# Computational Fluid Dynamics-Aided Simulation of Twisted Wind Flows in Boundary Layer Wind Tunnel
###### Abstract  
The twisted wind flow (TWF), referring to the phenomenon of wind direction varying with height, is a common feature of atmospheric boundary layer (ABL) winds, noticeably affecting the wind-resistant structural design and the wind environment assessment. The TWF can be effectively simulated by a guide vane system in wind tunnel tests, but the proper design and configuration of the guide vanes pose a major challenge as practical experience in using such devices is still limited in the literature. To address this issue, this study aims to propose an approach to determining the optimal wind tunnel setup for TWF simulations using a numerical wind tunnel, which is a replica of its physical counterpart, using computational fluid dynamics (CFD) techniques. By analyzing the mechanisms behind guide vanes for generating TWF based on CFD results, it was found that the design must take into account three key parameters, namely, (1) the distance from the vane system to the side wall, (2) the distance from the vane system to the model test region, and (3) the separation between the vanes. Following the optimal setup obtained from the numerical wind tunnel, TWF profiles matching both the power-law and Ekman spiral models, which, respectively, reflect the ABL and wind twist characteristics, were successfully generated in the actual wind tunnel. The findings of this study provide useful information for wind tunnel tests as well as for wind-resistant structural designs and wind environment assessment. twisted wind flow; wind tunnel testing CFD; numerical wind tunnel 2024149881  
ground. [PERSON] et al. [3] analyzed 4766 30-min-long data segments of synoptic winds collected by a wind profiler installed in a coastal area of China, concluding that the twist angle can reach 5\({}^{\circ}\) to 40\({}^{\circ}\) over the first 1000 m above ground. Based on the field measurement data of multiple tropical cyclones collected by Doppler radar wind profilers and anemometers in Hong Kong, [PERSON] et al. [4] and [PERSON] et al. [5] suggested that wind direction generally varies 20\({}^{\circ}\) to 40\({}^{\circ}\) within the first 1000 m in height, and the twist angle is highly dependent on the terrain condition, the wind speed, and the storm type (monsoons or typhoons). Based on these field observations, it can be readily understood that the TWF may impose significant asymmetrical loads on structures, and therefore the neglect of the TWF effect may lead to inaccurate calculations of the wind loads acting on super-tall structures, including skyscrapers and gigantic wind turbines [6; 7]. For instance, the variation in wind yaw angle may affect the drag and lift forces of the wind turbine blades, whilst such a TWF effect is usually neglected in turbine designs. Moreover, ignoring the TWF may also result in unreliable wind environment assessments, such as for pedestrian-level winds over complex urban and/or mountainous terrain [8]. As a widely accepted experimental approach, boundary layer wind tunnel testing is usually recommended or even mandatory for wind-resistant structural designs and wind environment assessments. To artificially replicate the TWF in conventional wind tunnels that employ passive devices (e.g., grids, spires, and ground roughness) to generate ABL winds, research efforts have been devoted over the past few decades. One of the most common approaches is to set up a guide vane system consisting of multiple vertical vanes with a curved surface to redirect the flow at the outlet of the wind profile development region (i.e., where the passive devices are installed). Given a properly designed guide vane system, the wind flow can be twisted to achieve the desired orientations at various heights, so that the flow in the downstream model test region follows a profile that has both the speed and direction varying with height in the required manner.