Akw Ahmed

Development of a vehicle–track model assembly and numerical method for simulation of wheel–rail dynamic interaction due to unsupported sleepers

In practice, it is not very uncommon to find railway track systems with unsupported sleepers due to the uneven settlement of a ballasted track system. These unsupported sleepers are among the major vibration excitations for a train and track system when a train moves forwards on a track. The vibration induced by unsupported sleepers can cause a large dynamic contact force between wheels and rails. For heavily loaded high-speed trains, the deteriorated sleeper support may lead to accelerated degradation of the railway track and vehicle components, and may thus impose safety risk to the operation. This paper presents analyses of a coupled vehicle–track assembly consisting of a roll plane vehicle model, a continuous track system model and an adaptive wheel–rail contact model. In order to improve the simulation efficiency, a numerical approach based on the central finite difference method is proposed in this investigation. The developed model assembly and proposed simulation method are utilised to simulate the vehicle–track dynamic interaction in the presence of unsupported sleepers. The dynamic response in terms of the dynamic wheel–rail interaction force due to one or multiple unsupported sleepers is studied. Important factors influencing the dynamic wheel–rail interaction force in the presence of sleeper voids are also investigated. The results show that the vehicle speed, the gap size and the number of unsupported sleepers primarily dictate the magnitude of impact load which can be significant.

Dynamic roll instability analysis of heavy vehicles using energy approach

In this paper, roll plane models of heavy vehicles are proposed to study their dynamic rollover properties. Equations of motions are derived using an energy approach. These equations are considered valid until the vehicle approaches its tip-over position. Dynamic rollover indicators in terms of Rollover Prevention Energy Reserve Factor (RPERF) and Critical Distance Ratio (CDR) are discussed. Dynamic rollover limits of a straight truck subject to different evasive manoeuvres are investigated using the proposed energy approach and the indicators. The dynamic rollover limits predicted by RPERF are slightly smaller than those predicted by CDR. It is concluded that the dynamic rollover limit of a vehicle is manoeuvre dependent, and the rollover limits at relatively high steering frequencies are considerably larger than the corresponding static rollover threshold of the vehicle. The dynamic rollover limits of the vehicle approaches the SRT from the upper side as the excitation frequency decreases. The SRT thus provides an underestimate of the anti-roll ability of the heavy vehicles under transient steering manoeuvres.

Modelling, validation and analysis of a three-dimensional railway vehicle–track system model with linear and nonlinear track properties in the presence of wheel flats

This paper presents dynamic contact loads at wheel–rail contact point in a three-dimensional railway vehicle–track model as well as dynamic response at vehicle–track component levels in the presence of wheel flats. The 17-degrees of freedom lumped mass vehicle is modelled as a full car body, two bogies and four wheelsets, whereas the railway track is modelled as two parallel Timoshenko beams periodically supported by lumped masses representing the sleepers. The rail beam is also supported by nonlinear spring and damper elements representing the railpad and ballast. In order to ensure the interactions between the railpads, a shear parameter beneath the rail beams has also been considered into the model. The wheel–rail contact is modelled using nonlinear Hertzian contact theory. In order to solve the coupled partial and ordinary differential equations of the vehicle–track system, modal analysis method is employed. Idealised Haversine wheel flats with the rounded corner are included in the wheel–rail contact model. The developed model is validated with the existing measured and analytical data available in the literature. The nonlinear model is then employed to investigate the wheel–rail impact forces that arise in the wheel–rail interface due to the presence of wheel flats. The validated model is further employed to investigate the dynamic responses of vehicle and track components in terms of displacement, velocity, and acceleration in the presence of single wheel flat.

An independently controllable active steering system for maximizing the handling performance limits of road vehicles

This study explores the effectiveness of an active independent front steering system capable of applying a corrective steering at each wheel selectively and in an independent manner. In doing this, it is possible to generate the required adhesion force for active steering control while ensuring that none of the tyres approaches saturation. A non-linear yaw-plane model of a two-axle truck with a limited number of roll degrees of freedom is used to evaluate the effectiveness of the active independent front steering under a range of steering manoeuvres. A simple proportional–integral controller is synthesized to track the reference response based on the neutral steering system as well as to limit the steering correction considering the saturation limit of the tyres, which is defined from the normalized cornering stiffness properties of the tyres. The directional responses obtained for the vehicle model integrating the active independent front steering controller are compared with those of the conventional active front steering system for each of the selected manoeuvres. The results show that, while both the control strategies can effectively track the target yaw rate of the vehicle, the proposed active independent front steering control can yield enhanced performance limits without any of the tyres approaching the saturation limit, irrespective of the road condition and the steering manoeuvre. Furthermore, the active independent front steering design permits some adhesion reserve for each wheel for generating additional traction and braking forces during a severe steering manoeuvre.

EXPERIMENTAL EVALUATION OF FRICTION COEFFICIENTS OF TYPICAL LOADS AND TRAILER DECKS UNDER VERTICAL VIBRATION

The freight transportation industry has been employing different types of load securement mechanisms to minimize the occurrence of load spills. In 1993, the Canadian Council of Motor Transport Administrators formed a load security research management committee to address the lack of a sound technical basis for the existing rules for load security on heavy vehicles. While the significant roles of friction forces arising within the cargo layers and between the cargo and the trailer bed has been recognized, there exists a lack of reliable data on the friction forces between typical loads and trailer decks, specifically under dynamic vehicular environment. The overall objective of this study was to characterize the friction properties of selected loads and deck surfaces under vertical vibration.

