Low Adhesion Conditions Research Articles

DEM modelling of surface indentations caused by granular materials: application to wheel–rail sanding

The presented surface indentation model is one step towards building a DEM model for wheel–rail sanding. In railways, so-called low-adhesion conditions can cause problems in traction and braking, and sanding is used to overcome this problem. Sand grains are blasted towards wheel–rail contact, fracture repeatedly as they enter the nip and are drawn into the contact and then increase adhesion. Research on this topic has mostly been experimental, but focussed on adhesion enhancement measurement. Thus, physical mechanisms increasing the adhesion are not well understood. Previous works involved experiments and DEM modelling of single sand grain crushing tests under realistic wheel–rail contact pressures of 900 MPa, focusing on sand fragment spread and formation of clusters of solidified fragments. In the experiments, indents in the compressing steel plates were also observed, which are also observed on wheel and rail surfaces in railway operation. These are now modelled by adapting an existing surface indentation model from literature to the case of surface indentations caused by granular materials. Two test cases are studied, and experimental spherical indentation tests for model parametrisation are presented. In a proof of concept, the mentioned single sand grain crushing tests under 900 MPa pressure are simulated including the surface indentation model. This work contributes to DEM modelling of wheel–rail sanding, which is believed to be a good approach to deepen the understanding of adhesion increasing mechanisms under sanded conditions.

CFD-DEM modelling of particle entrainment in wheel–rail interface: a parametric study on train characteristics

Rail-sanding is employed to improve the train’s wheel–rail traction loss in low adhesion conditions. This can significantly impede trains’ kinematics, operation, and performance by hindering the train’s acceleration and deceleration, resulting in delays and unreliability of transport system as well as causing safety risks and in the worst cases train collisions. Rail-sanding has its own merits in recovering the wheel–rail traction but can result in a sand wastage of more than 80% due to its low sand entrainment efficiency. In this research, computational fluid dynamics is coupled to discrete element modelling to study the behaviour of sand particles during rail-sanding. A parametric study based on the train characteristics, including train velocity, sand flow rate, and the geometry of the sander nozzle, is performed by comparing the entrainment efficiency of the sand particles. It is found that train velocities over 30 m/s result in the entrainment efficiency of almost zero. A moving air layer generated at the wheel–rail interface influences the lower bound of acceptable particle size range. The flow rate and nozzle geometry can be designed to enhance entrainment efficiency.

Adaptive MPC-Based Lateral Path-Tracking Control for Automatic Vehicles

Adaptive MPC-Based Lateral Path-Tracking Control for Automatic Vehicles

Coordinated control of path tracking and yaw stability for distributed drive electric vehicle based on AMPC and DYC

Maintaining both path-tracking accuracy and yaw stability of distributed drive electric vehicles (DDEVs) under various driving conditions presents a significant challenge in the field of vehicle control. To address this limitation, a coordinated control strategy that integrates adaptive model predictive control (AMPC) path-tracking control and direct yaw moment control (DYC) is proposed for DDEVs. The proposed strategy, inspired by a hierarchical framework, is coordinated by the upper layer of path-tracking control and the lower layer of direct yaw moment control. Based on the linear time-varying model predictive control (LTV MPC) algorithm, the effects of prediction horizon and weight coefficients on the path-tracking accuracy and yaw stability of the vehicle are compared and analyzed first. According to the aforementioned analysis, an AMPC path-tracking controller with variable prediction horizon and weight coefficients is designed considering the change of vehicle speed in the upper layer. The lower layer involves DYC based on the linear quadratic regulator (LQR) technique. Specifically, the intervention rule of DYC is determined by the threshold of the yaw rate error and the phase diagram of the sideslip angle. Extensive simulation experiments are conducted to evaluate the proposed coordinated control strategy under different driving conditions. The results show that, under variable speed and low adhesion conditions, the vehicle yaw stability and path-tracking accuracy have been improved by 21.58% and 14.43%, respectively, compared to AMPC. Similarly, under high speed and low adhesion conditions, the vehicle yaw stability and path-tracking accuracy have been improved by 44.30% and 14.25%, respectively, compared to the coordination of LTV MPC and DYC. The results indicate that the proposed adaptive path-tracking controller is effective across different speeds. Furthermore, the proposed coordinated control strategy successfully enhances the vehicle stability while maintaining good path-tracking accuracy even under extreme conditions.

