Contact Geometry Research Articles

Dynamic wheel–rail adhesion characteristics on wet curved tracks considering surface roughness

Wheel–rail rolling contact is a complex tribological issue, especially the existence of a thin water film on the rail surface, which leads to low adhesion problems for the traction and braking performance of the railway trains. The objective of this paper is to propose a non-Hertzian dynamic wheel–rail adhesion model for trains passing through wet curved tracks considering wheel–rail surface roughness. A vehicle-track coupling dynamics model was first developed to output the wheel–rail dynamic contact parameters under wet conditions. These parameters were subsequently utilized as dynamic inputs for the present wheel–rail adhesion model to simulate dynamic curve passing. The present numerical model includes the normal contact model and tangential model. For the normal contact issue, an elastohydrodynamic lubrication model was used to accurately determine the normal contact stress within the contact patch along the contact traces. Based on the normal contact characteristics, the advanced extended creep force model was used to compute the tangential stress. Finally, the dynamic wheel–rail adhesion coefficient can be obtained. This model considers the effects of time variation and changes in contact geometry, resulting in sudden increases or decreases in the adhesion coefficient at certain locations. At a speed of 400 km/h, a higher spin creepage occurs, leading to an increased displacement of the third-body layer and causing the maximum tangential stress to shift from one side of the contact patch to the other. Under the combined influence of the creepages, the maximum tangential stress occurs at the trailing edge of the contact patch.

Dynamical similarity in field theories

Abstract In previous work I have shown that Herglotz actions reproduce the dynamics of classical mechanical theories which exhibit dynamical similarities. Recent work has shown how to extend field theories in both the Lagrangian and de Donder-Weyl formalism to contact geometry [1–3]. In this article I show how dynamical similarity applies in field theory. This is applied in both the Lagrangian and Hamiltonian frameworks, producing the contact equivalents. The result can be applied to general relativity where I demonstrate how to construct a complete description of the dynamics, equivalent to those derived from the Einstein-Hilbert action, without reference to the conformal factor.

Multidimensional integrable systems from contact geometry

Upon having presented a bird’s eye view of history of integrable systems, we give a brief review of certain recent advances in the longstanding problem of search for partial differential systems in four independent variables, often referred to as (3+1)-dimensional or 4D systems, that are integrable in the sense of soliton theory. Namely, we review a recent construction for a large new class of (3+1)-dimensional integrable systems with Lax pairs involving contact vector fields. This class contains inter alia two infinite families of such systems, thus establishing that there is significantly more integrable (3+1)-dimensional systems than it was believed for a long time. In fact, the construction under study yields (3+1)-dimensional integrable generalizations of many well-known dispersionless integrable (2+1)-dimensional systems like the dispersionless KP equation, as well as a first example of a (3+1)-dimensional integrable system with an algebraic, rather than rational, nonisospectral Lax pair. To demonstrate the versatility of the construction in question, we employ it here to produce novel integrable (3+1)-dimensional generalizations for the following (2+1)-dimensional integrable systems: dispersionless BKP, dispersionless asymmetric Nizhnik–Veselov–Novikov, dispersionless Gardner, and dispersionless modified KP equations, and the generalized Benney system.

AN ENGINEERING MODEL FOR PRESSURE ASSESSMENT IN ROUGH CONTACTS

In many scenarios involving mechanical contacts loaded in the elastic-plastic domain, finding the pressure distribution and the related stresses without a detailed description of the plastic strains may be a sufficiently good approximation for the contact problem solution. This approach is considered in this work, which aims to achieve a rapid solution for the pressure distribution in elastic-plastic contacts without the solution of the residual part: the influence of the plastic strains on the contact geometry, i.e., the residual displacements, and on the subsurface stresses, i.e., the residual stresses, are not accounted for. Experimental works and numerical approaches from the literature prove that the pressure in elastic-plastic contacts is limited to roughly three times the yield strength of the softer material. The latter threshold is introduced in a previously developed computer code for the purely elastic contact, as an additional restriction. The elastic-plastic pressure is calculated from the solution of the purely elastic problem, but it is not allowed to exceed the aforementioned limit. Considering the iterative nature of the original elastic solver, algorithm convergence is not affected by the additional restriction. The technique is firstly applied to a Hertz-type contact to prove the solution viability. A roughness sample is then processed with the newly advanced computer program. The resulting pressure distribution is compared to the purely elastic case. Considering that the former was achieved with the same computational effort as the latter, it is clearly a more convenient solution for practical engineering purposes.

