Investigations on High-Speed Flash Boiling Atomization of Fuel Based on Numerical Simulations

Flash boiling atomization(FBA)is a promising approach for enhancing spray atomization,which can generate a fine and more evenly distributed spray by increasing the fuel injection temperature or reducing the ambient pressure.However,when the outlet speed of the nozzle exceeds 400 m/s,investigating high-speed flash boiling atomization(HFBA)becomes quite challenging.This difficulty arises fromthe involvement ofmany complex physical processes and the requirement for a very fine mesh in numerical simulations.In this study,an HFBA model for gasoline direct injection(GDI)is established.This model incorporates primary and secondary atomization,as well as vaporization and boilingmodels,to describe the development process of the flash boiling spray.Compared to lowspeed FBA,these physical processes significantly impact HFBA.In this model,the Eulerian description is utilized for modeling the gas,and the Lagrangian description is applied to model the droplets,which effectively captures the movement of the droplets and avoids excessive mesh in the Eulerian coordinates.Under various conditions,numerical solutions of the Sauter mean diameter(SMD)for GDI show good agreement with experimental data,validating the proposed model’s performance.Simulations based on this HFBA model investigate the influences of fuel injection temperature and ambient pressure on the atomization process.Numerical analyses of the velocity field,temperature field,vapor mass fraction distribution,particle size distribution,and spray penetration length under different superheat degrees reveal that high injection temperature or low ambient pressure significantly affects the formation of small and dispersed droplet distribution.This effect is conducive to the refinement of spray particles and enhances atomization.

Numerical simulation of sand load applied on high-speed train in sand environment

High-speed train running in the sand environment is different from the general environment. In the former situation, there will be sand load applied on high-speed train(SLAHT) caused by sand particles hitting train surface. This will have a great impact on the train stability, running drag and surface corrosion. Numerical simulation method of SLAHT in sand environment is studied. The velocity and mass flow rate models of saltation and suspension sand particles and the calculation model of SLAHT caused by sand particles hitting train surface are established. The discrete phase method is adopted for numerical simulating the process of saltation and suspension sand particles moving to train surface and generating sand load. By comparison with the field tests, the numerical simulation reliability is analysed. The theoretical formula of SLAHT changing with cross-wind and train speed is proposed. SLAHT changing law is analyzed. Research results indicate that SLAHT changing with cross-wind and train speed is a quadratic relationship. When train speed is constant, SLAHT increases quadratically with cross-wind speed improvement. When cross-wind speed is constant, SLAHT increases quadratically with train speed improvement.

Dynamic analysis of axle box bearings on the high-speed train caused by wheel-rail excitation

To explore the impact of wheel-rail excitation on the dynamic performance of axle box bearings,a dynamic model of the high-speed train including axle box bearings is developed.Subsequently,the dynamic response characteristics of the axle box bearing are examined.The investigation focuses on the acceleration characteristics of bearing vibration under excitation of track irregularities and wheel flats.In addition,experiments on both normal and faulty bearings are conducted separately,and the correctness of the model and some conclusions are verified.According to the research,track irregularity is unfavorable for bearing fault detection based on resonance demodulation.Under the same speed conditions,the acceleration peak of bearing is inversely proportional to the length of the wheel flat and directly proportional to its depth.The paper will contribute to a deeper understanding of the dynamic performance of axle box bearings.

Aerodynamic Analysis and Optimization of Pantograph Streamline Fairing for High-Speed Trains

A pantograph serves as a vital device for the collection of electricity in trains.However,its aerodynamic resistance can limit the train’s running speed.As installing fairings around the pantograph is known to effectively reduce the resistance,in this study,different fairing lengths are considered and the related aerodynamic performances of pantograph are assessed.In particular,this is accomplished through numerical simulations based on the k-ωShear Stress Transport(SST)two-equation turbulence model.The results indicate that the fairing diminishes the direct impact of high-speed airflow on the pantograph,thereby reducing its aerodynamic resistance.However,it also induces interferences in the flow field around the train,leading to variations in the aerodynamic resistance and lift of train components.It is shown that a maximum reduction of 56.52%in pantograph aerodynamic resistance and a peak decrease of 3.38%in total train aerodynamic resistance can be achieved.