Research on wheel-rail dynamic interaction of high-speed railway under low adhesion condition

Due to the inherent openness of the wheel-rail system, the wheel-rail interface is subject to the influence of a third medium, resulting in a low adhesion state. This study investigates the dynamic interaction between the wheel and rail under low adhesion conditions through the development of a numerical model employing anti-skid control strategies and infinite long track simulation technology. The model's validity is established by comparing its calculations with data from both the multi-body dynamics software UM and field tests. The numerical simulations uncover that the implementation of rail side lubrication significantly reduces longitudinal creep forces, thereby diminishing the wheelset's steering capabilities. Furthermore, it induces a notable increase in lateral wheel-rail interaction, consequently lowering safety indicators in vehicle operations. Rail side lubrication is found to substantially reduce rail side wear, resulting in an approximate 60% decrease in the maximum wear depth. Comprehensive friction control on the rail surface concurrently mitigates rail side and top surface wear. In instances of local low adhesion on both sides, the wheelset experiences pronounced stick-slip vibrations, leading to a deterioration in the longitudinal dynamic performance between the wheel and rail. Unilateral low adhesion situations prompt noticeable yawing and shaking motions within the low adhesion zone, resulting in evident lateral impact phenomena. Upon re-entering the normal adhesion zone from the low adhesion zone, the rail wear index experiences a significant peak, which means that the rails in this area are prone to severe local wear or fatigue.

DEM simulation of single sand grain crushing in sanded wheel–rail contacts

In railways, low adhesion conditions lead to problems in braking and traction. Sanding increases adhesion by blasting sand grains towards the wheel–rail contact. Which physical mechanisms increase adhesion during sanding is poorly understood, as research in this field has been almost exclusively experimental. This work is the first step in developing a DEM model describing the sanding process. Previous experimental studies involve single grain crushing tests focusing on initial breakage and repeated breakage under realistic wheel–rail contact pressures of 900 MPa. Tests on two types of rail sand under dry and wet contact conditions, showed different spread of fragments as well as the formation of clusters of solidified fragments at high stresses. The developed DEM model captures the observed behaviour for both types of sand, and both contact conditions. Combining experimental research and DEM modelling aims to improve the understanding of adhesion increasing mechanisms in sanded wheel–rail contacts.

Effect of Platoon Configurations on the Anti-Disturbing Performance

<div>In order to promote the actual application of the vehicular platoon, this study investigates the effect of the specific platoon configurations including predecessor following (PF), predecessor–leader following (PLF), and bidirectional following (BD), on the anti-disturbing performance from the linear to nonlinear perspective. First, based on the method of sensitivity of error propagation to the disturbance, a linear platoon model is established by considering an individual vehicle as a lumped-mass point. Then, the transfer function matrix from disturbance to spacing error is derived for sensitivity analysis. Finally, especially considering the inherent vehicle dynamics, the Burckhardt tire force model is adopted to construct a nonlinear platoon dynamics model for the nonlinear dynamics analysis. The results reveal the characteristics of each platoon configuration, as well as the design of control gains in terms of the anti-disturbing performance. The nonlinear dynamics property in high-adhesion conditions are generally similar to those of the sensitivity analysis based on the linear platoon model. However, some particular and complex phenomena different from the linear sensitivity analysis especially in low-adhesion conditions are only observed by the nonlinear dynamics analysis.</div>

Penetration of contaminated film on rails by ceramic particles and increase of traction force

Spinning and sliding caused by fallen leaves in autumn are problematic in terms of safety and timely operations. The results of braking tests using a vehicle equipped with an apparatus for applying ceramic particles as adhesion enhancers are reported. To ensure uniform conditions along the rail and minimise differences between test trials, low-adhesion conditions were implemented by applying paper tape to the top of the rail. The tangential coefficient at the beginning of the slide increased with the amount of ceramic particles applied. The larger the area of holes penetrated by the particles in the paper tape, the higher the tangential force during sliding. These findings will be useful in designing new adhesion enhancers and evaluating their performance.

