**Liang Lu 1,\*, Shirang Long <sup>1</sup> and Kangwu Zhu 2,3**


Received: 27 August 2019; Accepted: 8 October 2019; Published: 11 October 2019

**Abstract:** The oscillating flow field of the double nozzle flapper servo valve pre-stage is numerically analyzed through Large Eddy Simulation (LES) turbulent modeling with the previous grid independence verification. The vortex street flow phenomenon can be observed when the flow passes through the nozzle flapper channel, the vortex alternating in each side produces the periodical flow oscillation. The structural and flow parameter effects on the oscillating flow are emphasized, and it could be determined that the pressure on the flapper is nearly proportional to the flow velocity and inversely proportional to the actual distance between the flapper and the nozzle. On the other hand, the main frequency of oscillation decreases with the velocity and increases with the distance between the nozzle flapper. The main stage movement is further considered with a User Defined Function (UDF), and it could be determined that the influences of the structural and flow parameters on the flow oscillation are rarely changed, but the main frequencies drop, generally.

**Keywords:** double flapper nozzle servo valve; Karman vortex; self-sustained flow oscillation; computational fluid dynamics
