Study On Self-excited Oscillations In Grille-cavity Flow And Its Applications In High-speed Trains

A lot of equipment,constructions and vehicles contain ventilation grille,the grille and its internal space constitute the grille-cavity structure.Tangential flow past this kind of structure gives rise to grille-cavity flow.Under certain conditions,self-excited oscillations can be raised in these flow fields.The self-excited oscillations of the flow field are stable and periodic.This phenomenon can lead to structural vibration and noise.Besides,it will affect the operation of heat-removal system and air conditioning system.In addition,it contains complex flow phenomena and many basic problems of fluid mechanics.The research on this problem contains significance of engineering application and theoretical research.Taking the ground vehicles as the application object,this dissertation study self-excited oscillations in incompressible grille-cavity flow with numerical methods.The main content contains the study of the basic characteristics,excitation conditions,changing rules and control methods of the self-excited oscillations,as well as the application in the high-speed trains.The aim of this dissertation is finding out the characteristics of the oscillation frequency characteristics and spatial distribution,in order to deepen the understanding of the mechanism of this flow phenomenon.Another aim is proposing the prediction method of the oscillation frequency and amplitude,in order to provide technical and theoretical basis for solving the problems in the operation of vehicles caused by the self-excited oscillations and the related design and research work.The numerical simulation model of grille-cavity flow is established and is verified by particle image velocimetry method.The flow field change process was obtained by experiment and numerical simulation.The causes of the flow of the self-excited oscillation is analyzed.Using the method of spectrum,the time-frequency features and spatial distribution of different flow parameters are obtained,the basic characteristics of self-excited oscillation is discussed.The results show that the motion of the large-scale vortex leads to self-excited oscillations.The formation of large-scale eddies is closely related to the aggregation of vorticity around the grille.The oscillation frequency are same at different positions.Along the flow direction,the amplitude of pressure oscillation increases first and then decreases.The self-excited oscillation excitation conditions is studied.The main conclusions are as following:Under different conditions,the flow may be in non-oscillatory state,transition state and self-oscillation state.When the flow is relatively stable,it can be confirmed that the self-oscillations are excited if the flow oscillation amplitude is greater than zero and the oscillation amplitude change rate is close to zero,it can be confirmed that the self-oscillations are excited.The self-oscillation can be excited more easily when boundary layer momentum thickness at the separation edge ?d is smaller or the Reynolds number Re? is larger or the ratio between the length and the width of the grille gap G/H is larger.When the relative length of the cavity Lc/L<1.5 or relative height of cavity Hc/L>5,the cavity boundary will not affect the flow field oscillation.The flow field oscillation changes were studied,including the change of flow state and the oscillation mode.The oscillation frequency and amplitude under different flow states and oscillation modes are obtained.The main conclusions are as following:The flow state around the cavity grid can be distinguished by parameter ?=?Re??^1/2/??d/G?.With the increase of ?,the flow field will experience three flow states.Under different flow states,the variation of oscillation frequency and amplitude has great difference.With the increase of the ratio of G/H,the critical parameters of flow state transition ?₁ and ?₂ are both linearly decreasing.With the increase of ?,the critical parameter?L/G?j of mode shift decreased monotonously.The relationship between them can be expressed by exponential function.The oscillation control method is analyzed.The suppression effect of two typical control methods of the oscillation is evaluated.A method of controlling the flow field oscillation by changing the angle of grille rib is presented.Overall,adding a ramp at the separation edge can effectively reduce the oscillation intensity.However,the suppression effect of the method of cutting a ramp at the impingement edge is not obvious.Changing the grille rib angle can suppressed the oscillation well.The numerical simulation model of high-speed train equipment bay flow with consideration of the influence of ventilation grille is established.The simulation results show that the self-oscillation phenomenon can be observed near the ventilation grille of high-speed train equipment bay.Under the effects of the self-oscillation,periodic aerodynamic load is applied on the skirt board of equipment bay of the high-speed train.