Characterisation and tribological testing of recycled crushed glass as an alternative rail sand

In the UK Network Rail Environmental Sustainability Strategy 2020–2050, minimal waste and the sustainable use of materials are highlighted as core priorities. The ambition is to reuse, repurpose or redeploy all resources. In low adhesion conditions, sand particles are used to enhance traction throughout the network. However, sand is in danger of becoming scarce as many applications demand it. In this study, an alternative adhesion enhancing particle system made of recycled crushed glass is examined in terms of density, size, shape distribution, mineralogy, mechanical properties, and bulk behaviour to better understand their characteristics in comparison with the typical Great British rail sand currently in use and reported in the literature. Their effects on tribological behaviour and surface damage are also investigated using the High-Pressure Torsion test in dry, wet, and leaf-contaminated conditions. Both particle characterisation and tribological testing show promising results. Recycled glass particles provide an acceptable level of traction with a similar level of rail damage as typical rail sand. It is suggested to perform full-scale laboratory and field tests to further confirm the suitability of this material.

Combined single-pedal and low adhesion control systems for enhanced energy regeneration in electric vehicles: Modeling, simulation, and on-field test

In this paper, a single-pedal control algorithm was proposed, modeled, and theoretically analyzed to improve the driving range of electric vehicles and increase the intensity and frequency of energy regeneration. Specific analysis and strategy improvement were also conducted for the control problem under low adhesion conditions. The driving control function, economy, and control logic of the proposed single-pedal system were investigated using both simulations and real vehicle verifications under the New European Driving Cycle (NEDC). Based on simulations and real car testing, optimization of the single-pedal system under the traditional low adhesion control strategy was conducted, and then comprehensive comparisons were investigated. Results demonstrated that the proposed single-pedal control and its low adhesion condition control strategy are in line with the design requirements and effectively improved the vehicle economy, achieving a 24.9% energy regeneration rate. The optimized single-pedal low adhesion control strategy, according to the results, can effectively satisfy driving smoothness, economy, and stability under low adhesion conditions.

A numerical study on tread wear and fatigue damage of railway wheels subjected to anti-slip control

Tread wear and rolling contact fatigue (RCF) damage propagated on railway wheels are the two extremely important focal points as they can tremendously deteriorate wheel/rail interactions and hunting stability and destroy wheel surface materials, and subsequently, cut down the lifetime of the wheels. The on-board anti-slip controllers are of essence aiming to hold back the striking slipping of the powered wheelsets under low-adhesion wheel/rail conditions. This paper intends to investigate the impact of anti-slip control on wheel tread wear and fatigue damage under diverse wheel/rail friction conditions. To this end, a prediction model for wheel wear and fatigue damage evolution on account of a comprehensive vehicle-track interaction model is extended, where the wheel/rail non-Hertzian contact algorithm is used. Furthermore, the effect of frictional wear on the fatigue damage at wheel surface is considered. The simulation results indicate that the wheel/rail contact is full-slip under the low-adhesion conditions with braking effort. The wear amount under the low-adhesion conditions is observably higher than that under the dry condition. It is further suggested that the wheel tread is prone to suffering more serious wear and fatigue damage issues with a higher anti-slip control threshold compared to that with a lower one.

3D Spheroids — a Cellular Model for Studying the Effects of Hypoxia on the Epicardial Microenvironment

Fundamental research in recent years has allowed us to reassess the molecular and cellular mechanisms of cardiac ontogenesis and its repair after damage. The epicardium, the outer, tightly adjoining layer of the cardiac wall formed by epicardial mesothelial cells, collagen and elastic fibers, has gained special relevance as an important participant of reparative processes. Better insight into poorly understood epicardial function is challenged due to anatomical issues and lack of relevant cellular models.The aim of this study was to develop a spheroid 3D model of the epicardial microenvironment and determine responses of spheroids to hypoxia.Materials and methods. Spheroids were harvested in V-shaped culture dishes with a low adhesion coating. Immunofluorescent staining of cryosections, histological methods and real-time PCR were used for characterization of cultured spheroids.Results. We demonstrated that cultivation of cells under low adhesion conditions in V-shaped culture dishes resulted in the formation of spheroids with an average size of 136+21 µm and cell viability rates of over 98%. The cells in the spheroids cultured under normoxic conditions formed tight junctions and were characterized by a low level of proliferation and the ability to synthesize extracellular matrix proteins. Under hypoxia cells in the spheroids showed partial loss of intercellular contacts, acquired a spindle shape, started to express HIF1a, SNAIL, COL1Al and accumulate collagen. All these features demonstrated the activation of mesothelial(endothelial)-mesenchymal transition strongly resembling epicardial cellular responses to ischemia in vivo.Conclusion. An epicardial spheroid cell culture model suitable for study cellular responses to hypoxic environment was developed. This model can be used to clarify mechanisms regulating epicardial microenvironment and test new targeted candidate drugs.

