**5. Conclusions**

Equipped with vortex generators, the coil end aims to solve the local overheating of the oil-cooled motor, so as to increase the efficiency of the oil-cooled motor. The oil phase is used as the coolant in the fluid domain. Firstly, the numerical model is verified and compared with the experimental data in the references. On this basis, the improved dimensionless number *R* is used to optimize the attack angle, longitudinal distance, and transverse distance of the vortex generator. The main conclusions are shown below:


performance of *FD* with a longitudinal distance of4h(*FD-*4*h*) is more prominent, and when the Reynolds number is greater than 4000, the performance of *FU* with a longitudinal distance of4h(*FU-*4*h*) is superior.

3. Through the numerical simulation of the transverse distance of *FU-*4*h* and *FD-*4*h*, it can be concluded that, when the Reynolds number is less than 4000, the enhanced heat transfer performance of *FD-*4*h* is extraordinary, and when the Reynolds number is greater than 4000, the improved heat transfer performance of *FU* with a longitudinal distance of 4 h and transverse distance of 0.5 h (*FU-*4*h* − 0.5*h*) is better.

**Author Contributions:** Conceptualization, J.Z. and B.Z.; methodology, J.Z. and B.Z.; software, J.Z., S.Y. and B.Z.; validation, J.Z. and B.Z.; formal analysis, J.Z. and B.Z.; investigation, J.Z. and B.Z.; resources, B.Z., S.Y. and X.F.; data curation, J.Z. and B.Z.; writing—original draft preparation, J.Z.; writing—review and editing, J.Z., B.Z., S.Y. and X.F.; visualization, J.Z.; supervision, X.F.; project administration, X.F.; funding acquisition, X.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China, grant number 51879191.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

**Acknowledgments:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
