Acoustic Postprocessing of Multibody Simulations

SIMPACK allows models including flexible bodies to be set up and simulated efficiently in up to very high, relevant acoustic frequency ranges. In this article, a postprocessor module for SIMPACK which computes the sound power of a generic vibrating flexible component (so called structure-borne sound) is presented. The computed quantity is an indicator of the acoustic behaviour of the component and can also be used as an input for subsequent computations of sound radiation and propagation. Two application examples from railway engineering are presented

Zuverlässigkeitsbasierte Analyse der Seitenwindstabilität von Schienenfahrzeugen

Die Seitenwindstabilität spielt heutzutage eine entscheidende Rolle im Zulassungsverfahren von Schienenfahrzeugen und wird oft zur zentralen Randbedingung im Entwurfsprozess. In vielen Ländern, einschließlich Deutschland, basiert der Sicherheitsnachweis auf der numerischen Simulation des Fahrverhaltens unter ausgewählten Randbedingungen und enthält sowohl die fahrdynamischen und aerodynamischen Eigenschaften des Fahrzeugs als auch die zu erwartenden Windverhältnisse. Die Güte des Nachweises hängt deshalb von der Genauigkeit der verfügbaren Berechnungsmodelle ab. In dieser Hinsicht ist eine große Diskrepanz festzustellen: mittels der Mehrkörperdynamik ist es einerseits möglich, bei bekannten Randbedingungen das Verhalten des fahrenden Fahrzeugs sehr genau zu ermitteln; andererseits können die auf das Fahrzeug wirkenden aerodynamischen Lasten, die von der aerodynamischen Gestaltung des Fahrzeugs und dem Windszenario abhängen, nur grob geschätzt werden. Die Ursache liegt in der Komplexität der Strömung um das Fahrzeug und dem stochastischen Charakter des atmosphärischen Windes, die die Erstellung von Modellen und die Ermittlung der Werte der Systemparameter erschweren. In den Normen zur Seitenwindstabilität bleiben solche parametrischen Unsicherheiten grundsätzlich unberücksichtigt bzw. sie werden mit empirischen Sicherheitsfaktoren behandelt. In dieser Arbeit werden zunächst Verbesserungen in der Modellierung erarbeitet, sowohl auf der aerodynamischen als auch der fahrzeugdynamischen Seite. Es ergibt sich, dass im ersteren Fall komplexe Modelle unbedingt notwendig sind, um z.B. instationäre aerodynamische Vorgänge abzubilden, während im letzteren Fall einfachere Modelle als die üblichen kompletten Mehrkörpermodelle ausreichend wären und zum Beispiel die Anwendung der linearen Systemanalyse erlauben würden. Um die parametrischen Unsicherheiten in den Sicherheitsnachweis einzubeziehen, wird dann in dieser Arbeit die konventionelle Risikoanalyse mit Methoden aus der Zuverlässigkeitsanalyse gekoppelt. Solche Methoden, die in verschiedenen Formulierungen verfügbar sind und breite Anwendung in der Strukturmechanik finden, führen zur Quantifizierung des Risikos und erlauben deshalb letztendlich eine Reduktion der notwendigen Sicherheitsfaktoren. Dabei wird vorausgesetzt, dass eine statistische Beschreibung der Unsicherheiten gegeben ist. Obwohl die Erfüllung dieser Bedingung eine große Herausforderung darstellt, können wertvolle Aussagen auch bei minimaler verfügbarer Information gemacht werden. Darüber hinaus werden Sensitivitätsanalyse und Optimierung entsprechend dem vorgestellten Ansatzes formuliert und erweitert, um die Anwendung der Zuverlässigkeitsanalyse nicht nur im Nachweis- sondern auch im Entwurfsprozess zu ermöglichen. Die vorgestellten Methoden werden auf den realistischen Fall eines Hochgeschwindigkeitszugs angewandt.Nowadays, crosswind stability is a key topic for the homologation of railway vehicles and thus a pivotal boundary condition in their design process. In many countries, including Germany, the safety proof is based on the numerical simulation of the driving behaviour of the vehicle in extreme situations and must necessarily include the aerodynamic and driving performances of the vehicle as well as the wind conditions to be reasonably expected during operation. It follows that the quality of the safety proof depends on the accuracy of the available models. In this respect a deep gap can be observed: on the one hand high accuracy can be reached by multibody simulation in the investigation of the driving dynamics; on the other hand the aerodynamic loads acting on the vehicle, which depend on the vehicle shape and the wind scenario, can be estimated only with poor accuracy. The latter problem is due to the difficulties in the set up of models and the determination of the system parameters because of the complexity of the three dimensional flow around the vehicle and the implicit stochastic nature of atmospheric wind. In the present norms for crosswind stability such modelling uncertainties are usually not considered or are very empirically taken into account by safety factors. In this work some improvement in the modelling of the aerodynamic phenomena and the driving dynamics are firstly introduced. It could be observed that with regard to aerodynamics more complex models than usual are necessary, e.g. to cover unsteady phenomena; on the contrary, simpler models than usual can be sufficient for the analysis of the driving dynamics, allowing, for example, the use of linear system theory. Then, in order to include parametric uncertainty in the safety proof, the conventional risk analysis for crosswind stability has been coupled with methods from reliability analysis. Such methods, which are quite common in structural mechanics and are available in different formulations, lead to the efficient assessment of the risk and thus to a reduction of the safety factors, provided that a statistical description of the uncertainties is available. Even though the provision of such a description is often a challenge, good results can be also achieved on the basis of little available information. Furthermore, sensitivity analysis and optimisation can be reformulated on the basis of the proposed approach, so that reliability analysis can be integrated not only in the safety proof but also in the design process. The discussed methods have been tested on the real case of a German high speed train

