Cladding Wind Loads on Low-Rise Buildings: A Computational Fluid Dynamics Approach
Abstract
The lack of Computational Fluid Dynamics (CFD) guidelines for structural wind load evaluation significantly hinders its crucial application in research and practice. The basis for such guidelines is understanding how CFD parameters, such as mesh size, affect the simulated physical input and output parameters, such as velocity field and building aerodynamics. This dissertation addresses this gap by first establishing a relationship in modeling the approach flow and building aerodynamics that culminates into a set of CFD requirements for cladding wind load evaluation. Cladding load on low-rise buildings is selected for two main reasons. Cladding loads on low-rise buildings is critical when dealing with extreme wind hazards, such as hurricanes and tornados, where CFD can be applied. It is also the most error-sensitive compared to global wind loads, which makes it a conservative choice for validation and guideline developments.
The first problem addressed in this dissertation is to evaluate the influence of Large-Eddy Simulation (LES) mesh size and duration on the spectral frequency limits of the modeled velocity field. A relationship between the mesh size in the approach flow region of the LES domain and the resulting cut-off frequency in the modeled velocity spectra is proposed. A method based on a stationarity test is proposed to obtain the optimal duration for converged aerodynamics results. The proposed relationship was used to design the mesh throughout the thesis. The resulting velocity spectra show that the expression can accurately predict the cutoff frequency prior to running the simulation. For the optimal duration, results show that the optimal duration can be significantly shorter than full-scale 1-hour.
Second, the implications of LES filtering and its subsequent amplified viscosity on the modeled effective Reynolds number are investigated. The effective viscosity for separated reattaching flow over a forward-facing step (FFS) is defined as the mean sub-grid scale (SGS) viscosity maximized along the centerline of the separated shear layer (SSL). The FFS is placed in a uniform-smooth and turbulent-sheared inlet at varying levels of effective viscosity and corresponding effective Reynolds number. The Reynolds number effect on the separation bubble size, surface pressure statistics, flow profiles in the separation bubble, and characteristics of the SSL are used to evaluate the proposed definition compared to the conventional definition and vast data from the literature. The results show that the proposed Reynolds number definition based on the SGS viscosity characterizes the LES data more accurately than that defined based on the laminar viscosity.
The third part of the dissertation provides a set of CFD requirements, recommended workflow, and a benchmark validation for cladding load on a low-rise building. The requirements are based on the findings in this dissertation and the existing wind tunnel guideline, ASCE 49-21. New additions that are unique to CFD are also included. The recommended LES workflow is applied to a benchmark validation case for the cladding load on a flat-roof low-rise building. The results are evaluated according to the proposed requirements. The validation results show an excellent agreement with the target wind tunnel data with some explainable differences. The methodologies and results illustrate that, with thorough consideration of the physical requirements, CFD can be utilized to accurately simulate cladding wind loads.