## 1 Introduction  
Due to the non-renewability and polluting nature of traditional fossil energy, wind energy has become increasingly important in energy supply with its cleanliness and re-newability [1, 2, 3]. With the rapid development of wind power generation, the demand for power from wind turbines is increasing. However, the losses of permanent magnet synchronous generators will increase with the increase of the single-unit capacity of the generator. The increase in these losses leads to excessive temperature rise of the motor, which is the main cause of motor failure [4, 5, 6]. To address the heat generation issue in large motors, many scholars have conducted research on their cooling systems. [PERSON] et al. [7] conducted a thermal analysis of an 8 MW outer-rotor direct-drive wind turbine generator and employed internal coolant ducts for direct cooling of the stator windings. [PERSON] et al. [8] modeled and analyzed a high-power, high-torque-density direct-drive motor and proposed a novel winding molding method to enhance the cooling effect of the windings. [PERSON] et al. [9] optimized the cooling system of a steam turbine generator, proposing a strategy to adjust gas parameters to suppress temperature fluctuations in the windings. [PERSON] et al. [10] designed a cooling system combining a heat exchanger and a heat sink for a 2.5 MW permanent magnet wind turbine generator (PMSWG), addressing the issue of excessive temperature rise during operation. The fluid-structure interaction method based on computational fluid dynamics (CFD) can achieve accurate analysis of the temperature rise distribution and flow characteristics within the motor [11; 12; 13]. Numerical studies of the fluid in the cooling system are of great significance. Optimizing the cooling structure based on research results is crucial for enhancing the heat dissipation efficiency of generators. [PERSON] et al. [14] conducted numerical studies on the ventilation ducts structure of a 250 MW large hydroelectric generator, exploring the variation patterns of fluid temperature distribution, velocity, and heat transfer coefficient in the ventilation ducts. [PERSON] et al. [15] explored the ventilation system of a novel 350 MW air-cooled steam turbine generator, conducting flow and thermal analyses based on computational fluid dynamics and proposing a multi-chamber downstream cooling path for air-cooled generators. [PERSON] et al. [16; 17; 18] took the motor driven by a high-temperature gas-cooled helium reactor as the research object and proposed a method for calculating gas friction loss using Bernoulli's equation, analyzing the impact of ventilation ducts geometry on temperature distribution. [PERSON] et al. [19] analyzed the ventilation and thermal performance of a radially forced air-cooled fractional-slot concentrated-winding PMSWG. The study provided insights into the impact of the number of cooling ducts and ducts width on the fluid and temperature fields of the generator. [PERSON] et al. [20] analyzed a generator equipped with ventilation ducts, predicting the flow distribution within the stator ducts and proposing geometric designs to achieve higher mass flow rates and improved flow characteristics. At present, there have been some research results in the field of wind turbine research on optimizing the internal radial ventilation duct structure. However, further research is still needed on the optimization of radial ventilation duct structure and fluid characteristics in wind turbines, especially in the field of non-uniform core technology, where there are few research results. Further research can be conducted to improve the non-uniformity of internal cooling capacity and combined with multi physics coupling mechanisms and structural parameter collaborative optimization strategies. This study investigates a 4.5 MW direct-drive permanent magnet synchronous generator (DD-PMSG) incorporating forced air cooling. Based on the fluid-thermal coupling theory, an internal ventilation and cooling model for the wind turbine generator is established.