Research on Flower Pattern and Roll Positioning Optimization for Roll Forming Process of TA4 Coiled Tubing.

Titanium alloy coiled tubing (CT) has great superiority while operating in deep wells with severe conditions, but it faces serious challenges during roll forming like high springback, edge cracking and surface wearing. In order to utilize the high precision and high efficiency production of titanium alloy CT, it is urgently necessary to develop a detailed process design. Based on the W-forming strategy and the cubic curve of the strip edge projection, the flower pattern and roll design for the 21-pass roll forming process were completed. Different tube dimensions and roll positionings were researched regarding the aspects of contact state, stress-strain distribution and geometry ovality using numerical simulation. The thickness/diameter ratio of the tube that can be formed should be within an effective range: exceeding the range leads to edge cracking, and an insufficient ratio leads to high springback. Roll positioning significantly affects the contact state and stress distribution, but leads to no major differences in the tube geometry. Finally, the trial production of the TA4 titanium CT with a size of Φ50.8 × 4 mm was completed successfully and was consistent with the simulation results.

Dependence of terahertz photoconductive switch performance on metal contact geometry

This study investigates the emission and spectral characteristics of photoconductive THz switches employing coplanar stripline contact geometries fabricated on a GaAs substrate. The experimental results reveal how the power outputs as well as the spectral shape are significantly influenced by the strip width dimension. Utilizing the Drude–Lorentz conductivity model, photocarrier dynamics were analyzed through an RLC circuit framework, offering insights into how the contact design influences the spectral response. Our findings suggest that matching the photocurrent impedance to that of the metallic contacts is critical to improving the efficiency of these devices.

Fast symplectic integrator for Nesterov-type acceleration method

In this paper, explicit stable integrators based on symplectic and contact geometries are proposed for a family of non-autonomous ordinarily differential equations (ODEs) found in improving convergence rate of Nesterov’s accelerated gradient method. Symplectic geometry is known to be suitable for describing Hamiltonian mechanics, and contact geometry is known as an odd-dimensional counterpart of symplectic geometry. Moreover, a procedure, called symplectization, is a known way to construct a symplectic manifold from a contact manifold, yielding autonomous Hamiltonian systems from contact ones. It is found in this paper that a previously investigated non-autonomous ODEs can be written as a contact Hamiltonian system family. Then, by developing and applying a symplectization of non-autonomous contact Hamiltonian vector fields expressing the non-autonomous ODEs, novel symplectic integrators are derived. Because the proposed symplectic integrators preserve hidden symplectic and contact structures in the ODEs, they are expected to be more stable than the Runge–Kutta method. Numerical experiments demonstrate that, as expected, the second-order symplectic integrator is stable and high convergence rates are achieved.

Shot noise as a diagnostic in the ν = 2/3 fractional quantum Hall edge zoo

The ν=2/3 filling is the simplest paradigmatic example of a fractional quantum Hall state, which contains counter-propagating edge modes. These modes can be either in the unequilibrated regime or equilibrated to different extents, on top of a possible edge reconstruction. In the unequilibrated regime, two distinct renormalization group fixed points have been previously proposed, namely Kane–Fischer–Polchinski and Wang–Meir–Gefen. In the equilibration regime, different degree of thermal equilibration may occur, while charge is fully equilibrated. Here, we show that this rich variety of models can give rise to three possible conductance plateaus at e2/2h (recently observed in experiments), 5e2/9h (predicted here), and e2/3h (observed earlier in experiments) in a quantum point contact geometry. We identify different mechanisms for electrical shot noise generation at these plateaus, which provides an experimentally accessible venue for distinguishing among the distinct models.

In-Situ n-Type Doped Carrier-Injection Layers in GeSn Direct Bandgap LEDs for Methane Sensing

