Study Of The Flow-induced Vibration Characteristics In A Nozzle-flapper Valve Affected By The Disturbances Of Oil Viscosity And Pressure Pulsation

Electro-hydraulic servo system is widely applied in modern industry fields owing to its high power to mass ratio,accurate control,fast dynamic response and well load matching.As a bridge between the electron and hydraumatic convertor,the servo-valve is the core component of the electro-hydraulic servo system.The nozzle-flapper valve is often used as the pilot stage of the two-stage servo-valve because of its low movement inertia and highly sensitive.However,two problems frequently occur from these electro-hydraulic servo system in practical applications.On the one hand,it leads to oil temperature rising and oil viscosity decreasing in a short time.On the other hand,intense pressure pulsations at the nozzle inlet are often caused by the coupling between the high frequency pulsation of the pump and pipeline impedance.Up to several thousand hertz noise is often found from the nozzle-flapper valve in lots of engineering examples,which is closely associated with the flapper vibration and adversely flowinduced phenomena in the nozzle-flapper valve.Therefore,the influences of the external disturbances including the oil viscosity,the frequency and amplitude of the pressure pulsations at the nozzle inlet on the flow field in the nozzle-flapper valve and flapper assembly vibration are studied in this paper.In this paper,the dynamical equations of the flapper assembly considering the lateral force from the nozzle-flapper valve are established.The influences of the lateral force on the stability of the flapper assembly are analyzed.Furthermore,in view of the phenomena of the pressure pulsation nonlinearity propagation characteristics in the pipeline and cavitation transient in the nozzle-flapper valve,the mathematical model of the flow field in the nozzle-flapper valve in consideration of the oil viscosity and compressibility is established.The numerical simulation methods on the flow field characteristics in the nozzle-flapper valve considering the external disturbances of the pressure pulsations are presented based on this mathematical model.The numerical model of the nozzle-flapper valve including the connected pipeline is established in this paper.The flow field distribution in the pipeline and nozzle-flapper valve is simulated by Computational Fluid Dynamics(CFD)simulation at different oil viscosities.The influences of the oil viscosity on the cavitation transient,pressure pulsation and lateral force in the nozzle-flapper valve are numerically investigated.The relationships between the cavitation shedding,small scale vortex propagate,pressure pulsation and lateral force are analyzed.The CFD simulations of the nonlinearity propagation characteristics of the pressure pulsation in the pipeline regarding the nozzle-flapper valve as the load are performed by using CFD software.The change law of the pressure pulsation at the nozzle inlet is achieved by changing the pulsation frequency of the pump and pipeline length.The external disturbances including the frequency and amplitude of the pressure pulsation before the nozzle inlet are obtained.Then,the influences of the frequency and amplitude of the pressure pulsation disturbances at the nozzle inlet on the cavitation,pressure pulsation and lateral force in the nozzle-flapper valve are concluded by means of CFD simulations.The fluid-solid interaction(FSI)model of flow feild in the nozzleflapper valve and flapper assembly is established.The flow-induced vibration characteristics of the flapper assembly at these different external disturbances are simulated.Then,the numerical model with the Helmholtz resonator at the nozzle inlet is set up.The attenuation law of the pressure pulsation disturbance before the nozzle is numerically studied.The suppression influences of the attenuate external disturbance at the nozzle inlet on the cavitation-induced pressure pulsation,lateral force and flapper vibration in the nozzle-flapper valve are numerically analyzed.Based on the simulation results,the influences of the three different external disturbances on the flow field characteristics including cavitation,pressure pulsation in the nozzle-flapper valve are also investigated experimentally.On the one hand,the cavitation observation experiment of the nozzle-flapper valve is carried out.The cavitation transient captured by the camera achieves a good agreement with the simulation result at different oil viscosities,frequencies and amplitudes of the pressure pulsation disturbances at the nozzle inlet.On the other hand,the test experiments for the hydraulic control system are performed.The pressure pulsations at different positions,vibrations at the armature tip and the noise are measured at the three different external disturbances.The nonlinearity propagation characteristics of the pressure pulsation in the pipeline obtained in the simulation are verified by the experiments.The relationships and the variations between the cavitation-induced pressure pulsation,lateral force and flapper vibration affected by the three different external disturbances in the simulation are also validated by the experiment.Finally,the noticeable suppression influences of the resonator on the external disturbance of the pressure pulsation in the pipeline,flow field characteristics,the flow-induced vibration and high frequency noise in the nozzle-flapper valve are also proved in the experiments.