# Analysis of the Transonic Buffet Characteristics of Stationary and Pitching OAT15A Airfoil
## 1 Introduction  
In transonic flight, the interaction between the shock wave and the boundary layer has been one of the most significant research topics in the aerospace science community in recent decades [1]. Transonic buffet is a special type of interference between the shock wave and turbulent boundary layer, which is a self-sustained, low-frequency oscillation with the fluctuation of aerodynamic loads and belongs to the transonic-flow instability phenomenon. In transonic flow, a shock wave reciprocates on the lifting surface while interacting with the separated boundary layer, resulting in periodic oscillations of the flow field. The shock motion and the resulting flow field oscillations can significantly affect the overall aerodynamic forces and moments, which, in turn, could have an impact on the coupled elastic structure's overall performance and maneuverability. Therefore, shock buffet has been one of the most important factors affecting the design of transonic vehicles. Transonic shock buffet involves complex flow mechanisms, such as shock wave/ boundary layer interference, which are mostly investigated by numerical simulation and wind-tunnel experiments. [PERSON] [2] proposed a mechanism model based on acoustic wave propagation feedback, which was considered a feedback phenomenon of the pressure waveand trailing edge acoustic wave generated by the shock wave motion. This model is only able to effectively predict the buffet frequency of some airfoils [3; 4]. [PERSON] et al. [5; 6] proposed the theory of buffet onset, in which the linear stability analysis of the flow field attributes the self-excited oscillations of the shock wave to the occurrence of global unstable aerodynamic modes. This theory, based on unsteady mode analysis, was validated in subsequent wind-tunnel experiments [7; 8] and numerical simulations [9]. In early studies of transonic shock buffet flow, its structure was usually considered a rigid body, ignoring the interaction between the fluid and the structure. With the in-depth investigation of buffet flow, the study of the effect of structural motion on the shock buffet response characteristics has received the attention of many scholars [10; 11]. [PERSON] et al. [12; 13] investigated the effect of the pitching motion of Naca0012 airfoil on the transonic buffet load through numerical simulations and found the phenomenon of frequency lock-in; that is, when the amplitude of the structural vibration reaches a certain value, the frequency of the flow field response locks at the structural vibration frequency. Subsequently, [PERSON] and [PERSON] [14] used unsteady Reynolds-averaged Navier-Stokes (URANS) coupled with Computational Structural Dynamics (CSD) to investigate the effect of structural elasticity on the buffet response. It was found that the relative ratio of the natural frequency of the structure to the buffet frequency of the flow field, as well as the amplitude of the structural oscillation, directly affects the aeroelastic responses. Based on the aerodynamic Reduced Order Model (ROM) method, [PERSON] and [PERSON] [15] classified and analyzed the transonic buffet response from the point of view of the coupling between the fluid and structural modes. As seen in previous studies, when considering the effect of structural elastic motion on shock buffet, the research mainly focuses on the analysis of aeroelastic responses, such as aerodynamic force and structural displacement [16]. Nevertheless, there are relatively few studies on the response of the flow field and the flow modes within one period of structural motion. The current study focuses on the characteristic analysis of flow physics when a supercritical airfoil is in buffeting flow. This is carried out by simulating the transonic flow field responses of the OAT15A airfoil with a fixed or pitching state under flight conditions that exhibit strong shock oscillations. By analyzing and comparing the unsteady features of the flow field response in the two cases, insights into the nature of the buffeting flow are provided.