Browsing by Author "Flores, Paulo"
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- Implementation of a non-Hertzian contact model for railway dynamic applicationPublication . Magalhães, Hugo; Marques, Filipe; Liu, Binbin; Antunes, Pedro; Pombo, João; Flores, Paulo; Ambrosio, Jorge; Piotrowski, Jerzy; Bruni, StefanoThe development of wheel–rail contact models is an active topic of railway research with the dual objective of improving the accuracy of multibody simulations and reducing its computational effort. This paper extends the online Hertzian contact model, proposed by Pombo et al. (Veh. Syst. Dyn. 45: 165–189, 2007) to propose a non-Hertzian contact model. The new methodology presented here includes the following steps: (i) search of the points of contact; (ii) identification of the undeformed distance function; (iii) evaluation of the contact patch; (iv) calculation of the normal and tangential contact forces; (v) application of the contact forces in the multibody vehicle model. Among several contact models available in the literature, this non-Hertzian contact approach uses the Kik–Piotrowski model for the normal contact force, while the tangential forces are obtained from the interpolation of the available Kalker Book of Tables for non-Hertzian (KBTNH) contact. With the purpose to demonstrate the proper implementation and selection of parameters that define this new model, several contact analysis and dynamic simulations are performed in which the wheel S1002 and the rail UIC50 are considered. First, the contact analyses that determine the contact condition of different wheel–rail interactions serve to assess the accuracy of the Hertzian and non-Hertzian models with respect to the software of reference CONTACT. Second, the Hertzian and non-Hertzian models are utilised to perform dynamic simulations of a wheelset, a bogie and a vehicle running in tangent and curved tracks. In short, this work provides, not only a complete description of the implementation of a non-Hertzian contact model in a multibody code, but also suggests for the proper selection of the parameters that promote better accuracy and optimal computational efficiency.
- Implementation of a non-Hertzian Contact Model for Railway DynamicsPublication . Magalhães, Hugo; Marques, Filipe; Liu, Binbin; Pombo, João; Flores, Paulo; Ambrosio, Jorge; Bruni, StefanoThe development of wheel-rail contact models is an active topic of research, in which more accurate and reliable methodologies are required to improve the realism of multibody simulations while reducing the computational effort. However, their implementation in a multibody software consists of a challenging task for itself. This paper presents a generalized strategy for the implementation of non-Hertzian contact models. This new methodology is split into the following steps: determination of points of contact; identification of the undeformed distance function; determination of the contact patch; and calculation of the normal and tangential forces which are computed according to the contact conditions and the theories used. Here, the normal force is determined with the Kik-Piotrowski model, while the tangential forces are obtained from the interpolation of the Kalker Book of Tables for Non-Hertzian contact patches. To demonstrate the proper implementation of the new methodology, static and dynamic simulations have been performed. First, a static wheel-rail interaction at different slip conditions is simulated being observed a good agreement with respect to the results obtained from CONTACT. Secondly, a dynamic simulation of a bogie running in a tangent track is considered, in which the contact developed respect the Hertzian conditions. Thus, a comparison between the Hertzian approach and the proposed methodology is possible, being discussed the slight differences between the results obtained from both simulations. Besides the proposed implementation is 4.5 times slower than the Hertzian method considered in this work, for the case study presented in this work, the proposed methodology allows to deal with non-Hertzian contact.
- A new simplified approach to deal with conformal contact in railway dynamicsPublication . Marques, Filipe; Magalhães, Hugo; Liu, Binbin; Pombo, João; Flores, Paulo; Ambrosio, Jorge; Bruni, StefanoThe contact between a wheel and a rail in the context of railway dynamics is mostly the result of the interaction of two convex surfaces. However, when negotiating sharp curves and due to worn profiles, the conformal contact tends to occur. In this type of interaction, the contact zone cannot be contained in a single plane as opposed to the non-conformal case. Hence, a new methodology to deal with conformal contact in the framework of railway dynamics is proposed in this work. A curved axis in the lateral direction is considered, and it is used to measure the separation between profiles. Moreover, the contact patch is divided into strips in which it is locally planar, and the pressure distribution is estimated based on Kik-Piotrowski model. The interaction between a wheel and rail has been tested for four static cases. The preliminary results show that this methodology can be a reliable alternative to the use of more computationally intensive approaches as the boundary or finite element methods.
- On the generation of enhanced lookup tables for wheel-rail contact modelsPublication . Marques, Filipe; Magalhães, Hugo; Liu, Binbin; Pombo, João; Flores, Paulo; Ambrosio, Jorge; Piotrowski, Jerzy; Bruni, StefanoIn railway dynamics, the interpolation of lookup tables (LUTs) is a procedure utilized to reduce the computational effort when computing the wheel-rail contact forces. However, the generation of LUTs with multiple inputs and multiple outputs is a challenging task for which aspects such as their minimal size and uniform accuracy over the LUT domain have not been systematically addressed in the literature. Thus, this work presents a comprehensive methodology for a detailed analysis of general LUTs, and identifies ways to improve them. For that, an analysis of the variation of the input parameters is made and the interpolation error is assessed on the cells and edges of the original table, then, based on this analysis, two enhanced LUTs are proposed. The first one is approximately 5 times smaller than the original but holds similar accuracy. The second table exhibits half of the maximum interpolation error of the original LUT but holds an identical size. The methodology is demonstrated here using the recently published Kalker Book of Tables for Non-Hertzian contact (KBTNH) but it can be used by any other LUT approach in order to improve accuracy and/or to reduce size.
