# Dynamic Stall Characteristics of the Bionic Airfoil with Different Waviness Ratios
###### Abstract  
A dynamic stall will cause dramatic changes in the aerodynamic performance of the blade, resulting in a sharp increase in the blade vibration load. The bionic leading-edge airfoil with different waviness ratios, inspired by the humpback whales flipper, is adopted to solve this problem. In this study, based on the NACA0015 airfoil, the three-dimensional unsteady numerical simulation and sliding mesh technique are used to reveal the flow control mechanism on the dynamic stall of the bionic wavy leading edge. The effects of the waviness ratio on the dynamic stall characteristics of the airfoil are also investigated. The results show that the peak drag coefficient is dramatically reduced when a sinusoidal leading edge is applied to the airfoil. Although the peak lift coefficient is also reduced, the reduction is much smaller. When the waviness ratio R is 0.8, the peak drag coefficient of the airfoil is reduced by 17.14% and the peak lift coefficient of the airfoil is reduced by 9.20%. The dynamic hysteresis effect is improved gradually with an increasing waviness ratio. For the bionic airfoil with R = 1.0, the area of the hysteresis loop is the smallest. airfoil; bionic structure; stall characteristics; aerodynamic performance; flow control; numerical simulation +
Footnote †: journal: Applied  
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Footnote †: journal: Applied  
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Footnote 1: email: _Applied_  
Footnote †: journal: Applied  
experimental investigation to determine the influence of leading sinusoidal protrusions on two different NACA airfoils. For both airfoils, reducing the tubercle amplitude led to a larger stall angle and a higher maximum lift coefficient. The results also suggested that tubercles act similarly to conventional vortex generators. [PERSON] et al. [17] numerically investigated the effect of the wavy leading edge on the hydrodynamic characteristics of three-dimensional wings. They also suggested that the wavy leading-edge tubercle acts as a vortex generator. In addition, [PERSON] et al. [18] provided a necessary perspective for an overall understanding of the mechanism of flow modification by multiple leading-edge protrusions. [PERSON] et al. [19] adopted, as a new parameter for leading-edge protrusion, the asymmetry factor, which determines the peak position along the wavelength direction of the tubercle. Wind tunnel and visualization tests demonstrated that an asymmetric protuberance directly influences the spanwise flow and induces resistance. [PERSON] and [PERSON] [20] studied the hydrodynamic characteristics of a new type of hydrofoil modified by a twin protuberance. Compared with the baseline hydrofoil, the lift coefficient and maximum lift of the modified foil decreased before stalling, but the lift coefficient was higher after stalling. [PERSON] and [PERSON] [21] reviewed both active and passive flow control methods adopted by swimming fish and mammals, and considered numerical and experimental studies of the static aerodynamic characteristics of airfoils, in this context, dating from 1995 to 2004. They concluded that wavy leading-edge tubercles could increase lift and reduce drag. The above studies were all concerned about how a wavy leading edge affected the static characteristics of airfoils. However, we should also consider the effect of the wavy tubercle edge on the dynamic stall characteristics of airfoils. The effects of a wavy leading edge on a pitching NACA 0018 profile were investigated experimentally and numerically by [PERSON] et al. [22]. They found that the increase in lift on the upstroke was higher than that on the downstroke. Additionally, the increase in drag was less than the increase in lift. These numerical simulation results showed that stall is suppressed during the pitching motion of the wing. The static and dynamic characters of a NACA 634-021 airfoil with the sinusoidal leading edge were numerically simulated by [PERSON] et al. [23] using the Spalart-Allmaras (S-A) turbulence model.