GeSn-based group-IV alloys are attracting great attention in the Si photonics community, as they are considered to be compatible with Complementary Metal Oxide Semiconductor (CMOS) technology. Alloying germanium with more than 8% of tin (Sn) results in direct bandgap semiconductors, with some optical gain in devices such as lasers. Recently, room temperature optically pumped lasing was achieved thanks to high Sn content stacks. GeSn alloys can be used for near, short and mid wavelength infra-red spectral range operation, notably for gas detection. Current efforts are focused on electro-optical GeSn IR devices such as light-emitting devices (LEDs), electrically pumped lasers and photodetectors.In this paper, we evaluate the impact of various types of in situ n-type doped carrier injection layers on top of GeSn direct bandgap LEDs. More precisely, we compare the performances of GeSn:P and Ge:P –capped mid IR LEDs. Using reduced pressure chemical vapor deposition and metastable growth conditions (e.g. fast growth rates at low temperature), high crystalline quality GeSn layers were grown on 200 mm diameter Ge-buffered Si(001) wafers (Figure a). In situ n-type doped Ge layers (sample A) and GeSn layers (sample B) were used to inject carriers into direct band gap Ge0.87Sn0.13 active layers beneath (Figure b). I(V) curves (Figure c) and Electro-Luminescence spectra (Figure d) were compared for sample B (GeSn:P) and sample A (Ge:P). Very similar I(V) behaviors were observed under forward bias for both samples. However, a higher dark current was measured under reverse bias for sample B than for sample A. This could be due to a higher number of defects in the thicker capping layer of sample B (240 nm) compared to that of sample A (50 nm). However, a 2-fold increase in the EL signal was obtained for sample B than sample A (Figure d). Temperature dependence electroluminescence measurements and atom probe tomography data will be used to explain the electroluminescence behavior differences between Samples A and B.Finally, even higher Sn content LEDs were fabricated to emit light at 3.3 µm (Figure e). A direct band gap Ge0.846Sn0.154LED (Sample C) with an in situ n-type doped Ge cap was placed in a gas cell filled with diluted methane. Its emission overlapped well with the absorption of methane (Figure f). The methane detection limit of our setup was evaluated (data not shown). To further increase the emitted power of GeSn LEDs, current spreading was studied for different top contact geometries. Emission maps collected by an InSb camera at room temperature showed a definite dependence of light emission on electrical contact geometry (Figure g). Investigations are underway to further increase light extraction from LEDs and improve their performances for use in environmental sensors. Acknowledgement: This work was supported by the European Union’s Horizon 2020 LASTSTEP Project under the grant agreement ID: 101070208, the EDEN Carnot project and the CEA DRF-DRT Phare project. We gratefully acknowledge the clean room staff from LETI and IRIG for their technical support. Figure 1

On geometric bases for quantum A-polynomials of knots

A simple geometric way is suggested to derive the Ward identities in the Chern-Simons theory, also known as quantum A- and C-polynomials for knots. In quasi-classical limit it is closely related to the well publicized augmentation theory and contact geometry. Quantization allows to present it in much simpler terms, what could make these techniques available to a broader audience. To avoid overloading of the presentation, only the case of the colored Jones polynomial for the trefoil knot is considered, though various generalizations are straightforward. Restriction to solely Jones polynomials (rather than full HOMFLY-PT) is related to a serious simplification, provided by the use of Kauffman calculus. Going beyond looks realistic, however it remains a problem, both challenging and promising.

A rigidity result for coisotropic submanifolds in contact geometry

A rigidity result for coisotropic submanifolds in contact geometry

Contact geometry and contact time effect on 690TT alloy/405 stainless steel impact-sliding fretting wear processes

Due to the inevitable flow-induced vibration and turbulent excitation in steam generator (SG), it is impossible to avoid fretting wear or impact between steam generator tubes (SGTs) and anti-vibration bars (AVBs). Therefore, the approach of this work was to simulate the real fretting problems between SGTs and AVBs. Thus, the impact-sliding fretting wear tests of 690TT alloy/405 stainless steel (405 SS) were investigated under various contact times in tube/plate configuration and tube/cylinder configuration in air at room temperature, respectively. The results indicated that the influences of contact geometry and contact time on the peak radial force (Frp) were insignificant, while the value of peak tangential force (Ftp) was seen to increase with increasing contact time and was associated with the contribution of sliding wear to impact-sliding process. Meanwhile, the degree of wear damage increased with the increase of contact time, either line contact or point contact. The damage area of wear scar in tube/plate configuration was evidently larger than that of tube/cylinder configuration. Instead, the maximum wear depth of tube/cylinder configuration was larger than that of tube/plate configuration. Along the sliding direction, the “U" shape profiles were usually detected whether tube/plate configuration or tube/cylinder configuration. In general, both contact geometry and contact time had a direct influence on the behavior of wear debris, i.e., movement and oxidation.

