## 1 Introduction  
Dynamic stall, caused by the continuous change of the angle of attack, has a significant effect on aerodynamic performance. It was first discovered in helicopter flow fields, more than fifty years ago [1; 2; 3; 4; 5]. They showed that the blade experiences vastly different loads between static and dynamic stall and indicated that the aerodynamic loading is dominated by the intense vortices shed from the vicinity of the wing leading-edge at moderate to high pitching rates. [PERSON] et al. [6] further revealed that the initial vorticity of shed vortices is fed from the abrupt, unsteady separation of the turbulent boundary layer, not the laminar bubble. [PERSON] et al. [7] measured the surface pressure distribution, lift, drag, and pitching moment, and delineated static and dynamic characteristics of eight airfoils both in and out of stall. Numerical modeling was employed as a new research method and the experimental equipment was also updated. [PERSON] and [PERSON] [8] proposed a semi-empirical dynamic stall model based on the proper consideration of unsteady flow mechanisms. [PERSON] et al. [9] numerically studied the unsteady flow fields around airfoils, oscillating in pitch and associated dynamic stall phenomenon. They also presented a numerical method that can be used to calculate the full viscous flow, and the results showed that the formation of the leading-edge vortices can be delayed when the reduced frequency is increased. The means of particle image velocimetry was used in the experiment by [PERSON] et al. [10], and the result showed the characteristic features of the dynamic stall process, the formation and development of an organized vortex structure, for increasing incidences and the subsequent separation. The comparison between numerical and experimental studies was conducted for four main phases of the dynamic stall process, and the result was encouraging [11]. The computational fluid dynamic successfully captured the three-dimensional dynamic stall [12]. Then, it had a good application in studying the dynamic stall of delta wings. [PERSON] et al. [13; 14] implemented a finite, volume-based solver, with a rigid moving mesh and delayed detached eddy simulation (DDES), to analyze the effect of the reduced frequency (RF) on the burst point (BP), helical mode instability, pressure fluctuations, and dynamic pitching stability of a delta and double-delta wings (DW and DDW) in detail. They observed that all the pitching motions postponed the vortex breakdown, and the time-averaged location of the BP has different changes in three sections, divided by a pair of critical RFs, which has been found in several previous experiments. They also indicated, when the upstream speed of the BP is higher than the downstream speed, the variation in the pitching moment is ahead of the AOA, which leads to dynamic instability. Conversely, when the downstream speed is higher than the upstream speed, the DDW is dynamically stable. [PERSON] et al. [15] studied the unsteady aerodynamic characteristics of a 50\({}^{\circ}\) sweep delta wing performing pitching oscillations at angle of attack, between 0\({}^{\circ}\) and 60\({}^{\circ}\), in numerical and experimental methods. They found that both the lift hysteresis and an unsteady phenomenon of leading-edge vortex evolution were significantly influenced by the pitch rate of the wing. [PERSON] et al. [16] also indicated that the hysteresis effect of lift is magnified with the increasing reduced frequency. The virtual camber effect would dominate the flow field at a sufficiently high reduced frequency, which mitigates the impact of wing geometry on the lift characteristics. Except on aircrafts, the study of dynamic stall on horizontal axis wind turbines (HAWTs) was also conducted. [PERSON] et al. [17] analyzed how dynamic stall varies throughout the blade span of a 10 M HAWT, during yawed and unyawed operating conditions. [PERSON] et al.