## 1 Introduction  
Demand for renewable energy production has increased because of global climate change, and wind power potential is significant as an environmentally friendly energy resource. As the total cost of electricity generation is proportional to the sizes of wind turbines, the sizes of wind turbines have been steadily increasing with the development of materials and control techniques. Large rotors optimize electricity generation efficiency and maximize investment returns. As of 2021, the most powerful wind turbine was rated at 16 MW and had a blade length of 118 m and a height of 242 m. In particular, the offshore wind turbine market is expected to expand from its current market value more than USD 7 billion in 2020 to over USD 21 billion by 2026.  
A wind turbine's tower is the sole supporting system of its structure; it needs to resist horizontal wind and earthquake loads, as well as the vertical load of a large and heavy topside structure (rotor and nacelle). The tower's safety and reliability ensures the total system's safety and electricity generation efficiency. Wind loads on a wind turbine system are some of the dominant horizontal loads, and they cause severe horizontal vibrations. When wind turbine towers lose their expected and intended function by any environmental loads such as wind and earthquake loads, electricity generation efficiency is negatively affected, causing considerable economic losses and social impact. There have been many reports on severe wind events that have damaged wind turbine systems around the world [1, 2]. [PERSON] et al. [2] described a large number of serious tower failures resulting from design and operation errors or inappropriate performance assessment. An in-depth understanding of aerodynamic characteristics and failure mechanisms of a tower is necessary for both academic and related industrial communities. Dynamic analyses of wind turbine systems have mainly been verified by long-term field measurements, CFD, and numerical/analytical simulations. However, one of the challenging problems related with CFD and numerical/analytical simulations is a poor amount of dependable test and long-term field measurement data. Wind tunnel tests, one of the most reliable methods, have also been widely used to examine the aeroelastic and aerodynamic performance of wind turbines as well as electricity generation efficiency [3; 4; 5; 6; 7; 8; 9]. Problems related to Reynolds number have been important issues in conducting wind tunnel tests. However, [PERSON] et al. [9] pointed out that results from wind tunnel tests are acceptable once the aerodynamic characteristics of the topside structure are identified, even though Reynolds number is smaller than the full-scale one, implying that wind tunnel studies at low Reynolds numbers can still be used for validation and verification.  
With upsizing of wind turbines, tower safety is becoming increasingly important. Traditionally, circular cross-section towers have been used. However, limitations of circular cross-section towers with upsizing of wind turbines have become apparent, especially with regard to problems related to inadequate and uneconomic design strength. To solve these problems, researchers have proposed polygonal-cross-section towers, which have been used in Europe, but their wind-resistant performance has not been thoroughly investigated [10; 11; 12].  
Any polygonal cross-section can be used to support towers. In this study, the effects of polygonal cross-sections were investigated through aeroelastic and aerodynamic wind tunnel tests. Square and square helical (4), octagon (8), decagon (10), dodecagon (12), tetradacagon (14), and circular (\(\infty\)) were tested. The main parameters include reference mean wind speed at hub height \(U_{\textit{ref}}\), wind direction \(\theta_{\textit{ind}}\), azimuth angle \(\theta_{\textit{azi}}\), and pitch angle \(\theta_{\textit{pit}}\), as well as cross-sectional shape. Strong wind speed that is higher than cut-out wind speed was assumed where the blades were in the feathered position.