Improving the stability of high-speed trains using an innovative passive yaw damper

Ensuring lateral stability is essential for achieving excellent operation safety and ride comfort of high-speed trains. However, complicated operational environments, particularly various wheel-rail contact geometries, would result in the occurrence of hunting instability. This study presents an innovative passive yaw damper to improve the stability of a high-speed train undergoing extreme wheel-rail contact conicities. A sensitivity analysis of the vehicle stability is first performed to demonstrate the importance of the dynamic stiffness of yaw dampers, promoting the application of the innovative damper. Experimental characterisation procedures enable the detailed investigation into the static and dynamic characteristics of the innovative damper, which are compared with those of conventional passive dampers to highlight the difference. To simulate the dynamic behaviour of the yaw damper, a numerical modelling method based on the Maxwell model is proposed. Using this method, the effectiveness of the innovative damper is assessed in terms of improving the vehicle stability at various wheel-rail conicities. The results show that the innovative yaw damper can ensure carbody stability at low conicity and bogie stability at high conicity, superior to conventional dampers, which fail to balance the vehicle stability at distinct conicities and usually result in hunting instability in specific wheel-rail contact states.

Research on wheel–rail impact dynamics due to combined rail weld irregularities and polygonal wheel effects considering 3D contact geometry

The geometric irregularities of rail welds and polygonal wheels are common defects observed in high-speed railways, leading to the generation of high-frequency impact forces and vibrations in the wheel–rail contact system. This research establishes a novel vehicle-track coupled dynamic model that incorporates a 3D wheel–rail contact model using meshing grid and conjugate gradient methods to study the high-magnitude wheel–rail impact forces caused by rail weld irregularities and polygonal wheel combinations. The primary objective of this study is to develop a precise 3D contact model that incorporates the actual geometry of wheel and rail surface irregularities into the calculation of vehicle-track dynamic interactions. The study evaluates the effects of 3D rail weld irregularities and polygonal wheels on wheel–rail dynamic interaction by comparing results with those obtained from a conventional vehicle-track coupled model that considers a 2D contact model. The results show that the lengths and depths of polygonal wheels and rail weld irregularities, as well as vehicle speeds, significantly increase the wheel/rail dynamic impact forces. The results also indicate that the widening of irregularities not only significantly affects the wheel–rail contact force but also has an important influence on the contact stiffness.

An open-source framework for the generation of OpenSim models with personalised knee joint geometries for the estimation of articular contact mechanics

Musculoskeletal modelling pipelines typically use generic models scaled to individual’s anthropometry. The ability to represent variations in bone or joint geometry and alignment is highly limited. This may have a large effect, particularly when modelling contact between articular surfaces such as for the knee where articular contact mechanics are used to determine joint kinematics and the resulting cartilage contact pressures and locations. Here we describe a developed open-source framework for the personalisation of such models and compare dynamic simulation outputs. The framework involves three main steps: (1) positions personalised geometries from magnetic resonance imaging and replaces generic bone and contact geometries. (2) Repositions muscle and ligament attachments and via points and optimisation of wrapping surfaces to ensure physiological lengthening behaviour. Finally, (3) muscle and ligament properties are calibrated to ensure physiological behaviour. Following model creation, dynamic simulations from a single participant and gait trial were compared. Small changes in knee adduction/abduction and rotation angles were observed between models. Joint moment differences however were present in not only the knee but also hip and ankle joints. These differences resulted in changes in both the magnitude and location of knee joint contact pressure. The framework developed is automated and requires only minimal user interaction and is built using open-source software packages which can be freely downloaded and installed. The adoption of such personalised modelling approaches facilitates patient specific modelling and may provide more detailed information regarding disease progression, patient stratification and facilitate personalised rehabilitation and treatment planning.

Hybrid TLM‐CTLM Test Structure for Determining Specific Contact Resistivity of Ohmic Contacts

ABSTRACTVarious test structures can be used to determine the specific contact resistivity of ohmic contacts. The transmission line model test structure and circular transmission line model test structure are the most commonly used. The analytical expressions of the former are straightforward and effectively describe the electrical behaviour of a contact, while the concentric geometry of the latter eliminates complications during fabrication. In this article, we present a hybrid test structure that combines the advantages of the transmission line and the circular transmission line models. The analytical expressions of the new structure are presented, and its finite‐element modelling is undertaken. The effect of contact geometry on this test structure is also discussed. Using the presented test structure, determining contact parameters does not require any error corrections.

