## 3 Wind Tunnel Validation for CFD Simulations  
Prior to advancing with additional numerical analyses, it is essential to validate the CFD simulation model against empirical data. In this study, wind tunnel experiments were performed on a constructed stator winding prototype to assess its thermal behaviour under specific airflow conditions. For this, the experimental setup was divided into three segments, including two lateral sections that offered structural integrity and enclosed the central testing segment, which mirrors the genuine stator winding arrangement, as shown in Figure 4. The lateral segments were fabricated using high-temperature resin, chosen for its thermal resistance of up to 238 \({}^{\circ}\)C, ensuring the model's durability under high temperatures. The central testing section employed materials that closely simulate those found in the actual machinery, including iron teeth and copper windings, to achieve a realistic representation. This section was composed of five iron teeth units and three copper winding units, supplemented by two 3D-printed winding blocks designed to minimise thermal leakage at the sides. The configuration of the middle section meticulously aligned with that of the original simulation design, incorporating slot liners but omitting impregnation to mirror the simulated conditions as accurately as possible. It is noteworthy that the rotor was excluded from the CFD model as well as from the experimental framework, based on the premise that rotor-associated losses are not under consideration for this particular analysis. This decision ensures that the CFD simulation  
Figure 3: The CFD simulation domain with boundary conditions. model directly corresponds with the experimental setup, focusing solely on the stator's thermal performance for validation purposes. Our experimental investigation was performed within an open-return wind tunnel. The dimensions of the tunnel's test section were specified as 350 mm in height, 450 mm in width, and 3000 mm in length. Depending on the Reynolds number, the turbulence intensity (TI) within this section ranged from 0.4% to 0.6%, and the airflow velocity could be adjusted up to 35 m/s. The setup of this experimental arrangement is detailed in Figure 5, utilising the generated data to corroborate the numerical simulations and to refine the model as needed. Through this calibration process, the model is enhanced for more accurate CFD analyses concerning the motor assembly. Figure 4: Schematic illustration of the experimental setup and the assembled model for wind tunnel testing, which serves as a basis for the CFD simulation domain and boundary conditions. Figure 5: The testing setup and configuration of the wind tunnel experiment. The solid arrowhead represents the direction of airflow within the wind tunnel, while hollow arrowheads function as indicators, pointing to and labelling specific experimental instruments. Five Type-K plug-in thermocouples were used in temperature monitoring within the experiment, strategically placed at five distinct locations to capture temperature variations accurately. These positions are illustrated in Figure 6, and ensured that all end-winding measurement sites were positioned on the downstream side to prevent any disturbance to airflow. Precision in the temperature measurements was optimised by situating all sensors at the central unit of each tooth and winding assembly. Additionally, ambient temperature readings were collected to establish temperature differentials across all five designated points. A prerequisite for data collection required the wind tunnel's operation for a minimum duration of 60 min, ensuring the establishment of a steady-state condition prior to logging any measurements.