Study On Gas-liquid Two-phase Flow And Damping Characteristics Of Landing Gear Oleo-pneumatic Shock Absorber

At present,the research on the performance of the landing gear oleo-pneumatic shock absorber is mostly based on the single-phase flow of oil,which cannot well reflect the internal gas-liquid two-phase flow mechanism of the shock absorber during its movement,so there are certain limitations.In this paper,a certain type of landing gear oleo-pneumatic shock absorber was taken as the research object,and its simplified calculation model was established to numerically simulate the transient gas-liquid two-phase flow inside the shock absorber during its movement.On this basis,the heat transfer characteristics of gas-liquid two-phase flow and damping characteristics were studied.This research can provide some references for the structural design and performance optimization of landing gear oleo-pneumatic shock absorber.The main research contents of this paper are as follows:The basic principle of CLSVOF（coupled level set and volume-of-fluid）method was expounded,and the CLSVOF method and VOF（volume-of-fluid）method were respectively used to preliminarily simulate the internal transient gas-liquid two-phase flow in the compression process of the shock absorber.The results show that compared with VOF method,CLSVOF method can more accurately simulate the distribution and variation of oil-gas twophase in the nitrogen chamber.The CLSVOF method can weaken the spurious currents near the oil-gas two-phase interface in the relatively static state of the shock absorber,and reduce the numerical calculation error.At the same time,it also shows that the importance of improving the numerical calculation method of the gas-liquid two-phase interface motion process to weaken the spurious currents.This numerical simulation verifies that the CLSVOF method is more advantageous than the VOF method.At normal temperature,the calculation method and parameter setting were improved,and the Peng-Robinson real gas model and the CLSVOF gas-liquid two-phase interface capture method were used to numerically simulate the whole motion process of the shock absorber.On this basis,the damping characteristics of the shock absorber were studied.The numerical simulation accurately describes that the oil flows out of the main orifice in the form of turbulent jet and collided with the upper wall of the nitrogen chamber,resulting in complex phenomena such as oil-gas two-phase mixing and wall attached flow.In the study of damping characteristics,the peak value of oil damping force calculated on the basis of the flow field analysis is lower than the engineering results when the discharge coefficient values is fixed.Moreover,there is a certain hysteresis characteristic compared with the engineering results.Large amount of oil damping energy loss is concentrated in the shock absorber compression process.Considering the importance of the shock absorber compression process to the study of shock absorber performance,the influence of the compressibility of oil at different initial temperatures on the damping force was studied for the shock absorber compression process.The results show that the gas-liquid two-phase flow state is quite different at different initial temperatures.The damping force curve is not ordered distribution,not strictly following the initial temperature conditions and showing complex variability.Temperature and oil compressibility factors affect the change of damping force by influencing gas-liquid two-phase flow state and flow field pressure respectively.For the complex gas-liquid two-phase flow and heat transfer in the nitrogen chamber of the shock absorber,the effects of temperature and oil compressibility on the internal flow and heat transfer process were studied from the perspective of wall average heat transfer coefficient.The results show that the average heat transfer coefficient of the upper wall,the lower wall and the side wall of the nitrogen chamber have their own variation laws during the compression process of the shock absorber.At different initial temperatures,due to the gas-liquid two-phase flow state,the flow field development law is different,and the variation law of the wall heat transfer coefficient is significantly different.Considering the compressibility of oil affects the calculation of flow field pressure,which will affect the prediction of wall heat transfer coefficient.