# Computational Fluid Dynamics-Aided Simulation of Twisted Wind Flows in Boundary Layer Wind Tunnel
###### Abstract
One of the first attempts was made by Prof. [PERSON] and his research group from the University of Auckland, who evaluated the aerodynamic performance of downwind yacht sails by installing plastic vanes [9]. Despite the drawbacks, such as high turbulence intensities and dips in mean wind speeds at regular heights, this guide vane system delivered promising test results that well agreed with the data obtained by numerical simulations and field measurements. The successful application of this guide vane system has not only spurred studies in sail aerodynamics [10; 11], but has also garnered attention from the research field of civil engineering. In recent years, a few wind tunnel tests have been conducted to investigate the TWF effect on structures and the wind environment [12; 13] In particular, [PERSON] et al. [8] fabricated five wooden vanes to simulate the TWF at the pedestrian-level height caused by mountainous and densely built city terrain, indicating that the TWF noticeably modifies flow features such as asymmetric wind speed distributions and reduced wind speed near an isolated building or building groups. [PERSON] et al. [14] generated two TWFs with the characteristics of thousand-meter-high ABL winds using four vanes and pointed out that the value of wind load applied to a super-tall building under the TWF effect highly depends on the total twist angle instead of being a specific value. [PERSON] et al. [15] developed two one-piece molded-fiberglass vanes to investigate the TWF effect on a typical super-tall building, highlighting that the TWF significantly affects the distribution of extreme cladding pressures and local wind loads. Nevertheless, the wind tunnel investigation of the TWF is still lacking, and one of the major difficulties it encounters is the design of a proper guide vane system. As an alternative approach to investigating the wind effect, numerical simulation using computational fluid dynamics (CFD) techniques has also been widely adopted in numerous studies. Compared with typical wind tunnel testing, numerical simulations have prominent advantages, such as the detailed illustration of flow fields, which is especially important to the investigation of complex flow structures, e.g., the TWF. To numerically simulate the pedestrian-level wind field under the TWF effect, [PERSON] et al. [16] introduced new inflow boundary conditions to model the wind speed profiles of the TWF. [PERSON] et al. (2017) and [PERSON] and [PERSON] (2018) incorporated the Coriolis force term in the momentum equations to numerically generate the TWF profiles, so that the influence of the Coriolis force on the TWF could be clearly demonstrated by comparing the TWF profiles with the reference profiles generated without this term. However, most of these studies generated the TWF profiles directly as the inlet boundary condition, and thus they hardly provided useful advice for the design of guide vane systems in wind tunnel tests. On the other hand, by combining advanced CFD techniques with the skills and experience in wind tunnel testing accumulated over the decades, the concept of a numerical wind tunnel has emerged. A numerical wind tunnel mimics its physical counterpart by setting the inlet boundary condition as a uniform flow and creating models of passive devices (e.g., grids, spines, and ground roughness) in the computational domain, allowing the desired ABL wind profile in the downstream area to be generated in a similar manner to that in an actual wind tunnel. [PERSON] and [PERSON] (2019) established a numerical wind tunnel using models of spires and ground roughness, which allowed them to test solar panels at various geometric scales with ease. [PERSON] et al. (2020) modeled spires and ground roughness to generate ABL winds, suggesting that the simulation results of wind speed and turbulence intensity well agree with those obtained from wind tunnel tests. To investigate the performance of a novel device for the wind tunnel simulation of transient gust-front outflow, [PERSON] et al.