Sanded Wheel–Rail Contacts: Experiments on Sand Crushing Behaviour

In railway operation, the sanding process is used to overcome low adhesion conditions in the wheel–rail contact. In the literature, previously conducted research has been experimental, e.g., measuring adhesion coefficients (ACs) under different contact conditions (dry, wet, …) or applying different sands. Under dry conditions, sanding can reduce measured ACs, while under wet conditions different types of rail sand can leave ACs unchanged or increase adhesion. Despite active research, the physical mechanisms causing the change in ACs under sanded conditions are still poorly understood. A possible remedy is the development of advanced models of sanding including local effects. As a basis for such a model, this study presents experimental results concerning single grain crushing behaviour of two types of rail sand under dry and wet contact conditions. Firstly, initial breakage behaviour is investigated with focus on the particle fragments’ size and spread as only fragments within the running band are available to influence the AC during roll-over. Secondly, single grain crushing tests are conducted under realistic wheel–rail load showing the formation of solidified clusters of sand fragments, as well as their size and thickness. This information is important for understanding mechanisms and for future physics-based modelling of the sanding process in wheel–rail contacts.

Experimental Investigation of a Maneuver Selection Algorithm for Vehicles in Low Adhesion Conditions

Winter conditions, characterized by the presence of ice and snow on the ground, are more likely to lead to road accidents. This paper presents an experimental proof of concept, with a 1/5th scale car platform, of a maneuver selection scheme for low adhesion conditions. In the proposed approach, a model-based estimator first processes the high-dimensional sensors data of the IMU, LIDAR and encoders to estimate physically relevant vehicle and ground conditions parameters such as the inertial velocity of the vehicle <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$v$</tex-math></inline-formula> , the friction coefficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> , the cohesion <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$c$</tex-math></inline-formula> and the internal shear angle <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\phi$</tex-math></inline-formula> . Then, a data-driven predictor is trained to predict the optimal maneuver to perform in the situation characterized by the estimated parameters. Experimental results show that it is possible to 1) produce a real-time estimate of the relevant ground parameters, and 2) determine an optimal maneuver based on the estimated parameters between a limited set of maneuvers.

A year-round analysis of railway station overruns due to low adhesion conditions

Key Performance Indicators, such as station overruns and delay minutes, are used to assess the performance and punctuality of the GB railway. They can be used to quantify the effects of low adhesion, but the majority of previous analysis has been constrained to the autumn season. A Python script has been created in this work to extract 11 years of detailed passenger and freight station overrun data, throughout the entire year. The information gathered includes time and date, location, direction, vehicle type, railhead conditions and subsequent delay minutes caused by the incident. Although the majority of low adhesion related overruns occur in the autumn season due to leaf fall, this work has highlighted the number of low adhesion related issues that occur throughout the year where there are no visible signs of contamination. This work gives an overview of this new dataset and looks at some key trends in the data but the granular detail available means that future case studies could be carried out in specific locations, linked to geographic and meteorological data, to assess when and why low adhesion is occurring. From an operational perspective, the dataset could then be used as a daily updated assessment of the effectiveness of low adhesion mitigation methods.

Numerical study of wheel/rail dynamic interactions for high-speed rail vehicles under low adhesion conditions during traction

The evolution of wheel-rail dynamic interaction of a high-speed railway vehicle is essential for the simulation of wheel-rail rolling contact damage under low adhesion conditions. The main objective of this study is to reproduce the dynamic wheel-rail interaction behaviour under various interfacial contaminations by experimental and numerical modelling methodology. Firstly, the wheel-rail adhesion characteristics under different interfacial contaminations were obtained. Subsequently, a simplified numerical methodology was proposed to extract the parameters from the high-speed adhesion tests to modify the simplified theory of Kalker (FASTSIM). The initial slop reduction factor and the slip-velocity-dependent coefficient of friction (COF) were introduced based on the high-speed experimental curves. Furthermore, the modified wheel-rail rolling contact model was incorporated into a longitudinal vehicle-track coupled dynamic model. The wheel-rail dynamic interaction behaviours under different operational conditions were attained. Finally, the wheel-rail wear performances were primarily investigated for typical traction coefficients under wet conditions. Results show that the wheel-rail creep force drops dramatically when the wheel enters the low adhesion zone (LAZ). There is a sudden increase in the creep force when the adhesion recovers. In addition, the wheelsets begin to slide and have severe wear at LAZ when the traction coefficient is larger than 0.1 at high speed.

