CFD modelling of atmospheric boundary layer flow and wind effects on high-rise structures and tall buildings

Abstract

The topics covered in this dissertation include two main parts: one focuses on computational fluids dynamics (CFD) modelling of atmospheric boundary layer (ABL) flow, and another is concerned with CFD modelling of wind effects on high-rise structures and tall buildings. The-state-of-the-art of CFD applications in these topics is reviewed and their main challenges are summarized. Simulations of ABL flows have been performed by numerous researchers with Reynolds Averaged Navier-Stokes (RANS) approach. However, two important issues, which require being resolved firstly, are the horizontal inhomogeneity of the simulated ABL flows and the appropriate RANS model. In this study, a novel approach, which justifies a decrease of the atmospheric turbulence properties with height, is proposed to prevent the horizontal inhomogeneity. Furthermore, it is applied to simulate the ABL flows over a flat open terrain and the numerical results are validated through comparisons with the experimental measurements. Also, a numerical test on the simulation of wind flow over a triangular ridge is conducted to identify an appropriate turbulence model for the simulation of flow separation and reattachment among various turbulence models. The RNG k-ε model is found to be quite satisfactory through comparisons with the well-documented wind tunnel testing results. Finally, the novel approach together with the modified standard k-ε model on the basis of the high Reynolds-Number RNG k-ε model is applied to investigate the wind flow fields over complex terrain through detailed comparisons with the wind tunnel and long-term field measurements. And improved results are provided by the newly proposed method in combination with the high Reynolds-number RNG model. For applications of CFD in wind engineering with unsteady approaches including large-eddy simulation (LES) and hybrid RANS/LES simulation, it is essential to reproduce physically realistic inflow turbulence for accurate estimation of wind effects on buildings and structures. In this study, assessments of four inflow turbulence generation methods (including three turbulence synthetic methods and the recycling method) for turbulent boundary layer are carried out. Advantages and disadvantages of these methods are discussed in details and improvements on the recycling method are also given. Consequently, recommendations on generating inflow turbulence for wind effects on buildings and structures are provided. In the flow field around a circular or elliptical cylinder, the roughness condition of the cylinder surface is fairly important. However, it is quite cumbersome to model the surface roughness in wind tunnel experiments. In this study, the ability of several detached eddy simulation (DES) approaches for the prediction of wind effects on a complex high-rise structure with elliptical shape is examined in comparison with the results of wind tunnel testing and the improved recycling method is used to generate the appropriate inflow turbulence. Using the k-ω Shear-Stress-Transport (SST) based DES, the rough wall with two different roughness heights and smooth wall are implemented on the surface of the high-rise structure. It is found that the surface roughness is of direct influence on the prediction of wind loadings with DES. In computational wind engineering (CWE), although it is critical and essential to examine the reliability and accuracy of CFD modelling, there are still fairly rare in-depth validation studies of the LES for fluctuating wind loads on buildings and structures in urban environment. In this study, the combined study of the LES, wind tunnel testing and full-scale measurement is conducted for estimation of wind effects on a super-tall building in complex urban area. The main objective is to explore a useful approach for the accurate estimation of wind effects on structures in urban environment. The LES techniques and numerical treatments employed in this study are proved to provide reasonably good results compared with the wind tunnel and field measurements. Moreover, the study also identifies the dependence of the sustainability of inflow turbulence on the adequate grid resolution between the inflow plane and the object building site.

Date of Award	16 Feb 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Qiusheng LI (Supervisor)