Impact of atmospheric boundary layer inhomogeneity in CFD simulations of tall buildings

Abstract

Recently, there has been a growing interest in utilizing computational fluid dynamics (CFD) for wind analysis of tall buildings. A key factor that influences the accuracy of CFD simulations in urban environments is the homogeneity of the atmospheric boundary layer (ABL). This paper aims to investigate solution inaccuracies in CFD simulations of tall buildings that are due to ABL inhomogeneity. The investigation involves two steps. In the first step, homogenous and inhomogeneous ABL conditions are generated in an empty computational domain by employing two different modelling approaches. In the second step, the homogenous and inhomogeneous conditions are each applied to an isolated tall building, and simulation results are compared to investigate impact of ABL inhomogeneity on wind load predictions. The study finds that ABL inhomogeneity can be a significant source of error and may compromise reliability of wind load predictions. The largest magnitude of inhomogeneity error occurred for pressure predictions on the windward building surface. Shortening the upstream domain length reduced inhomogeneity errors but increased errors due to wind-blocking effects. The study proposes a practical approach for detecting ABL inhomogeneity that is based on monitoring sensitivity of key output metrics to variations in upstream domain length.

Optimising the computational domain size in CFD simulations of tall buildings

“…The increase in pressure on the windward face is uniform at all heights of the building. This suggests the absence of ABL inhomogeneity errors, which are largest near the ground and decrease with building height (Abu-Zidan et al, 2020). The downstream length l d had minimal influence on surface pressure.…”

“…Controlling ABL inhomogeneity in this study is important because both ABL inhomogeneity errors and domain errors are influenced by upstream domain length l u . A fuller discussion of the relationship between ABL inhomogeneity errors and upstream domain length has been presented by Abu-Zidan et al (2020).…”

“…The modified wall function is based on aerodynamic roughness height z 0 and can accurately produce ground roughness conditions that are necessary for maintaining homogeneity of the ABL. Full details for this modelling approach can be found in a previous study by the authors (Abu-Zidan et al, 2020).…”

“…It is important to note that quantifying wind-blocking error in isolation is not possible without achieving horizontal homogeneity of the atmospheric boundary layer (HHABL). Wind-blocking errors are sensitive to upstream domain length l u , and so too are ABL inhomogeneity errors (Abu-Zidan et al, 2020). If inhomogeneity errors are present in the simulation, then l u will influence both wind-blocking errors and inhomogeneity errors simultaneously, such that increasing l u will minimise wind-blocking error while maximising inhomogeneity errors (and vice versa).…”

“…To our knowledge, no such study has been performed for tall building applications. Furthermore, domain studies in the literature do not demonstrate horizontal homogeneity of the atmospheric boundary layer (ABL) which is needed to provide confidence that domain errors are investigated independently of ABL inhomogeneity errors since both domain errors and ABL inhomogeneity errors are sensitive to upstream domain length (Abu-Zidan et al, 2020). Many studies do not address the issue at all (Buccolieri and Di Sabatino, 2007;Ramponi and Blocken, 2012;Xiang and Wang, 2007), while others acknowledge this issue but do not quantify its impact on their findings (Ai and Mak, 2015;Revuz et al, 2012).…”