MS21.1 - Vehicle Dynamics

As the strong wind effects, especially in mountain terrain and coastal regions, it may lead to road vehicle accidents while passing through long-span bridges. Many researchers have pointed this out and presented some related research work to analyze the coupled dynamic interactions of wind-vehicle-bridge system.This study mainly contributed to investigate the vehicle driving safety and physical comfort passing through a bridge tower region, considering strong cross wind environment variations in bridge tower regions.A long-span suspension bridge (Honghe Bridge) with a main span of 700 m was taken as research background.Honghe Bridge was located in southwest of China, a typical mountain canyon terrain with large undulations, and the terrain of the bridge site presented an “S” canyon cove landform.Considering the typical terrain impacts on the wind microclimate on the girder deck, the characteristics of the mean wind and turbulence wind at the mid-span and bridge tower regions were measured and analyzed.According to field measurement results, the typical wind curve model in tower regions along the bridge was proposed, as well as variation characteristics of strong turbulent and power spectral in bridge regions.Because strong shielding effects of the bridge tower on the incoming oblique wind flow, the wind speeds were drastically reduced while entering the tower region.However, the wind speed increased again after leaving the bridge tower regions which exhibited a significant acceleration effect on wind speed while leaving tower region.Compared to the standard wind curve model in tower regions suggested by Wind Load Specification, the measured results shown that the typical cross wind characteristics may impact vehicle dynamic responses in a large extent.A two-axle vehicle model was utilized to analyze coupled dynamic interaction between vehicle and bridge under such a typical time-varying cross-wind condition.The vertical, lateral, rolling, yawing, and pitching dynamic responses of a moving vehicle were discussed to gain a deeply comprehensive of system coupled dynamics.Influences of wind speed, turbulence intensities, vehicle speed, and dynamic bridge responses on a passing vehicle were also discussed.The road roughness in plane, driver reaction, and aerodynamic interference between the moving vehicle and bridge were included as well.The responses of vehicle passing by tower and main span were analyzed based on considerations of above factors.Driving safety and physical comfort analysis were also performed to revise the currently used limitation of driving speed on long-span bridge, and the definition of rolling and the side-slide accidents.Results shown that the lateral and rolling responses of vehicle model were greatly boosted by the cross-wind variations in tower regions along the bridge.The responses of vehicle model decreased sharply since the shielding effects of bridge tower on crosswind.However, the responses of vehicle model shown a significantly increasement while it gets out of bridge tower region, as a result of the acceleration effects of tower on cross-wind.The dynamic responses of vehicle model revealed that the vehicle driving safety influenced by wind microclimate, especially in bridge tower regions under influences of complex terrains.

Short pitch corrugation (hereinafter corrugation) is one of the major rail defects in railway tracks worldwide. It is recognized as a (quasi-) periodic undulation of the rail surface with shiny crests and dark valleys. Corrugation excites high-frequency wheel-rail dynamic forces, induces strong vibration and noise, and accelerates the degradation of railway components, which significantly increase the maintenance cost. Despite extensive theoretical and experimental research in recent decades, corrugation development mechanism remains unclear. This work proposes an integrated approach to identify the corrugation formation process, including field observation and measurement, numerical simulation employing a three-dimensional (3D) finite element model (FEM), and experimental validation using an innovative downscale V-Track test rig The field observation and measurement show that corrugation wavelength falls typically in the range of 20-80 mm and insensitive to trains speed variation. Corrugation can develop in the embedded rail system with continuous support where the pinned-pinned resonance is absent. Corrugation with visible initial excitation (e.g., rail joints, squats) usually decay after several waves, while corrugation with invisible excitation often occurs with a much longer distance. This work focuses on the latter corrugation. To investigate the corrugation formation mechanism, a 3D FE vehicle-track interaction model is employed which can simultaneously consider the contact mechanics and structural dynamics, as well as their interplay. The rail damage mechanism is assumed as wear. The simulation results indicate that with nominal track parameters, corrugation cannot initiate, and a predefined corrugation will also be erased by the wheel-rail dynamic interaction. However, by introducing an initial excitation to the vehicle-track system from the degraded fastenings, corrugation can initiate and consistently grow up to 80 um. The reproduced corrugation agrees with the field observation in terms of the wavelength and periodicity. Further analysis shows that longitudinal compression modes are responsible for corrugation initiation, and the consistency between the longitudinal compression and vertical bending eigenfrequencies of the wheel-track system is required for consistent corrugation growth. To validate the FEM simulation results, we perform the corrugation experiment using the V-Track test rig. Compared to other testing facilities, the V-Track can comprehensively simulate both the wheel-rail contact and the high-frequency dynamics of the real vehicle-track system. During the corrugation experiment, the fastening clips are loosened at certain locations of the ring track to simulate fastening degradation and introduce dynamic effects to the vehicle-track system for corrugation initiation. Overall, the FEM simulation and the V-Track experiment both highlight the significance of rail longitudinal vibration modes on corrugation formation.

