## 4 Discussion
### Flow Field Details  
The flow field details of the reference rotor, GA case rotor and the result rotors after modification by the agent were analyzed and compared in this section, indicating that the improvements were convictive and explicable. The combination of sweep, lean, and change in other design variables contributed to the improvements. The forward sweep of rotor blades can improve the efficiency and the stable operating range, according to [PERSON] et al. [39]. The sweep feature changed the shape of the passage shock, so the shock loss and pressure ratio of GA case 1 were reduced. [PERSON] [40] affirmed that the shock waves in the tip region of the transonic rotor are essential for the pressure ratio, so the local pressure ratio of the GA case decreased at the tip region. The agent improved \(\chi_{in}\), reduced \(\chi_{out}\), and simultaneously changed \(\beta_{y}\) to increase the pressure ratio. Figure 18 shows the isentropic Mach number distribution at 70% and 90% spans. The GA step decreased the isentropic Mach number of the suction surface, showing that the shock wave was weakened. Then, the RL agent moved the peak isentropic Mach number location toward the leading edge, with the value remaining unchanged. At the pressure surface, the isentropic Mach number near the leading edge declined upon the GA process and reduced even further upon the agent's modification. Figure 17: Deviation of the local performance distribution in different spans: (**a**) pressure ratio, (**b**) efficiency, and (**c**) absolute flow angle. The pressure gradient perpendicular to the end wall must be zero, according to the illustration of [PERSON] and [PERSON] [41]. Therefore, back sweep at the tip region would reduce the load of the rotor near the trailing. The relative Mach number of the reference rotor, GA case 1, and the rotor after 17 steps of modification of agents was plotted in Figure 19 to visualize the separation region of the rotors. The load reduction and the airfoil segment angle change downsized the separation near the tip, which is one of the reasons why the local \(\eta\) increased. Then, the separation did not increase after agent modification, which helped maintain efficiency. As marked in the 90% span surface, the shock wave at the leading edge and in the passage could be identified. The forward sweep feature diminished the Mach number before the shock waves compared to the reference, as shown in Figure 19b, which reduced the shock loss and the pressure rise. After modification by RL agents in Figure 19c, the shock wave at the leading edge became stronger and consequently increased the pressure ratio, while the passage shock wave remained nearly unchanged. In addition, since the shock waves changed, the peak relative Mach number at 50% and 70% span also lowered slightly. A combined lean feature was also introduced and modified. According to [PERSON] and [PERSON] [42], a lean feature caused unloading near the end wall and overloading around the mid-span if the angle between the end wall and the suction surface was obtuse. [PERSON] et al. [43] explained that lean features work because they generate a pressure gradient and drive low-energy flow through the radial direction. Figure 20 showed the radial velocity \(\mathrm{Vr}\) distribution at the 80% length axial surface to analyze the radial secondary flow. The air near the suction surface flowed from the hub to the shroud because of the centrifugal force and was influenced by the radial pressure surface. The maximum \(\mathrm{Vr}\) near the suction surface and the minimum \(\mathrm{Vr}\) at the same spanwise location were plotted for each rotor, and it was apparent that \(\mathrm{Vr}\) declined after introducing the lean feature. As the RL agent modified the rotor further, \(\mathrm{Vr}\) increased slightly because the lean feature was slightly diminished. Figure 18: Isentropic Mach number distribution through the chord line at spans of 70% and 90%: (a) 70% span; (b) 90% span.