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

Carrarini, Antonio

English

Institute of Transport Research:Publications

4Acoustic Postprocessing of Multibody SimulationsSIMPACK allows models including flex-ible 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 computa-tions of sound radiation and propaga-tion. Two application examples from railway engineering are presented.The noISe ProbleMThe widespread attention for noise ex- posure in modern society, accompa-nied by legislative and normative in-tervention on different levels, encour-ages the development of numerical tools for the acoustic analysis of me-chanical systems. In this context, the concept of a comprehensive "acoustic management" is becoming extremely important. But, in practice, the ex post acoustic improvement of existing sys-tems can be a tough and costly task. It follows that the consideration of acoustic requirements by simulation in the earliest possible stage of the pro-duct development is an essential issue. STATe of The ArTDue to the intrinsic complexity of the involved physical phenomena and the different fields which must be consid-ered (sound generation, radiation, propagation), only a few computa-tional methods and tools for acoustic analysis are available. Solely considering structure-borne sound, Finite Elements encompassing approaches based on modal decomposition are typically used. However, these tools are mainly restrict-ed to the frequency domain and to the analysis of steady phenomena. Moreo-ver, the set-up is often strongly simpli-fied and the vibrating body considered as "stand-alone". This means that the boundary conditions and the applied loads are only a rough approximation of the real operative environment. The MulTIbody APProAChOnce a multibody simulation is per-formed, the dynamics of the flexible bodies included in the model are known. In particular, the motion of every point on the surface of the bodies can be computed from the time histories of the modal coordinates. On this basis, the sound power on the body’s surface (structure-borne sound) can be evalu-ated with very little additional compu-tational effort and without setting up dedicated acoustic models. Moreover, according to the time dependent ap-proach incorporated in multibody sim-ulation, transient phenomena can be efficiently studied.In general, the multibody model does not need to be adapted for acoustic analysis: the user only has to ensure that the modal description of the acoustic relevant flexible bodies cov-ers the acoustic frequency range of interest. The simulation itself is also a completely conventional one, with all the advantages of the integrated envi-ronment offered by SIMPACK.The quantity computed by the post-processor can be interpreted as the intensity of the acoustic source (the vibrating body) and is generally not equivalent to the sound intensity per-ceived by a virtual receiver. In fact, the radiated sound power cannot be computed because this would require the knowledge of the radiation effi-ciency. This parameter can, however, only be determined by a subsequent, computationally expensive analysis of the fluid/structure interface to be done in an appropriate Finite Element or Boundary Element code. Moreover, the propagation of sound from the ra-diating body to the receiver should be computed, too.However, the knowledge of the sound power on the surface is sufficient for most engineering applications. This is the case for a relative acoustic as-sessment of mechanical parts with re-spect to design variations or modified boundary conditions. Additionally, the sound power on the surface directly leads to an estimate of the radiated power if the radiation efficiency can » CuSToMer APPlICATIonSAntonio CarrariniGerman Aerospace CentreSIMPACK»news, Publication June 2009Fig. 1: Exemplary instantaneous  distribution of the sound power  level over a railway wheel (frame  from a time animation)Fig. 2: Time/frequency diagram of the sound power for the switch crossing case. The wheelset hits the rail gap at t = 0.55 s5be deduced from experience, previous analyses or empiric formulas; this is possible in many engineering cases.Different kinds of analysis and repre-sentation of the results are possible. For example, the distribution of sound power over the vibrating body can be studied (Fig. 1), and classical time/frequency diagrams, particularly use-ful for transient phenomena, can be drawn (Fig. 2).APPlICATIon fIeldSThe acoustic postprocessing can be vir-tually performed on every vibrating body which is known or suspected to generate noise. The largest amount of possible applications is in the fields of machinery (gearwheels, casings, frames, etc.) but also peculiar prob-lems can be addressed, as in railway engineering (see the examples below).In many cases, the acoustic post-processing can be seen as an addition-al exploitation of the results of a high-frequency simulation. But, in other situations, the package “multibody simulation + acoustic postprocessing” can be seen as a proper acoustic anal-ysis tool, especially when other ap-proaches are not satisfactory. Such a situation occurs when unsteady phe-nomena must be studied or when the mechanical system in which the vibrating body is integrated plays an important role. This is the case for the railway wheelset presented below: the complete vehicle must be taken into account to correctly reproduce the bearing and rail/wheel contact forces acting on the vibrating wheelset.APPlICATIon exAMPle:SwITCh CroSSIngThe first example deals with a railway vehicle running through a switch. It has been chosen for its extremely tran-sient character, which clearly demon-strates the capabilities of the acous-tic postprocessor. In fact, the rail gap in the middle of the switch (called "frog") causes an impact of the wheel rolling over it. The induced vibration of the wheelset eventually generates impulsive noise. A generic passenger coach model was used, one wheelset having been modelled as flexible with a sufficient number of eigenmodes. S-variable rail profiles were deployed to model the switch on the basis of the standard SIMPACK Rail contact module. Apart from the frog geometry, the rails are perfectly smooth.The results are plotted as a time/fre-quency diagram in Fig. 5, highlight-ing a strong peak around 2100 Hz. The question arises, which modes are mainly responsible for the noise? An analysis of the modal contributions to the sound power is reported in Fig. 3. As the phenomenon is transient, the modal contributions are also a func-tion of time. The diagram clearly shows that the contributions from the modes no. 51/52 dominate the response.APPlICATIon exAMPle:rAIl roughneSSThe roughness of rails and wheels and the resulting vibration of the wheelsets is responsible for the notorious rolling noise, the most dramatic environmental problem of modern railways. This is particularly true for freight trains, be-cause the wheels are made very rough by the block brakes; on the contrary, pas-senger coaches, which use disc brakes, have smoother wheels and are quieter.On the basis of real measured rail roughness data (Fig. 4, courtesy of Prof. Hecht, TU Berlin, Fachgebiet Schienenfahrzeuge), the fluctuations in the normal contact force between wheel and rail were computed using PCM (Polygonal Contact Model, also available in SIMPACK as force element) on a simplified vehicle model. These fluctuations were then super-imposed to the standard W/R contact forces on a complete railway vehicle model with flexible wheelsets. The run over a short rough rail section pre-ceded and followed by smooth rail sec-tions was simulated (Fig. 5).ConCluSIonSThe proposed acoustic postprocessor for SIMPACK is a valuable novel tool for a quick analysis of the acoustic be-haviour of vibrating bodies in generic multibody models. Its capabilities have been successfully tested on real, com-plex application cases from railway en-gineering.» CuSToMer APPlICATIonSAntonio CarrariniGerman Aerospace CenterFig. 3: Time/eigenmode diagram showing the modal contributions to the sound power for the switch crossing case Fig. 4: Rough rail surface (interpo-lation of measured data) used for the PCM computations SIMPACK»news, Publication June 2009Fig. 5: Time/frequency diagram of the sound power for the rough rail case. For t < 0.3 s and t > 0.5 s the rail is smoothOn The WebFor more information, please see: www.dlr.de

Acoustic Postprocessing of Multibody Simulations

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