Research On The Vibration Characteristics Of The Aircraft Hydraulic Energy System

The aircraft hydraulic system is a complex integrated system with multiple redundancies and high power density.Each set of the hydraulic system consists of hydraulic energy system and its load system.The hydraulic energy system is the power core of the integrated system,which mainly includes the pumps,the pipelines,and the pressurized reservoir.Since both the pumps and the pipelines are major sources of severe mechanical and fluid vibrations,vibration has always been a serious problem for the aircraft hydraulic energy system.Moreover,the higher the pressure and the power of the system are,the more serious the vibration problem is.Therefore,related research topic has been an international academic frontier and hotspot.In this thesis,fundamental research has been carried out on the vibration characteristics of the aircraft hydraulic energy system,which not only has high value in academic research,but also has a great potential to the engineering application of improving the performance of the domestic large aircraft hydraulic system.The innovative research in this thesis is summarized as follows:1.A time domain calculation method has been proposed for the axial and transverse fluid structure interaction vibration model of three-dimensional pipelines.The time partial derivatives in the pipelines model were transformed into the first-order non-homogeneous linear differential equations by finite difference approximation.Meanwhile,an analytic integral value of the load term was obtained through polynomial approximation.Then this value was substituted into a transfer matrix for assembling and solving.Hence,in this time domain calculation method,only time partial derivatives were approximated by finite difference,and length partial derivatives were solved by analytic formula.In addition,transfer matrixes were used to assemble the pipelines model,and the dimension would not increase with the length segments.In the meantime,the present time domain calculation methods for the fluid structure interaction vibration model of pipelines are mostly based on the characteristic line,which are only applicable to simple pipelines.Besides,the time step and the segmentation length of the present methods must both be consistent with the characteristic equations.Under these conditions,when these present methods are applied in the complex pipelines,there will be increases in computational errors and the dimension of the model because of the very intricate multiple characteristic lines intersections.However,by adopting the method proposed in this thesis,not only the errors are smaller than the traditional characteristic line method,but also it is unnecessary for the time step and the segmentation length to be consistent with the characteristic equations.Because the transfer matrix form is introduced,the dimension of the pipelines model and the solution difficulty remain unchanged.Thus,the time step and segmentation length of the proposed method can be arbitrarily modified under numerical stability conditions to realize the variable step algorithm.Also the difficulty and complexity of the calculated time domain and frequency domain characteristics of pipelines system are greatly simplified by the uniformity and compatibility of this method.For instance,by comparing the acceleration test values of the typical pipelines,it was found that the calculated errors of this time domain calculation method of the pipelines inlet and midpoint are 2.34%and 1.56%respectively,which are less than the commercial finite element software 7.05%and 4.02%,and the parallel calculation time is only 11.6%of the commercial finite element software.2.The complicated hydraulic energy system with hoses,filters and accumulators has been fully modeled by unifying the time domain and frequency domain procedures,and the system characteristics have been calculated and analyzed.In this thesis,with consideration of hoses,hard pipes,filters,accumulators and load valves,a complex hydraulic energy system model was established by applying the time domain discrete transfer matrix method for the first time,and its time domain and frequency domain results were simultaneously solved.Hence,the key components of the hydraulic energy system were completely contained in this system model,and the steps and forms of the time domain and the frequency domain were unified in the model assembly and solution procedures.In contrast,most current researches on pipelines system characteristics are focused on the modeling of unattached pipeline structures or long hoses,and their time domain and frequency domain analysis are independent.On this account,without the tedious processes of independent modeling and solving in time domain and frequency domain for the hydraulic energy system,this system model can be widely used in modeling and analyzing complicated systems than the present existing methods.Furthermore,the simulation results shown that the four fluid dynamic resonance points,66 Hz,256 Hz,786 Hz,and 414 Hz,are also the forced vibration resonance points of the pipeline structure.These four frequencies were equal to the characteristic frequency of the filter,the fundamental frequency and the third-order frequency of the pipeline before the filter,and the characteristic frequency of the pipelines before the accumulator,respectively.Particularly,the filter has an isolation effect on the vibration that the high frequency pulsation is attenuated into low frequency fluctuation.Thus this low frequency fluctuation is suppressed by the accumulator,and the pressure and flow fluctuations at the system outlet are