Transient mixed thermal elastohydrodynamic lubrication analysis of aero ball bearing under non-steady state conditions

A transient mixed thermal elastohydrodynamic lubrication model of bearing considering the thermal effect and the changes in velocity, load, and contact geometry is proposed. Based on the bearing dynamic and lubrication analysis, the effects of bearing rotational speed and start-stop conditions on the dynamic behavior and lubrication performance are studied. Results show that when the bearing rotational speed exceeds a certain critical value, the increasing speed will reduce the film thickness, especially the outer raceway. The increasing bearing acceleration can cause a higher slide-roll ratio and film temperature at the initial start-up process. Compared with the start-up process, the lubrication change in the shut-down process has a similar opposite trend, but the overall lubrication performance is better.

A Non‐Iterative Wheel‐Rail Contact Geometry Determination Algorithm and Its Application

AbstractTo ensure a more accurate simulation of the wheel‐rail contact state in vehicle‐track interaction, a non‐iterative contact geometry determination algorithm is proposed. This algorithm decouples lateral movement and roll, independently calculates the minimum wheel‐rail gap on both sides, and overcomes the limitation of simultaneous wheel‐rail contact on both sides. It can be applied to single‐side and two‐side wheel‐rail separation problems. Additionally, by converting track irregularity and rail deformation into the reverse displacement of the wheel, it avoids real‐time calculation of the wheel‐rail spatial position, thereby improving calculation efficiency. The effectiveness of the algorithm is verified through Universal Mechanism software and field measurements. Subsequently, the influence of the measured wheel‐rail profile, rail cant, and rail gauge on the wheel‐rail contact characteristics is analyzed. Finally, vehicle shaking caused by track irregularity and wheel‐rail contact is examined from the perspective of wheel‐rail matching. The study demonstrates that the non‐iterative algorithm is suitable for online simulation by pre‐processing the contact geometry parameters. When the track gauge and rail cant increase, the distribution of wheel‐rail contact points becomes more concentrated. Furthermore, when the excitation frequency of wheel‐rail matching is close to the track irregularity excitation frequency, it causes vibration amplification, resulting in lateral vehicle shaking.

A Photovoltaic Self-Powered Volatile Organic Compounds Sensor Based on Asymmetric Geometry 2D MoS2 Diodes

Transition metal dichalcogenides have gained considerable interest for vapour sensing applications due to their large surface-to-volume ratio and high sensitivity. Herein, we demonstrate a new self-powered volatile organic compounds (VOC) sensor based on asymmetric geometry multi-layer molybdenum disulfide (MoS2) diode. The asymmetric contact geometry of the MoS2 diode induces an internal built-in electric field resulting in self-powering via a photovoltaic response. While illuminated by UV-light, the sensor exhibited a high responsivity of ∼60% with a relatively fast response time of ∼10 sec to 200 ppm of acetone, without an external bias voltage. The MoS2 VOC diode sensor is a promising candidate for self-powered, fast, portable, and highly sensitive VOC sensor applications.

α,ω-Alkanedibromides Form Low Conductance Chemisorbed Junctions with Silver Electrodes.

Chemical groups capable of connecting molecules physically and electrically between electrodes are of critical importance in molecular-scale electronics, influencing junction conductance, variability, and function. While the development of such linkage chemistries has focused on interactions at gold, the distinct reactivity and electronic structure of other electrode metals provides underexplored opportunities to characterize and exploit new binding motifs. In this work we show that α,ω-alkanedibromides spontaneously form well-defined junctions using silver, but not gold, electrodes through application of the glovebox-based scanning tunneling microscope-based break junction method. We systematically evaluate, through a series of additional studies, whether these molecular components form physisorbed or chemisorbed contact geometries, and if they undergo secondary chemical reactions at the silver surface. Critically, we find that the same junctions form when using different halide, or trimethylstannyl, terminal groups, suggestive of an electronically transparent silver-carbon(sp3) contact chemistry. However, the experimental conductance of the junctions we measure with silver electrodes is ∼30× lower than that observed for such junctions comprising gold-carbon(sp3) contacts, which does not align with predictions based on first-principles calculations. We further exclude the possibility that the proposed silver alkyl species undergo α- or β-hydride elimination reactions that result in a distinct contact chemistry through conductance measurements of control molecules that cannot undergo such processes. Applying insights provided from prior temperature-programmed desorption studies and a robust series of atomistic simulations, we ultimately propose that in these experiments we measure alkoxide-terminated junctions formed from the reaction of the chemisorbed alkyl with oxygen that is coadsorbed on the silver surface. This work, in demonstrating that high conductance contact chemistries established using model gold electrodes may not be readily transferred to other metals, underscores the need to directly characterize the interfacial electronic properties and reactivity of electrode metals of wider technological relevance.