## 1 Introduction  
Wind power generation technology has been maturely developed in the past decades. Airfoil is a basic element of a wind turbine blade, and its aerodynamic characteristics have a major influence on the wind energy conversion efficiency. Among the conventional rotor aerodynamic design strategy, the blade add-ons were of particular interest to further improve wind energy efficiency. Therefore, mounting flap to the airfoil trailing edge was one of the most feasible methods to improve the aerodynamic performance of wind turbines. In addition to power production, such a technique can also effectively reduce the aerodynamic loads both of wind turbine blades and tower. If a sophisticated controller was implemented, the flap can further reduce turbulence-induced fatigue loads, so that longer lifetime was guaranteed. After the pioneering work of [PERSON] [1], a large number of studies have been conducted to explain the phenomena induced by the presence of this device. More recently,research objects were most focused on airfoil attached various shapes of flaps, such as Gurney flaps [2; 3], triangular flaps [4], separate trailing edge flaps [5; 6] and deformable trailing edge flaps [7; 8]. For experiment tests, [PERSON] et al. [9] and [PERSON] et al. [10] studied the aerodynamic effect of a Gurney flap based on the airfoil through wind tunnel experiments. [PERSON] et al. [11] carried out a wind tunnel test of the lift force and pitching moment coefficients of both trailing edge flaps and Gurney flaps with different shape parameters. Based on a 5 MW reference wind turbine, [PERSON] et al. [12] designed and optimized a trailing edge flaps such that the blade mass can be further reduced but still maintain the desired power performance. [PERSON] et al. [13] explored the flow mechanisms of a flap at a high deflection angle. When the flap works in a separated flow region, he provided some ideas for realizing instantaneous action or alleviating extra aerodynamic loads on wind turbines. [PERSON] et al. [14] studied the flap tip vortexes and characterized the flow structures behind a flap in a low-speed wind tunnel by using particle image velocimetry. [PERSON] et al. [15] designed a trailing edge flap by using a single medium plasma driver which resulted in a higher lift force. [PERSON] et al. [16] designed an active controller of a trailing edge flap on a 5 MW reference wind turbine and proved that the flap could effectively control aerodynamic loads. [PERSON] et al. [17] also conducted experiments on a flaps noise drop. According to the wind tunnel tests based on a full-size rotor, [PERSON] et al. [18] found that flaps could also be used to control noise and vibration such that the noise generated by blade vortex interaction could be reduced by 6 dB. For numerical simulations and theoretical analyses, [PERSON] et al. [19] fitted a semi-empirical equation by summarizing the performance of a large number of flaps. [PERSON] et al. [20] numerically solved the unsteady flow field of Gurney flaps at high Reynolds numbers through the discontinuous Galerkin method. By analyzing dynamic characteristics of an airfoil with Gurney flaps through the numerical simulation, [PERSON] et al. [21] found that flaps were capable to reduce unsteady aerodynamic loads of wind turbines. [PERSON] et al. [22] numerically simulated the airfoil with trailing edge flaps using the immersed boundary method and found that flaps could be combined with the paddle movement to adjust the aerodynamic loads of a wind turbine airfoil. [PERSON] et al. [23] investigated the trailing edge flap together with an aeroelastic analysis. It was noted that the trailing edge flap could be a smart device to control aeroelastic deformation. All the above researches on the flaps were built on the uniform inflow of low turbulence intensity. In the past, there were few studies carried out by flow over flaps under high turbulence intensity [24; 25], however, such a flow condition often occurs on wind turbines operating in a wind farm.