The effect of top of rail lubricant composition on adhesion and rheological behaviour

The effect of top of rail lubricant composition on adhesion has been investigated using a laboratory ball-on-disc tribometer. Rheological properties were analysed using viscosimeter and high pressure torsion device. As a base medium, a biodegradable ester oil with bentonite thickener was selected. Added particles for friction modification were aluminium oxide, zinc oxide, copper sulfide and solid lubricants molybdenum disulfide and graphite. The effect of these components in the base medium on adhesion was evaluated. It was found that the most dominant component was the solid particles for friction modification. Based on the results, top of rail lubricant substances were prepared and tested. The best performing substances provided the optimal level of adhesion. These substances also showed resilience to overdosing, which caused commercial products to provide very low adhesion conditions. The rheological investigation confirmed the very low adhesion is controlled by elastohydrodynamic regime while the stable values are a result of transition to boundary lubrication.

Study on the safety speed of vehicle turning with considering the driving safety evaluation

Safety speeds estimation, as an indispensable link in the development of the aided or automatic drive, receives wide attention recently. Due to uncertain disturbances of vehicle driving, it is a challenging task to estimate the safety speed credibly. This work proposes a modified estimation approach to predict the turning safety speed by combining the static drive safety evaluation with the dynamic vehicle speed calculation. First, the driving safety state is evaluated considering the coupling of driver-vehicle-road-environment, where a comprehensive evaluation is obtained by combing the analytic hierarchy process and entropy weight analysis method. Then, a turning critical speed is calculated based on the vehicle driving dynamics considering both sideslip and rollover. The estimation of turning safety speed is achieved by modifying the critical speed with a safety correction factor obtained from the driving safety state evaluation. Finally, cases discussion on driving safety states evaluation, as well as the critical speed verification and safety speed analysis, are carried out. The results verify the validity of the driving safety state evaluation and critical speed calculation. The safety speeds have a reasonable safety margin according to the driving safety state evaluation. The maximum differences between the safety speeds and critical speeds are about 26.45% for buses and 26.39% for cars under low adhesion conditions, showing sufficient reliability for safety speed estimation.

The traction behaviour of high-speed train under low adhesion condition

To clarify the train-track traction behaviour in the low adhesion zone, a three-dimensional (3D) train-track coupling dynamic model was developed, in which the train was modelled as a rigid multi-body system with 8 vehicles. The vehicles are connected through coupler devices, which are simplified as nonlinear springs and dampers. The modified FASTSIM algorithm is utilized to obtain the wheel/rail creep force, which can consider the interfacial conditions. A new anti-slip control algorithm was proposed where the traction power could be adjusted to ensure the maximum utilization of wheel/rail adhesion. This control algorithm is achieved by iterating among the traction power, rotational speed and rotational acceleration of each wheelset. Numerical simulation results showed that the traction force would be reduced to a certain level to achieve maximum admissible force when the anti-slip control algorithm is activated. The rotational speed, traction force, creep force, creepages, and coupler force in contaminated conditions were investigated. The results show that the model can effectively simulate the traction process under the low adhesion state, which provides a certain theoretical reference for railway applications.

An improved degraded adhesion model for wheel–rail under braking conditions

Purpose The purpose of this paper is to develop an improved adhesion model to better reproduce the low adhesion condition of the anti-skid control for rail vehicles under braking condition. Design/methodology/approach In view of the low adhesion characteristics for rail vehicles under braking conditions, the Polach adhesion model was improved based on the sliding power and sliding energy. The wheel–rail low adhesion model suitable for braking condition was given. The analysis of braking anti-skid control under emergency braking condition was carried out through the co-simulation, and compared with the test data; the effectiveness and practicability of the improved low adhesion model were verified. Findings The results showed that the improved adhesion model is simple and efficient and the parameters involved are less, and it can be directly applied to the real-time simulation of anti-skid control in the process of train braking. Originality/value This paper can provide a theoretical reference for the reasons of change and improvement of adhesion between wheel and rail caused by the adjustment of braking force under anti-skid control, which can fulfill a need to the study of sliding energy on the contact surface, the removal effect of pollutants on the wheel–rail surface and the improvement and recovery of adhesion caused. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-07-2020-0244/