Influence of a flexible wheelset on the dynamic responses of a high-speed railway car due to a wheel flat

Large magnitude impact loads caused by wheel flats may excite various vibration modes of wheelsets employed in high-speed trains and thereby contribute considerably to the dynamic response of vehicles. In this study, the wheelset is modeled as a flexible body using the modal approach, which is integrated to a multibody dynamic model of the high-speed train coupled with a flexible track slab model. The multibody dynamic model is formulated for a typical high-speed train consisting of a car body, two bogie frames, and four wheelsets. The track is modeled considering the rail as a Timoshenko beam discretely supported on a flexible track slab. The effects of the wheelset flexibility on the dynamic response are illustrated through comparisons with those obtained with a rigid wheelset considering different vehicle speeds and sizes of the wheel flat. Subsequently, the effects of wheel flats on the vehicle–track system are evaluated in terms of the wheel–rail impact forces, axle-box vertical acceleration, and dynamic stress developed in the wheelset due to a haversine wheel flat. The results suggest that the wheelset flexibility can lead to significantly higher axle-box vibration and wheelset axle stress compared to a rigid wheelset in the presence of a wheel flat.

A normalised measure of relative roll instability for open-loop rollover warning

A new measure, termed as Normalised Roll-response of Semitrailer Sprung Mass (NRSSM), based upon the normalised roll response of the sprung and unsprung masses through online monitoring of roll deflections, is identified and proposed as a superior measure of relative roll instability. The proposed NRSSM was found to be less sensitive to variations in design and operating conditions, while the measurability and lead-time performance of the metric were found to be superior when compared with the existing measures. The performance of this metric is analysed and a two-tier warning strategy, incorporating the NRSSM and the semitrailer lateral acceleration response, is proposed to enhance the reliability and lead-time of the warning strategy, while reducing the possibilities of false warnings.

ASSESSMENT OF OPEN-LOOP ROLLOVER CONTROL OF ARTICULATED VEHICLES UNDER DIFFERENT MANOEUVRES

In this paper, a comprehensive three-dimensional heavy vehicle model is developed to investigate the effectiveness of an open-loop roll instability control. The steering system compliance, roll steer, bump steer, Ackerman steer and wrap steer are incorporated in the vehicle model, along with a comprehensive tyre model and ABS algorithm. Time delays due to a drivers reaction and the transportation lag of the braking system are characterized by a variable called the reaction delay. The rollover indicators in terms of roll safety factor, tractor and trailer lateral accelerations and roll angles, and the rearmost axle roll angle are investigated for their effectiveness for open-loop roll stability control in various cornering and evasive manoeuvres, road conditions, braking efforts, and different reaction delays.

Influence of friction wedge characteristics on lateral response and hunting of freight wagons with three-piece bogies

In this study, the nonlinear damping characteristics of friction wedges in the secondary suspension of a freight wagon are investigated considering nonsmooth unilateral contact, multiaxis motions, slip–stick conditions, and geometry of the wedges. The parameters of the contact pairs within the suspension were identified to achieve smooth and efficient numerical solutions, while ensuring adequate accuracy. A simulation model of the friction wedge was formulated and analyzed, which revealed highly nonlinear dependence on vertical, roll, and lateral motions between the bolster and the side frames. The friction wedge model was integrated into the multibody dynamic model of a three-piece bogie to study the effects of wedge properties on hunting characteristics. The resulting 114-degrees-of-freedom wagon model incorporated constraints due to side bearings, axle boxes, and the center plates, while the wheel–rail contact forces were obtained using the FASTSIM algorithm. The simulation results were obtained to study hunting properties of the wagon in terms of critical speed and the predominant oscillation frequency, and the effects of wedge friction and geometry on stability characteristics of the freight car. The results showed subcritical Hopf bifurcation of dynamic responses of the wagon. Moreover, an increase in the wedge angle, friction coefficient, and springs free length resulted in a higher critical speed.

Hunting Analysis of a Partially-Filled Railway Tank Car

General purpose railway tank cars similar to road tankers are known to transport liquid cargo in partial-fill state due to variations in liquid cargo density and governing axle load limits. It is widely reported that the cargo movements constitute additional forces and moments that could strongly affect the wheel-rail interactions and coupling forces, and thereby the directional dynamics of the wagon. In this study, the linear slosh theory is used to describe the liquid cargo movement in the roll plane by a simple pendulum, which is integrated into a comprehensive nonlinear multi-body model of a three-piece truck to study the effects of liquid cargo slosh on lateral dynamics of the tank car. The model also incorporates the nonlinear secondary suspension restoring and damping forces, attributed to friction of the wedges, using the non-smooth contact method in addition to the geometric constraints of various components. The wheel/rail contact forces are simulated considering non-elliptical wheel-rail contact using the FASTSIM algorithm. The lateral dynamic responses of the multi-body model of a freight car with partially filled liquid load and an equivalent rigid cargo are evaluated to study the effect of cargo movement on the critical speed and the wheelset hunting oscillations frequency. The results obtained considering different fill ratios of the liquid cargo suggest that the fluid slosh yields additional damping effect on the lateral dynamics of the car. Liquid cargo movement within partly-filled tank car could thus yield a beneficial influence on the wheelset hunting. This was evidenced from the phase relationship between the lateral oscillations of the pendulum and the bogie/wheelset. Consequently, a partially filled tanker resulted in relatively higher critical hunting velocity compared to that of the wagon with equivalent rigid cargo.Copyright