**6. Conclusions**


the modal experiment and the theoretical one determined by the analytical method, showing that good agreement with the first-order natural frequency of the jet system was found.

**Author Contributions:** Conceptualization, X.Y. and C.W.; methodology, X.Z.; investigation, X.Z. and C.W.; writing—original draft preparation, X.Y.; writing—review and editing, C.W. and X.Z.; supervision, L.Z. and Y.Z.

**Funding:** This work is funded by the National Natural Science Foundation of China (No. 51805468, 51805214), Natural Science Foundation of Hebei Province (No. E2017203129), Open Foundation of the State Key Laboratory of Fluid Power and Mechatronic Systems (No. GZKF-201820), Basic Research Special Funding Project of Yanshan University (No. 16LGB001), and China Postdoctoral Science Foundation (No. 2019M651722). The authors would also like to thank the reviewers for their valuable suggestions and comments.

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