Numerical and Experimental Analysis of the Aerodynamic Torque for Axle-Mounted Train Brake Discs

As the velocity of a train increases,the corresponding air pumping power consumption of the brake discs increases proportionally.In the present experimental study,a standard axle-mounted brake disc with circumferential pillars was analyzed using a 1:1 scale model and a test rig in a wind tunnel.In particular,three upstream velocities were selected on the basis of earlier investigations of trains operating at 160,250,and 400 km/h,respectively.Moreover,3D steady computational fluid dynamics(CFD)simulations of the flow field were conducted to compare with the wind tunnel test outcomes.The results for a 3-car train at 180 km/h demonstrated:(1)good agreement between the air resistance torques obtained from the wind tunnel tests and the related numerical results,with differences ranging from 0.95%to 5.88%;(2)discrepancies ranging from 3.2 to 3.8 N·m;(3)cooling ribs contributing more than 60%of the air resistance torque;(4)the fast rotation of brake discs causing a significantly different flow field near the bogie area,resulting in 25 times more air pumping power loss than that obtained in the stationary brake-disc case.

Numerical simulation and optimization of aerodynamic uplift force of a high-speed pantograph

Aiming at the problem that aerodynamic uplift forces of the pantograph running in the knuckle-downstream and knuckle-upstream conditions are inconsistent,and their magnitudes do not satisfy the corresponding standard, the aerodynamic uplift forces of pantographs with baffles are numerically investigated, and an optimization method to determine the baffle angle is proposed. First, the error between the aerodynamic resistances of the pantograph obtained by numerical simulation and wind tunnel test is less than 5%, which indicates the accuracy of the numerical simulation method. Second, the original pantograph and pantographs equipped with three different baffles are numerically simulated to obtain the aerodynamic forces and moments of the pantograph components.Three different angles for the baffles are-17°, 0° and 17°.Then the multibody simulation is used to calculate the aerodynamic uplift force of the pantograph, and the optimal range for the baffle angle is determined. Results show that the lift force of the baffle increases with the increment of the angle in the knuckle-downstream condition, whereas the lift force of the baffle decreases with the increment of the angle in the knuckle-upstream condition. According to the results of the aerodynamic uplift force, the optimal angle of the baffle is determined to be 4.75° when the running speed is 350 km/h, and pantograph–catenary contact forces are 128.89 N and 129.15 N under the knuckledownstream and knuckle-upstream operating conditions,respectively, which are almost equal and both meet the requirements of the standard EN50367:2012.

Numerical Simulation of High-Speed Water Entry of Cone-Cylinder

Numerical method by solving Reynolds-averaged Navier-Stokes equations is presented to solve the vertical high-speed water entry problem of a cone-cylinder. The results of the trajectory and cavity shape agree well with the results obtained by the analytical model from literatures. The velocity of the projectile decays rapidly during the penetration,which is about 90% losing in 80D penetration depth. Pressure distributions are also discussed and the results show that the largest pressure appears on the tip of the cone and the lowest pressure occurs inside the cavity and causes vapor generation. For inside the cavity,there is always a supplement of air from outside before the splash closed,after that,the cavity is mainly filled with vapor.

Numerical simulation of formaldehyde distribution characteristics in the high-speed train cabin

The global concern over indoor air pollution in public vehicles has grown significantly.With a focus on enhancing passengers’comfort and health,this study endeavors to investigate the distribution characteristics of formaldehyde within a high-speed train cabin by employing a computational fluid dynamics(CFD)model which is experimentally validated in a real cabin scenario.The research focuses on analyzing the impact of air supply modes,temperature,relative humidity,and fresh air change rate on the distribution and concentration of formaldehyde.The results demonstrate that the difference in average formaldehyde concentration between the two air supply modes is below 1.3%,but the top air supply mode leads to a higher accumulation of formaldehyde near the sidewalls,while the bottom air supply mode promotes a more uniform distribution of formaldehyde.Furthermore,the temperature,relative humidity,and fresh air change rate are the primary factors affecting formaldehyde concentration levels,but they have modest effects on formaldehyde’s distribution pattern within the cabin.As the temperature and relative humidity increase,the changes in formaldehyde concentrations in response to variations in these factors become more evident.Importantly,the formaldehyde concentration may surpass the standard limit of 0.10 mg/m^(3)if the fresh air change rate falls below 212 m^(3)/h.This research provides a systematic approach and referenceable results for exploring formaldehyde pollution in high-speed train cabins.

