the wind tunnel test result.In order to verify effectiveness of RANS method,we evaluate the mean wind pressure coefficient of solar panels,which are placed on the top of the tall buildings,and study the influence of the parapet wall and distance between each panel.(7)The LES Samagorinsky turbulence model is used to simulate the wind pressure of high-rise building with an aspect ratio of 9 in B-type and D-type landform.Then compare the results of LES,RANS and wind tunnel tests.The wind load characteristics of super high-rise buildings under B and D landforms are summarized.The conclusions show that,although LES method takes more amount of grids and more computing resources,but the LES method shows good stability and its superiority.Compared with wind tunnel test and RANS method,the LES method can learn more details of wind pressure on the surface of tall building.In the end,we compare the wind pressure coefficients of the backward of tall buildings with different high-width ratio,such as 4,5,6,and 9(They have same section).The results show that three-dimensional flow around high-rise buildings with an aspect ratio of 9 is the weakest.