## 1 Introduction  
The bird strike is a common hazard of aircraft which has a significant effect on their operation safety. According to the statistics of the Federal Aviation Administration (FAA), wildlife strikes have damaged more than 271 aircraft and have led to more than 292 deaths worldwide between 1988 and 2019 [1]. As shown in Figure 0(a), the wildlife strike frequency on aircraft has been constantly increasing since 1990. In 2019, 17,760 wildlife strikes took place, among which the proportion of bird strike was 94.4%. As shown in Figure 1b, the proportion of engine damage is the highest, reaching 26%, namely, the aero engine is a frequent occurrence area for bird striking [2]. When a bird impacts a jet engine, the fan is struck first, which may cause the failure of fan blades and induce secondary faults. Research on the effect of bird strike on the operation of an aero engine is of great importance to the working safety and reliability of the aero engine. In early bird strike studies, experimental methods were the most valid and popular means of gaining knowledge about bird strike. However, the experimental tests are expensive and difficult to perform [3]. Therefore, numerical simulation methods have been widely used and fully developed in the past 30 years [4]. Several simple geometries have been adopted to represent the shape of a bird. The geometry of the substitute bird models are of four main types: straight-ended cylinder, hemispherical-ended cylinder, ellipsoid, and sphere [5,6]. The hemispherical-ended cylinder has been the most commonly used geometry due to its more accurate results [7]. More recently, bird models with realistic geometries based on CT scan images have replaced the geometric model due to the more accurate results [8,9]. In the early simulations based on finite element analysis, the bird impact load was modelled as a uniform pressure vs. time at the impact area [10]. Afterwards, bird models based on Lagrangian, Eulerian, Arbitrary Lagrangian-Eulerian (ALE), and Smoothed Particles Hydrodynamics (SPH) formulations were developed and used in the bird strike simulations [11, 12, 13, 14, 15, 16, 17, 18]. As for the target fan model, the initial approach was to use a plate with a thickness [19,20]. In more recent studies, fan blades with realistic geometries have become more common, whereafter transient response of rotating is obtained under bird-slicing impact [21, 22, 23]. Meanwhile, different cases of bird strikes with different bird mass and velocity, impact location and timing, and rotary speed have been investigated on the rotary engine [24,25]. In this paper, the SPH bird impactor model has been chosen and the finite element method on a fan model with realistic geometry is applied. Bird strikes often have a dramatic effect on the aircraft's engines. The impact dynamics analysis under bird strike (which can be considered as foreign object impacts) can cause severe deformation in the fan blades, leading to impact-induced force generation [21]. Bird strike experiments on the engine primary compressor under different bird strike conditions were investigated by [PERSON] et al., and a significant effect on the engine performance was found [24]. A bird strike creates a substantial impact force, which may apply significant imbalanced loads on the fan shaft, leading to problems occurring in the rotor performance [26]. Figure 1: Number of wildlife strikes to civil aircraft and the proportion of civil aircraft components damaged by wildlife: (**a**) number of wildlife strikes to civil aircraft during 1990–2019, (**b**) proportion of civil aircraft components damaged by wildlife during 1990–2019. Transient impacts on the rotor system can affect the stability of the whole system. Adding shock excitation to the bearing location of the rotor system can result in a significant impact on the rotor system [27]. The transient impact caused by airflow excitation has an obvious influence on the dynamic response of the rotor system [28].