Development of a simulation model for dynamic derailment analysis of high-speed trains

The running safety of high-speed trains has become a major concern of the current railway research with the rapid development of high-speed railways around the world.The basic safety requirement is to prevent the derailment.The root causes of the dynamic derailment of highspeed trains operating in severe environments are not easy to identify using the field tests or laboratory experiments.Numerical simulation using an advanced train–track interaction model is a highly efficient and low-cost approach to investigate the dynamic derailment behavior and mechanism of high-speed trains.This paper presents a three-dimensional dynamic model of a high-speed train coupled with a ballast track for dynamic derailment analysis.The model considers a train composed of multiple vehicles and the nonlinear inter-vehicle connections.The ballast track model consists of rails,fastenings,sleepers,ballasts,and roadbed,which are modeled by Euler beams,nonlinear spring-damper elements,equivalent ballast bodies,and continuous viscoelastic elements,in which the modal superposition method was used to reduce the order of the partial differential equations of Euler beams.The commonly used derailment safety assessment criteria around the world are embedded in the simulation model.The train–track model was then used to investigate the dynamic derailment responses of a high-speed train passing over a buckled track,in which the derailmentmechanism and train running posture during the dynamic derailment process were analyzed in detail.The effects of train and track modelling on dynamic derailment analysis were also discussed.The numerical results indicate that the train and track modelling options have a significant effect on the dynamic derailment analysis.The inter-vehicle impacts and the track flexibility and nonlinearity should be considered in the dynamic derailment simulations.

NUMERICAL AND EXPERIMENTAL INVESTIGATION OF WAVE DYNAMIC PROCESSES IN HIGH-SPEED TRAIN/TUNNELS

Numerical and experimental investigation on wave dynamic processes induced by high-speed trains entering railway tunnels are presented. Experiments were conducted by using a 1:250 scaled train-tunnel simulator. Numerical simulations were carried out by solving the axisymmetric Euler equations with the dispersion-controlled scheme implemented with moving boundary conditions. Pressure histories at various positions inside the train-tunnel simulator at different distance measured from the entrance of the simulator are recorded both numerically and experimentally, and then compared with each other for two train speeds. After the validation of nonlinear wave phenomena, detailed numerical simulations were then conducted to account for the generation of compression waves near the entrance, the propagation of these waves along the train tunnel, and their gradual development into a weak shock wave. Four wave dynamic processes observed are interpreted by combining numerical results with experiments. They are: high-speed trains moving over a free terrain before entering railway tunnels; the abrupt-entering of high-speed trains into railway tunnels; the abrupt-entering of the tail of high-speed trains into railway tunnels; and the interaction of compression and expansion waves ahead of high-speed trains. The effects of train-tunnel configuration, such as the train length and the train-tunnel blockage ratio, on these wave processes have been investigated as well.

Numerical simulation and control of welding distortion for double floor structure of high speed train

The welding heat source models and the plastic tension zone sizes of a typical weld joint involved in the double floor structure of high speed train under different welding parameters were calculated by a thermal-elastic-plastic FEM analysis based on SYSWELD code.Then,the welding distortion of floor structure was predicted using a linear elastic FEM and shrinkage method based on Weld Planner software.The effects of welding sequence,clamping configuration and reverse deformation on welding distortion of floor structure were examined numerically.The results indicate that the established elastic FEM model for floor structure is reliable for predicting the distribution of welding distortion in view of the good agreement between the calculated results and the measured distortion for real double floor structure.Compared with the welding sequence,the clamping configuration and the reverse deformation have a significant influence on the welding distortion of floor structure.In the case of30 mm reverse deformation,the maximum deformation can be reduced about 70%in comparison to an actual welding process.

Modeling and simulation of high-speed passenger train movements in the rail line

In this paper, we propose a new formula of the real-time minimum safety headway based on the relative velocity of consecutive trains and present a dynamic model of high-speed passenger train movements in the rail line based on the proposed formula of the minimum safety headway. Moreover, we provide the control strategies of the high-speed passenger train operations based on the proposed formula of the real-time minimum safety headway and the dynamic model of highspeed passenger train movements. The simulation results demonstrate that the proposed control strategies of the passenger train operations can greatly reduce the delay propagation in the high-speed rail line when a random delay occurs.

