## 4 Thermal Design Investigations with the Validated CFD Model
### Effects of Turbulence Intensity  
Turbulent flow, characterised by chaotic changes in the pressure and flow velocity, significantly enhances the mixing of fluid particles, thereby improving heat transfer rates from the motor components to the cooling medium. Forced convection, a mechanism through which fluid motion is generated by an external source (such as fans or pumps), further augments this effect by actively removing heat from the surface of the stator windings. This concerted action of turbulent flow and forced convection ensures the efficient dissipation of heat, crucial for maintaining the operational integrity and extending  
Figure 8: An Iso-Surface created across the stator windings.  
Figure 9: Rise in temperature of winding hotspot at air speeds ranging from 10 m/s to 100m/s. The ambient air temperature is 288 K.  
the lifespan of permanent magnet motors. Recognising the complexity and importance of these thermal dynamics, we highlight their application in the thermal design of stator windings, presenting a more detailed understanding of the strategies that can be employed to enhance cooling performance in electrical machines.  
Prior studies [21; 22] have shown that an increase in turbulence intensity enhances the heat transfer coefficient, thereby improving cooling effectiveness on surfaces such as plates or cylinders. Building on this foundation, in our study, an investigation into the effect of freestream turbulence intensity on heat transfer across stator windings surfaces was conducted. Figure 10 illustrates the surface temperature distributions on the stator windings under conditions of low turbulence (0.5%), moderate turbulence (10%), and high turbulence (40%), at a wind speed of 10 m/s.  
The findings highlight the significant role of freestream turbulence intensity on localised heat transfer. With a constant air speed of 10 m/s, the maximum surface temperature on the upwind-side end winding with a turbulence intensity of 0.5% is found to be 12 K higher than at a turbulence intensity of 40%, and 9 K higher on the downwind-side end winding. In the low-turbulence scenario, it is noted that laminar flow separation occurs prominently above the stator tooth, creating a large separation area above the stator core back. This separation hinders the airflow from efficiently dissipating heat away from the stator, thus lowering the heat transfer coefficient on the surface and resulting in suboptimal cooling performance. As the turbulence intensity increases to 10%, the previously open flow separation zones on the core back and beneath the rotor transform into closed separation areas, known as laminar separation bubbles in fluid dynamics, markedly affecting the thermodynamic and aerodynamic efficiency [23]. With a turbulence intensity of 40%, the adverse-pressure-gradient induced flow separation is minimised due to a more energetic flow, resulting in extensive areas of attached flow on both the upper and lower surfaces. The reduction in the separation bubble's size and its shift towards the very leading edge of the stator and rotor enhance the surface heat transfer coefficients, thus significantly improving the cooling efficiency.  
Increasing the turbulence intensity to apply this effect is comparatively straightforward. Globally, inflow turbulence can be induced upstream of the heat source by employing turbulence grids, while controlling the overall turbulence intensity through varying the grid sizes. Additionally, local effects can be adjusted by altering the local turbulence intensity, potentially through the implementation of micro vortex-generators [24] or by modifying the local surface curvature [25].  
Figure 10: Winding temperature distributions at airflow speed of 10 m/s with an inflow turbulence intensity of (**a**) 0.5%, (**b**) 10%, or (**c**) 40%. The flow direction is from left to right in all of the cases.