There are currently no evidence-based regulations to set appropriate speed limits or enforce the closure of road bridges under strong crosswinds in Norway. To provide a scientific basis for future regulations, a test vehicle has been equipped with an anemometer, global navigation satellite system and inertial measurement unit and has been driven across several bridges under extreme crosswinds. The collected data describes the resultant vehicle-driver response to crosswind perturbations. The most significant values of lateral acceleration and yaw rate have been observed in response to easily identifiable gusts created in the wakes of bridge towers. The spatial periods of these significant gusts typically lie in the range 2 to 20 vehicle lengths, which has previously been identified as a range that is critical to vehicle crosswind sensitivity. Additionally, at the typical Norwegian speed limit of 80 kph, these spatial periods translate to frequencies that lie in the range 0.5 to 2 Hz, which has previously been identified as a range in which drivers are likely to amplify the handling response to perturbations (as opposed to attenuation below 0.5 Hz and a neutral effect above 2 Hz). Identifiable gusts of other dimensions/frequency have been observed outside these ranges as well and the resultant responses are of a much lower amplitude. Concrete empirical evidence is therefore presented to support the theory on critical gust dimensions/frequency for vehicle handling stability. A force identification method is proposed that allows the transient aerodynamic loads to be identified such that the contribution of the driver to unstable handling response can be better characterised.

Magnetic track brakes (mtbs) are additional braking systems used in railway vehicles at emergency situations and low adhesion conditions. While the mtb is active, the electromagnets create magnetic attraction forces between the rail and the brake and occurring friction forces in the mtb—rail contact are transmitted to the bogie. At low velocities, mtbs were typically deactivated to avoid stopping jerks. However, trends of increasing operational speed put highest demands on the braking performance of railway vehicles and require activation until full stop. This can cause harmful self-excited vibrations of the mtb. Some passive countermeasures are already published, [1], but may not always be reasonable, because of constraints in the design or negative aspects regarding weight or braking performance. To overcome these drawbacks and to gain deeper system understanding, strategies for active vibration control of an mtb are investigated, not addressed in literature so far. This paper builds on preliminary work from [1], where the stability behavior of the mtb was studied in detail and on considerations from [2], where the input of mechanical energy, during one oscillatory period, is used for stability investigations. There, an active control of the normal force on a basic friction oscillator was presented, which minimizes the input energy and diminish the vibrations. In this study, the electromagnet of the mtb serves as actuator and the electric voltage as the regulated variable of the implemented state feedback. Since the states of the coupled electromagnetic-mechanical system cannot be measured directly a state observer based on the electric current (system output) and voltage (system input) is used. To ensure the functionality of the controller, necessary requirements of the system, such as controllability and state-observability are discussed. Different approaches to diminish self-excited vibrations are studied, based on reducing the energy fed into the oscillating system. Considering a minimal model of the mtb the input energy depends on the electromagnetic-mechanical coupling and the friction force in the mtb—rail contact, which were both identified as self-excitation mechanisms in [1]. With a harmonic approach for the states the input energy is calculated for the linearized system. The obtained equation shows dependencies of the phase shift between magnetic flux and the oscillatory mechanical motion, the model parameters and the steady state values. For certain parameters an optimal phase shift between flux and motion is identified to minimize the calculated energy. These findings are used together with the electromagnetic model to obtain a control law for the input voltage. By comparison of the numerical simulations, the obtained control strategies applied on the linearized and the non-linear model, are evaluated with respect to effectiveness and necessary energy effort. [1] Tippelt D. et al. Modelling, analysis and mitigation of self-excited vibrations of a magnetic track brake, Proceedings of the Institution of Mechanical Engineers, Part F: Journal of Rail and Rapid Transit, 2021 [2] Popp K. Modelling and control of friction-induced vibrations. Mathematical and Computer Modelling of Dynamical Systems. 2005.