Dynamic simulation and safety evaluation of high-speed trains meeting in open air

Dynamic responses of a carriage under excitation with the German high-speed low-interference track spectrum together with the air pressure pulse generated as high-speed trains passing each other are investigated with a multi-body dynamics method.The variations of degrees of freedom（DOFs：horizontal movement,roll angle,and yaw angle）,the lateral wheel-rail force,the derailment coefficient and the rate of wheel load reduction with time when two carriages meet in open air are obtained and compared with the results of a single train travelling at specifie speeds.Results show that the rate of wheel load reduction increases with the increase of train speed and meets some safety standard at a certain speed,but exceeding the value of the rate of wheel load reduction does not necessarily mean derailment.The evaluation standard of the rate of wheel load reduction is somewhat conservative and may be loosened.The pressure pulse has significan effects on the train DOFs,and the evaluations of these safety indexes are strongly suggested in practice.The pressure pulse has a limited effect on the derailment coefficien and the lateral wheel-rail force,and,thus,their further evaluations may be not necessary.

Experimental and Numerical Investigation on the External Aerodynamic Noise of High-Speed Train

Aerodynamic noise is the dominant noise source of the high-speed train.It not only seriously affects the passenger comfort and people’s normal life along the railway line,but also may cause fatigue damage to the surrounding equipment and buildings.This manuscript carried out the simulation and experimental study on the external aerodynamic noise of high-speed train,in order to increase the understanding of the noise and hence to be better able to control it.The on-line tests were performed to verify that it is reasonable to simplify the high-speed train model.The turbulent air flow model was then developed,and the external steady flow field was computed by Realizable k-εturbulence model.Based on the steady flow field,aerodynamic noise sources on the train surface and the external transient flow field were calculated by broadband acoustics source model and large eddy simulation(LES)respectively.The pressures on the train surface were obtained from the results of the transient model.Considering the transient flow field,the far-field aerodynamic noise generated by the high-speed train was finally obtained based on Lighthill-Curle theory.Through the comparison between simulations and on-line tests,it is shown that the numerical model gives reliable aerodynamic noise predictions.This research is significant to the study and control of the aerodynamic noise of high-speed train.

A study on aerodynamic noise characteristics of a high-speed maglev train with a speed of 600 km/h

Purpose–This study aims to explore the formation mechanism of aerodynamic noise of a high-speed maglev train and understand the characteristics of dipole and quadrupole sound sources of the maglev train at different speed levels.Design/methodology/approach–Based on large eddy simulation(LES)method and Kirchhoff–Ffowcs Williams and Hawkings(K-FWH)equations,the characteristics of dipole and quadrupole sound sources of maglev trains at different speed levels were simulated and analyzed by constructing reasonable penetrable integral surface.Findings–The spatial disturbance resulting from the separation of the boundary layer in the streamlined area of the tail car is the source of aerodynamic sound of the maglev train.The dipole sources of the train are mainly distributed around the radio terminals of the head and tail cars of the maglev train,the bottom of the arms of the streamlined parts of the head and tail cars and the nose tip area of the streamlined part of the tail car,and the quadrupole sources are mainly distributed in the wake area.When the train runs at three speed levels of 400,500 and 600 km$h1,respectively,the radiated energy of quadrupole source is 62.4%,63.3%and 71.7%,respectively,which exceeds that of dipole sources.Originality/value–This study can help understand the aerodynamic noise characteristics generated by the high-speed maglev train and provide a reference for the optimization design of its aerodynamic shape.

Numerical Study on Aerodynamic Performance of High-Speed Pantograph with Double Strips

Pantograph is a critical component of the high-speed train.It collects power through contact with catenary,which significantly affects the running safety of the train.Pantograph with double collector strips is one common type.The aerodynamic performance of the collector strips may affect the current collection of the pantograph.In this study,the aerodynamic performance of the pantograph with double strips is investigated.The numerical results are consistent with the experimental ones.The error in the aerodynamic drag force of the pantograph between numerical and experimental results is less than 5%.Three different conditions of the strips are studied,including the front strip,the rear strip,and the double strips.Results show that the presence of the front strip will affect the lift force of the rear strip,and reduce the resistance of the rear strip under the opening condition.Meanwhile,the rear strip has few effects on the front strip under the opening operation condition.The law of the resistance for the interaction between two strips under the closing condition is similar to the opening one.

