# Numerical Study on Dynamic-Stall Characteristics of Finite Wing and Rotor
## 1 Introduction  
To satisfy the flight requirements of a helicopter in forward flight, rotor blades are designed to perform pitching, flapping, and rotation movements. Therefore, rotor blades work in unsteady aerodynamic environments compared with a fixed-wing aircraft. As a result, the aerodynamic characteristics of helicopter rotor are much more complex, and the dynamic-stall phenomenon [1; 2] usually occurs on the rotor blades of a helicopter in forward flight. Consequently, many negative influences, such as stall flutter and vibration and noise increase could arise due to dynamic-stall effects. Therefore, dynamic-stall characteristics have significant influence on the aerodynamics performance of a helicopter. Hence, studies for improving aerodynamic rotor performance would directly face the dynamic-stall phenomenon. As a result, investigations of the dynamic-stall phenomenon have received considerable attention in recent years. Aiming to develop a full understanding of rotor dynamic-stall characteristics, many investigations on two-dimensional (2D) rotor airfoil dynamic stall were done in the last 40 years to reveal the physical characteristics of this phenomenon, including experimental [3; 4; 5], theoretical [6; 7], and numerical methods [8; 9; 10]. However, the dynamic-stall characteristics of an airfoil are obviously different from those of a rotor. As a result, some studies on the three-dimensional (3D) dynamic-stall characteristics of a finite wing or rotor were done by employing experiment and numerical methods. [PERSON] [11] researched the 3D flow structures that arise from 2D airfoil/wall interactions by using the smoke-wirevisual method, and a simple vortex image model was established to describe the 3D vortex structures of the dynamic stall. [PERSON] [12] also performed an experiment to investigate the dynamic-stall characteristics of a 3D wing. The test results indicated that the wing-tip vortex could delay dynamic stall due to the reduction of the effective angle of attack (AoA) for an unswept wing. Interaction between the tip vortex and wing was reduced for a swept wing. Later, by using Particle Image Velocimetry (PIV) technology, [PERSON] [13] measured the 3D flowfield of a fully equipped helicopter model. The measured results revealed that a large-scale dynamic-stall vortex appeared at a 50%-60% radius on the retreating blade, and rotor rotation would have a stabilizing effect on the formation and convection of the dynamic-stall vortex. [PERSON] [14; 15; 16] investigated the 3D flowfield over a retreating blade by using the Stereoscopic Particle Image Velocimetry method under a dynamic-stall condition. This research demonstrated that centrifugal effects played a strong role on radial flow in the separated flow regions, and affected dynamic-stall vortex formation. Recently, [PERSON] [17] performed a new experiment by employing the Differential Infrared Thermography (DIF) method to investigate the dynamic-stall characteristics of a rotor. This study indicated that the DIF method could differentiate the stalled and attached flow with no contacted-blade surface treatment. [PERSON] [18] investigated the dynamic-stall characteristics of a rotor blade with parabolic-tip geometry. The experiment results indicated that the tip vortex could reduce the lift near the blade tip under a static condition. However, the lift remained higher, near the blade tip, under the dynamic-stall condition due to the influence of vorticity accumulation. With the development of computer technology, the computational fluid dynamics (CFD) method has the advantages of less time consumption and lower cost compared with the experimental methods. Therefore, the numerical method was widely used in 3D dynamic-stall studies. By employing CFD, a numerical study about the 3D dynamic stall of a wing with an NACA0015 airfoil was performed by [PERSON] [19]. It was indicated that wind sweep could delay dynamic-stall vortex formation and decrease aerodynamic force peaks compared with a 2D condition.