kept at a small value.When the system is operated at 22 Hz,the characteristic frequency of the accumulator,almost all flow fluctuations in the pipelines system are transiently absorbed by the accumulator.3.The accurate mathematical model of high frequency vibration of the swash plate variable displacement mechanism in the high speed piston pump has been developed.Most of the neglected dampings,long and slim flow channels and load modules in the swash plate variable mechanism were considered for the first time in the accurate mathematical model.Then the influences of structural parameters on the high frequency vibration characteristics of the swash plate variable mechanism were systematically analyzed,and the local sensitivities of all structural parameters to the high frequency vibration of the swash plate were obtained.Because of the sensitivity analysis of all structural parameters,it was easy to propose the optimization measures for attenuating the high frequency vibration of the swash plate.Since the present major research models of the swash plate vibration characteristics of the piston pump are relatively simple,the influence of a high frequency vibration on the swash plate is analyzed either only in considering the torques of the pumping pistons and the variable piston or in studying low frequency dynamic characteristics.As the accurate model is more complicated and more comprehensive than the present ones,it can be used to analyze accurately the high frequency vibration characteristics of the swash plate and obtain optimization measures for the high frequency vibration attenuation.According to the simulation results,the vibration amplitude of the swash plate could be greatly attenuated by either appropriately increasing the outlet volume,the control valve spring stiffness,the variable piston action area,and the variable piston arm length or by reducing the control valve spool diameter and the variable piston volume.Among these measures,the optimization effects of the control valve spool diameter,the outlet volume,and the control valve spring stiffness were the best,and the corresponding ratios of the swash plate vibration attenuation amplitudes to the parameter variation amplitudes were 215%,155%,and 130%,respectively.The thesis is organized as follows:In chapter 1,the application of the aircraft hydraulic energy system is outlined.The research progress of mechanical vibration and fluid pulsation related to the aircraft hydraulic energy system is expounded.The background and significance of the subject are analyzed,and the main research contents and the difficulties of this research are introduced.In chapter 2,the vibration characteristics of the aircraft piston pump are analyzed.The complex time domain dynamic model of the piston pump is established and verified by experiments.The frequency domain model of the piston pump swash plate vibration including the various types of cavity flow resistance,long thin pipe and load module inside the piston pump is established and verified by experiments.The relationship between the vibration of the swash plate variable mechanism and the fluid pulsation is analyzed in time domain.The influence of various parameters of the swash plate variable mechanism of the piston pump on the swash plate vibration is studied.The technical approach to reduce the swash plate vibration is explored through the parameter sensitivity analysis.In chapter 3,the aircraft hydraulic pipeline system model is proposed.A complex fluid-structure coupling 14-equations model of straight pipe and bend pipe unit is established.The effects of bourdon coupling,quasi-constant friction,non-constant friction,centrifugal force and Coriolis force are considered in the model.A fluid-structure coupling model of the hose with complex viscoelastic wall is established.Models of typical hydraulic accessory filters and accumulators are established through structural analysis.In chapter 4,the calculation method of aircraft hydraulic energy pipeline system is proposed.The time domain calculation method based on discrete time transfer matrix model and the frequency domain calculation method based on Laplace transform transfer matrix model are derived.The long tube transfer equation of the multi-segment unit combination and the numerical solution of any position in the pipe section are derived.Complex boundary conditions and typical pipeline structure models have been established,including various types of support constraints,branch structures,and space rotation.The impact test of a simple bend pipe system is designed,and the ANSYS fluid-structure coupling simulation project is established to verify the accuracy of the model.In chapter 5,the comprehensive characteristics of aircraft hydraulic energy system are analyzed.The dynamic characteristics of complex viscoelastic hoses,filters and accumulators are calculated,and the effects of the structural parameters of these components on the dynamic characteristics are analyzed.Complex models of plane pipeline system and space pipeline system are established to verify the accuracy of the model.The influence of key parameters such as the stiffness and the position of clamps on system characteristics are studied.A typical hydraulic energy system model is established,and the time-domain and frequency-domain characteristics of fluid and structure in the hydraulic energy system are studied.The effects of hose,filter,accumulator and pipeline layout on the characteristics of the hydraulic energy system are discussed.In chapter 6,the previous chapters are summarized.The research achievements and innovation is highlighted.Suggestions are also provided for the future work.