## 3 Wind Tunnel Validation for CFD Simulations
Upon conducting the experiments at an airflow speed of 10 m/s and a turbulence intensity of 0.5%, temperature increases across various locations were recorded, with findings detailed in Figure 7. In response to these empirical outcomes, simulation parameters were fine-tuned to align with the experimental conditions, facilitating a direct comparison between the simulated and actual temperature rises, as illustrated in the same Figure. The strategy of employing an equivalent anisotropic lumped material to simulate the composite coils within the CFD framework demonstrated remarkable effectiveness. This approximation method successfully mirrored the thermal behaviour observed in the experiments, especially in predicting temperature rises on the surface of both middle and end windings. This comparison underscores the validity of the simulation approach, highlighting its potential as a reliable predictor of thermal dynamics in practical applications. The  
Figure 6: Thermal monitoring positions demonstrated in an extracted unit. Position 2 (P2) is at the symmetrical position of Position 1 (P1). Hollow circles indicate the positions where the surface temperature is monitored, while solid circles denote the positions where the temperature is monitored 5 mm below the surface. Figure 7: Comparison of experimental and numerical results at five monitoring positions. close match between the simulated results and experimental data reinforces the accuracy of the CFD model, suggesting that the simplifications made for the simulation did not significantly compromise the integrity of its predictions regarding thermal performance. The CFD results slightly overestimated the temperature of the stator at Position 1 and Position 5, but generally provide a good match. The small overestimation indicates that the heat transferred through conduction (winding \(\rightarrow\) slot liner \(\rightarrow\) stator) in the experiment is less than that in the CFD simulation, as Positions 1 and 5 are in the stator rather than the winding. The reduced conducted heat in the experiment is due to the unavoidable existence of air pockets inside and around the windings in the experimental coil. These air pockets introduce extra thermal resistance to the winding block, reducing the thermal conductivity between the winding and the slot liner/stator and hence leading to a lower temperature in the stator compared to the CFD simulations. Other reasons should also be considered, such as the errors introduced by simulating heat transfer between different materials, including lumped winding blocks, slot liners, and stator tooth units, and the accuracy of the measuring positions due to the small size of the tooth. It is noted that the reduced thermal conductivity due to the existence of air pockets should result in overestimated temperature values in the winding in the experimental testing, whilst the CFD results match the experimental measurements quite well at Positions 2, 3, and 4. This is possibly due to the factors such as the roughness of the winding surface and effective contact surface area between the winding and the air. In real life, the winding surfaces are formed through many turns of the copper wire. The surfaces are hence not flat, but rather full of surface irregularities introduced by the individual conductors. These \"local bumps\" effectively increase the roughness of the winding surface, as well as the contact area with the airflow, leading to higher heat transfer coefficients and hence a lower temperature value in the winding in the experiment testing. This underestimation counteracts the overestimation due to the air pockets, resulting in matching results at the winding position. The CFD simulation model receives robust validation from the wind tunnel experiments across all five monitored positions, once stable temperature increments are realised experimentally. The differences observed between the empirical findings and the numerical predictions remain within a narrow margin of 4 K for each of the monitored locations.