## 2 Challenges in Wall-Bounded Aerodynamic Testing  
In wind engineering research, accurately emulating wind attributes in a controlled environment is crucial to understanding a structure's reaction to designated wind scenarios. This necessitates the initial replication of wind flow characteristics as per a standardized methodology. Following this, aerodynamic examinations are executed to produce wind-induced pressures and forces acting upon the facades and the structure while upholding the principles of similitude [43]. To satisfy these prerequisites, diverse techniques are employed for simulating atmospheric boundary layer (ABL) flows, including laboratory and computational approaches [44].  
Wind tunnel modelling, assumed to be indispensable for assessing wind-induced pressures and loads on structures, has been considered fundamental for numerous decades. However, challenges arise in replicating turbulence and associated physics accurately, especially in wall-bounded laboratory settings [45]. Achieving spectral fidelity with authentic wind patterns at high Reynolds numbers has been a challenge, impacting the accuracy of laboratory experiments [46]. Scaling factors introduce complexities [26,47]. However, larger test models in wall-bounded test sections can alleviate some issues but introduce disparities [47].  
The aerodynamic characteristics of structures immersed in turbulent flows are intricately linked to wind patterns' spatial and temporal dynamics [48]. Understanding peak loads and the spatial pressure interrelation necessitates realistic flow production aligned with full-scale conditions. Challenges arise in replicating the complete turbulence spectra at considerable scales [49, 50, 51].  
Disparities exceeding 50% in aerodynamic metrics have been observed across research laboratories, emphasizing the importance of novel and advanced wind testing [52]. The feasibility of accurately estimating wind-induced loads for design considerations solely reliant on small-scale wall-bounded wind testing outcomes is questioned [53]. The realistic replication of low-frequency turbulence poses a challenge in conventional wall-bounded laboratory experimentation [54].  
Figure 2 provides an overview of the observed lack of large-scale turbulence, specifically low-frequency turbulence, in wall-bounded laboratory testing scenarios. In subfigure (a), at a scale of 1:300, the high turbulence flow aligns well with the intended target, demonstrating a successful match. However, in subfigure (b), at a scale of 1:10, the high-turbulence flow deviates significantly from the target, indicating a lack of congruence. Subfigure (c) emphasizes the issue with the normalization of frequency (\(x\)-axis) using the flow integral length scale. It is important to note that while this approach may seem intuitive, it can potentially lead to misleading conclusions as it conceals the underlying deficiency in turbulence production, a recurrent issue in prior wall-bounded laboratories.  
Figure 2: Lack of large-scale turbulence (low frequency turbulence) in wall-bounded laboratory testing: (**a**) scale 1:300 (the high turbulence flow matches with the target), (**b**) scale 1:10 (the high turbulence flow does not match with the target), and (**c**) normalizing the frequency (\(x\)-axis) using the flow integral length scale. Using the flow integral length scale to normalize the frequency can be misleading as it conceals the lack of turbulence (an issue common in previous wall-bounded studies).  
Small-scale testing presents challenges such as spectra mismatch, Reynolds number effects, blockage issues. Besides, concerns about solar panel damage, and cladding failures on low- and high-rise buildings raise questions about the previous aerodynamic testing methods and design approaches that have been followed for decades [34; 55; 56; 57; 58]. Addressing these challenges necessitates advanced studies, and motivates the consideration of a novel testing approach, which is crucial for the safety and dependability of structures when subjected to wind loads.