- Síntese de alto nível em FPGAPublication . Véstias, Mário; Flores, Paulo; Cláudio de Campos Neto, HorácioAs metodologias e as ferramentas de projeto de sistemas digitais têm evoluído com o objetivo de conseguir circuitos melhores e mais eficientes. Com o aumento da complexidade dos sistemas digitais, surgiu uma nova dimensão no desenvolvimento destes sistemas relacionada com a eficiência de projeto. É necessário lidar com o aumento crescente da complexidade dos circuitos, com a redução do tempo disponível para o projeto, com a integração de equipas de projeto e com a dinâmica de evolução das novas tecnologias de circuito integrado. A introdução de linguagens de descrição de hardware (HDL – Hardware Description Language) foi fundamental para dar resposta ao aumento da complexidade dos sistemas digitais. Numa metodologia de projeto de sistemas digitais baseada em linguagens de descrição de hardware, os circuitos são descritos com uma HDL (p. ex., VHDL ou Verilog). As vantagens deste tipo de abordagem ao projeto de hardware são várias, como a documentação formal do circuito, a integração com ferramentas de síntese e de simulação, a reutilização de código, a portabilidade, entre outros. Os fluxos de projeto de circuito digital baseados em linguagens de descrição de hardware e as ferramentas de síntese e de simulação foram fundamentais para dar resposta ao projeto de sistemas digitais com crescente complexidade. Com o evoluir da tecnologia de circuito integrado e a complexidade das aplicações, bem como a redução do tempo de projeto, os fluxos de projeto baseados em linguagens de descrição de hardware tornam-se cada vez mais difíceis de conseguir e com maiores custos. Para lidar de forma eficiente com esta complexidade, é necessária uma abordagem à síntese de sistemas digitais a um nível de abstração mais elevado. Este nível de abstração foi conseguido com a síntese de alto nível (HLS – High Level Synthesis). A síntese de alto nível permite traduzir funcionalidades descritas numa linguagem de alto nível, por exemplo a linguagem C ou a C++, de forma automática, análogo ao trabalho realizado pelos compiladores de software. O conceito começou a ser investigado no início dos anos 80, tendo surgido algumas ferramentas comerciais na segunda metade da década de 90. Contudo, foi só a partir da década seguinte que houve um investimento forte de várias empresas na terceira geração de ferramentas de síntese de alto nível que alavancou o sucesso da síntese de alto nível. Um dos fatores que ajudou neste processo foi a tecnologia de hardware reconfigurável, permitindo não só um projeto rápido, como também a implementação rápida do circuito em hardware. A HLS tornou-se numa importante ferramenta de projeto, com os maiores mercados a utilizarem a HLS no projeto dos seus circuitos integrados. A presente obra procura contribuir para o crescimento da HLS ao disponibilizar conteúdos necessários à formação de quem pretende trabalhar nesta área.
- A three-dimensional approach for contact detection between realistic wheel and rail surfaces for improved railway dynamic analysisPublication . Marques, Filipe; Magalhães, Hugo; Pombo, João; Ambrosio, Jorge; Flores, PauloThe wheel-rail contact modeling problem assumes a preponderant role on the dynamic analysis of railway systems using multibody systems formulations. The accurate and efficient evaluation of both location and magnitude of the wheel-rail contact forces is fundamental for the development of reliable computational tools. The wheel concave zone might be a source of numerical difficulties when searching the contact points, which has been neglected in several works. Here, it is demonstrated that the minimum distance method does not always converge when the wheel surface is not fully convex, being an alternative methodology proposed to perform the contact detection. This approach examines independently the contact between each wheel strip and the rail, where the maximum virtual penetration is determined and associated with the location of the contact point. Then, an Hertzian-based force model is considered for both normal and tangential forces. The results obtained from dynamic simulations show that the minimum distance method and the proposed methodology provide a similar response for simplified wheel profiles. However, the new approach described here is reliable in the identification of the contact point when realistic wheel profiles are considered, which is not the case with the minimum distance method.
- Utilization of non-conformal wheel surfaces for railway dynamicsPublication . Marques, Filipe; Magalhães, Hugo; Pombo, João; Ambrosio, Jorge; Flores, PauloThe dynamic analysis of railway systems using multibody systems formulation is nowadays a reliable tool for several studies, in which the wheelrail contact interaction has significant importance. The definition of contacting geometries affects directly the dynamic response of the system. For sake of simplicity and computational efficiency, some authors simplify the wheel profile to avoid its concave part. This work intends to study the impact of utilizing a simplified non-conformal profile instead of a real wheel profile. For this purpose, an Hertzian-based contact force model is used to compute either the normal and creep forces. A wheelset rolling over a straight track is used as demonstration case. The results show that although the location of the contact points is similar, the use of simplified wheel profile may have an extensive impact on the dynamic response of the system.