**4. Conclusions**

In this paper, the CFD approach with LES turbulent modeling is utilized for the double flapper nozzle servo valve oscillating flow numerical analysis. The vortex street flow phenomenon could be observed when the flow passes the nozzle flapper channel, the vortex alternating in both sides producing the periodical flow oscillation. With detailed discussion, it could be determined that:

(1) Without the influence of the main valve, the greater the inlet velocity is, the greater the average pressure on the flapper is, and the lower the main frequency of pressure oscillation is.

(2) Without the influence of the main valve, the larger the actual distance between the nozzle and the flapper is, the smaller the average pressure on the flapper is, and the larger the main frequency of pressure oscillation is.

(3) When considering the influence of the main valve, the change trend of hydraulic power and the main frequency of the servo valve flapper are rarely changed, but the main frequency of the pressure oscillation is significantly lower than that without considering the main valve.

**Author Contributions:** Investigation S.L. and L.L; Simulation and Analysis S.L.; Methodology L.L.; Software S.L.; Writing and Editing S.L., K.Z. and L.L.; Validation L.L. and K.Z.

**Funding:** The authors are grateful to the National Natural Science Foundation of China (no. 51605333 and no. 51805317) for financial support.

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

## **Nomenclature**

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