Numerical Analysis of Railway Rolling Noise Generation

Due to its environmental and thus social relevance, railway noise has gained in the last years large importance and has been object of large legislative and normative intervention on different levels, [1, 2, 3, 4]. As a consequence, the acoustic behavior of railway vehicles is becoming a central topic for manufacturers and transportation companies, which need to developed noise optimal configurations and noise abatement concepts. This finally leads to a need for analysis tools which allow the acoustic properties of vehicles and tracks to be analyzed or predicted in an efficient and economic way, [5]. In this context, the concept of a comprehensive “acoustic management” has been developed, see for example [6]. Moreover, many standards have been defined, e.g. [7, 8, 9]. The actual rolling noise problem encompasses different topics: rail/wheel contact dynamics, wheelset and rail (partially including track) structural dynamics, sound radiation, sound propagation. An overview can be found in [10, p. 483 et seqq.]. Different established computer tools covering one of the above mentioned topics exist (e.g. SIMPACK, ANSYS, SYSNOISE, SOUNDPLAN, ...). However, some topics can still be addressed only by approximations or by the simplifying neglection of some physical effects, [10, p. 565]. This was for example the case of the structural dynamics of the rotating wheelset as a part of a complete vehicle model, which has been very recently solved, [11]. At the moment, the only commercially available software aiming to cover the whole subject of rolling noise is TWINS [12]. In the present report two aspects the overall problem are addressed: the wheel/rail contact and the structural dynamics of the wheelset. These topics cover the part of the overall problem that can be roughly designated as rolling noise generation

Application of SimSound to an analytical example and to real cases of railway rolling noise

The implementation of SimSound as a tool for the acoustic postprocessing of multi-body simulations including flexible bodies has been described in a previous report, Carrarini (2008a). In the present one, the application of SimSound to an analytical example and to two real cases of railway noise is reported: - Plate: A simply supported rectangular plate is excited by a point force. As an exact analytical solution for this problem can be given, this case has been used to check the implementation. - Switch gap: A railway vehicle crosses a switch; the impact due to the wheel rolling over the rail gap in the central part of the switch (the so called frog) causes strong vibrations of the wheelsets and thus impulsive noise. - Rough rail: A railway vehicle runs over rough rails. A similar example was reported in Carrarini (2008b) but in the present work real measured roughness data (courtesy of Prof. Hecht, TU Berlin, Fachgebiet Schienenfahrzeuge) has been used