Numerical research on effect of overlap ratio on thermal-stress behaviors of the high-speed laser cladding coating

High-speed laser cladding technology, a kind of surface technology to improve the wear-resistance and corrosion-resistance of mechanical parts, has the characterizations of fast scan speed, high powder utilization rate, and high cladding efficiency. However, its thermal-stress evolution process is very complex, which has a great influence on the residual stress and deformation. In the paper, the numerical models for the high-speed laser cladding coatings with overlap ratios of 10%,30%, and 50% are developed to investigate the influence rules of overlap ratio on the thermal-stress evolution, as well as the residual stresses and deformations. Results show that the heat accumulation can reheat and preheat the adjacent track coating and substrate, resulting in stress release of the previous track coating and decreased longitudinal stress peak of the next track coating. With the overlap ratio increasing, the heat accumulation and the corresponding maximum residual stress position tend to locate in the center of the cladding coating, where the coating has a high crack susceptibility. For a small overlap ratio of 10%, there are abrupt stress changes from tensile stress to compressive stress at the lap joint, due to insufficient input energy in the position. Increasing the overlap ratio can alleviate the abrupt stress change and reduce the residual deformation but increase the average residual stress and enlarge the hardening depth. This study reveals the mechanism of thermal-stress evolution, and provides a theoretical basis for improving the coating quality.

Numerical simulations of kinetic formation mechanism of Tangjiashan landslide

Tangjiashan landslide is a typical high-speed landslide hosted on consequent bedding rock. The landslide was induced by Wenchuan earthquake at a medium-steep hill slope. The occurrence of Tangjiashan landslide was basically controlled by the tectonic structure, topography, stratum lithology, slope structure, seismic waves, and strike of river. Among various factors, the seismic loading with great intensity and long duration was dominant. The landslide initiation exhibited the local amplification effect of seismic waves at the rear of the slope, the dislocation effect on the fault, and the shear failure differentiating effect on the regions between the soft and the hard layers. Based on field investigations and with the employment of the distinct element numerical simulation program UDEC （universal distinct element code）, the whole kinetic sliding process of Tan iashan landslide was represented and the formation mechanism of the consequent rock landslide under seismic loading was studied. The results are helpful for understanding seismic dynamic responses of consequent bedding rock slopes, where the slope stability could be governed by earthquakes.

Step-by-Step Numerical Prediction of Aerodynamic Noise Generated by High Speed Trains

In this paper,the unsteady flow around a high-speed train is numerically simulated by detached eddy simulation method(DES),and the far-field noise is predicted using the Ffowcs Williams-Hawkings(FW-H)acoustic model.The reliability of the numerical calculation is verified by wind tunnel experiments.The superposition relationship between the far-field radiated noise of the local aerodynamic noise sources of the high-speed train and the whole noise source is analyzed.Since the aerodynamic noise of high-speed trains is derived from its different components,a stepwise calculation method is proposed to predict the aerodynamic noise of high-speed trains.The results show that the local noise sources of high-speed trains and the whole noise source conform to the principle of sound source energy superposition.Using the head,middle and tail cars of the high-speed train as noise sources,different numerical models are established to obtain the far-field radiated noise of each aerodynamic noise source.The far-field total noise of high-speed trains is predicted using sound source superposition.A step-by-step calculation of each local aerodynamic noise source is used to obtain the superimposed value of the far-field noise.This is consistent with the far-field noise of the whole train model’s aerodynamic noise.The averaged sound pressure level of the far-field longitudinal noise measurement points differs by 1.92 dBA.The step-by-step numerical prediction method of aerodynamic noise of high-speed trains can provide a reference for the numerical prediction of aerodynamic noise generated by long marshalling high-speed trains.

A Fast Approach for Predicting Aerodynamic Noise Sources of High-Speed Train Running in Tunnel

The aerodynamic noise of high-speed trains passing through a tunnel has gradually become an important issue.Numerical approaches for predicting the aerodynamic noise sources of high-speed trains running in tunnels are the key to alleviating aerodynamic noise issues.In this paper,two typical numerical methods are used to calculate the aerodynamic noise of high-speed trains.These are the static method combined with non-reflective boundary conditions and the dynamic mesh method combined with adaptive mesh.The fluctuating pressure,flow field and aerodynamic noise source are numerically simulated using the abovemethods.The results showthat the fluctuating pressure,flow field structure and noise source characteristics obtained using different methods,are basically consistent.Compared to the dynamic mesh method,the pressure,vortex size and noise source radiation intensity,obtained by the static method,are larger.The differences are in the tail car and its wake.The two calculation methods show that the spectral characteristics of the surface noise source are consistent.The maximum difference in the sound pressure level is 1.9 dBA.The static method is more efficient and more suitable for engineering applications.