# Effect of Impact and Bearing Parameters on Bird Strike with Aero-Engine Fan Blades
## 4 Results and Discussion
### Bird Initial Velocity and Fan Rotational Speed Influence  
The influence of the bird's initial velocity (three values considered in [PERSON]'s experiment) and the fan's rotational speed (three typical values) on bird strikes was also investigated (Figure 10). According to the curve of bird impact force variation, the peak values of the bird's impact force decrease with the decrease in the initial velocity (359, 278 and 218 kN for 253, 225, and 116 m/s). The difference in flight velocity leads to different contact duration with the blades as well as different sizes of cut bird blocks, see Figure 10e. The peak force of cases V1 (i.e., 225 m/s) and V2 (i.e., 116 m/s) occur later in the contact force curve as compared to case G (i.e., 253 m/s), and they also have a higher number of peaks (Figure 10a).  
Figure 10b shows that decreasing the bird velocity from 253 to 225 m/s has little effect on bearing force. However, reducing the bird's initial velocity to 116 m/s has a significant effect on the recorded bearing force due to the differences it causes in the number of bird blocks. The case of V2 (i.e., 116 m/s) includes an additional big block of bird impacting the fan at t = 5.5 ms, resulting in a large bearing force of 247 kN, which can easily cause damage to the bearing system. As it can be seen in Figure 10d, there is not much difference in the maximum level of von Mises stress for different initial bird velocities. However, different numbers and sizes of bird blocks have led to different blade deformations.  
Comparing the magnitude of bird contact force at the same initial bird velocity but at different fan rotational speeds shows that decreasing the rotational speed of the fan decreases the impact force. The maximum impact force of cases G, R1 and R2 are 359, 66.9 and 63.7 kN, respectively. As compared to the effect of a decrease in the bird's initial velocity, a decrease in rotational speed has a greater effect on the value of impact force. It can be seen in Figure 10b,c that decreasing the rotational speed has a significant reducing effect on the bearing forces (90.1, 44.1, and 13.9 kN for rotational speeds of 600, 395, and 88 rad/s) and the stress levels (1140, 553, and 281 MPa for rotational speeds of 600, 395, and 88 rad/s). The maximum stress of cases G, R1, R2 were similar after the bird strike, reaching 1280, 1190 and 1191 MPa, beyond the yield stress of the blade. In Figure 10d,e, the von Mises stress maximums are close at different initial bird velocities, but the extents of blade damage and bird deformations are different, the red area of stress at high bird velocities (253 and 225 m/s) is significantly wider than at low bird velocity (116 m/s). Therefore, it can be considered that when the bird's initial velocity differences are significant, the greater the bird velocity, the more obvious the blade deformation.  
The bearing force is mainly determined by the fan's rotational speed, and the bird's initial velocity has little influence on it. The kinetic energy change in Table 7 shows that the fan blades lose higher levels of kinetic energy as the fan's rotational speed increases. The bird gains high levels of kinetic energy in the cases of G, V1, and V2 (57.16, 60.12 and 69.08 kJ, respectively). However, the bird only gains 8.75 kJ kinetic energy in the case of R2, and it even loses 5.29 kJ in the case of R1. The engine energy loss has little relationship with the bird velocity (Table 7). Therefore, it can be concluded that the greater the bird's initial velocity is, the more extensive the blade deformation becomes.