ACOUSTIC POSTPROCESSING FOR MULTIBODY SIMULATIONS INCLUDING FLEXIBLE BODIES

In this paper an acoustic postprocessor for multibody simulations is presented. The tool computes the sound power on the surface of a generic vibrating flexible body. The radiated sound power is not computed because this would require the knowledge of the radiation efficiency. Such parameter can be determined by an ulterior analysis including the fluid/structure interface, to be performed by an appropriate Finite Element or Boundary Element analysis. The sound power computed by the presented tool can be directly used for the relative acoustic assessment of mechanical parts with respect to design variations or modified boundary conditions. Moreover, the computed sound power directly leads to an estimate of the radiated power – and thus eventually of the noise level – if the radiation efficiency can be deduced from experience or empiric formulas, which is the case of most engineering problems. As an application example, the acoustic behavior of a railway wheelset is studied. The wheelset is modeled as a flexible body and integrated in the multibody model of a complete vehicle. Two operative cases have been considered: first, the vehicle driving through a switch, with the resulting impulsive noise due to the wheel rolling over the rail gap in the middle of the switch (“frog”); secondly, the rolling noise due to wheel and rail roughness – a dramatic problem especially for freight vehicles

Reliability based analysis of the crosswind stability of railway vehicles

Nowadays, crosswind stability is a key topic for the homologation of railway vehicles and thus a pivotal boundary condition in their design process. In many countries, including Germany, the safety proof is based on the numerical simulation of the driving behaviour of the vehicles in extreme situations and must necessarily include the aerodynamic and driving performances of the vehicle as well as the wind conditions to be reasonably expected during operation. It follows that the quality of the safety proof depends on the accurency of the available models. In this respect a deep gap can be observed: on the one hand high accuracy can be reached by multibody simulation in the investigation of the driving dynamics; on the other hand the aerodynamics loads acting on the vehicle, which depend on the vehicle shape and the wind scenario, can be estimated only with poor accuracy. The latter problem is due to the difficulties in the set up of models and the determination of the system parameters because of the complexity of the three dimensional flow around the vehicle and the implicit stochastic nature of atmospheric wind. In the present norms for crosswind stability such modelling uncertainties are usually not considered or are very empirically taken into account by safety factors. In this work some improvement in the modelling of the aerodynamic phenomena and the driving dynamics are firstly introduced. It could be observed that with regard to aerodynamics more complex models than usual are necessary, e.g. to cover unsteady phenomena; on the contrary, simpler models than usual can be sufficient for the analysis of the driving dynamics, allowing, for example, the use of linear system theory. Then, in order to include parametric uncertainty in the safety proof, the conventional risk analysis for crosswind stability has been coupled with methods from reliability analysis. Such methods, which are quite common in structural mechanics and are available in different formulations, lead to the efficient assessment of the risk and thus to a reduction of the safety factors, provided that a statistical description of the uncertainties is available. Even though the provision of such a description is often a challenge, good results can be also achieved on the basis of little available information. Furthermore, sensitivity analysis and optimisation can be reformulated on the basis of the proposed approach, so that reliability analysis can be integrated not only in the safety proof but also in the design process. The discussed methods have been tested on the real case of a German high speed train

Efficient models and techniques for the computational analysis of railway vehicles in crosswinds

Nowerdays, crosswind stability is a key topic for the homologation of railway vehicles and thus a pivotal boundary condition in their design proccess. In most countries the safety proof is based on the numerical multibody simulation of the driving behaviour of the vehicle assuming a worst case wind scenario. The conservativeness of this approach is aggravated by the uncertainties in the parameters of the simulation models. In this work some possible improvement of the safety proof are firstly discussed, mainly concerning the computational model of the vehicle. An alternative approach based on reliability techniques is then presented and discussed, finally leading to a more efficient assessment of the risk and thus to a reduction of the safety factors