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Guj L.,Rieter Automotive Management AG | Culla A.,University of Rome La Sapienza | Sestieri A.,University of Rome La Sapienza
Proceedings of ISMA 2010 - International Conference on Noise and Vibration Engineering, including USD 2010 | Year: 2010

In this paper the solution of a composite periodic longitudinal bar is investigated in the framework of the homogenization and stochastic techniques. A two scale asymptotic homogenization technique is used to provide the equivalent mechanical characterization of the bar. Such approach isolates a macro and microscale problem. The macro-scale problem describes the dynamics of the bar, while the micro-scale problem supplies the equivalent Young modulus and density. The micro-scale is made of two different materials: the portion of each material in the micro-scale is known in a statistical sense and a probability density function (pdf) is assumed to take into account such uncertainty. Consequently, the macro-scale material properties are random and their probability density functions are calculated in closed form. The random properties of the material represent the coefficients of the governing equation of the bar. The pdf of the solution (eigensolution) is obtained analytically. A Monte-Carlo simulation provides an alternative solution compared to the provided one.

Bertolini C.,Rieter Automotive Management AG | Guj L.,Rieter Automotive Management AG
SAE International Journal of Passenger Cars - Mechanical Systems | Year: 2011

The Diffuse Field Absorption Coefficient (DFAC) is a physical quantity very often used in the automotive industry to assess the performance of sound absorbing multilayers. From a theoretical standpoint, such quantity is defined under rather ideal conditions: the multilayer is assumed to be infinite in extent and the exciting acoustic field is assumed to be perfectly diffuse. From a practical standpoint, in the automotive industry the DFAC is generally measured on samples having a relatively small size (of the order of 1m2) and using relatively small cabins (in the order of 6-7m 3). It is well known that both these factors (the finite size of the sample and the small volume of the cabin) can have an influence on the results of the measurements, generating deviations from the theoretical DFAC. The widely used Transfer Matrix Method (TMM) allows the evaluation of the theoretical DFAC and can, in some implementations, approximately take into account the finite size of the sample by means of a suitable analytical correction. Within this method, though, the exciting acoustic field is always assumed to be ideally diffuse or, in any case, given by the superposition of a set of uncorrelated plane waves impinging on the sample with incident angles within a certain predefined range. This paper intends to present numerical investigations that allow going beyond this modeling technique. In a first part of the paper, this is done using an analytical model consisting of a rectangular cavity having the dimensions of a small cabin. The reverberation time of this cavity is evaluated with and without an absorbing sample placed on its floor and, from these data, the corresponding DFAC is calculated using Sabine's law. Results from this model are presented and compared with results coming from testing. Using this analytical model, it is already possible not only to evaluate the effect of the finite size of the sample, but also the effect of its positioning on the cavity's floor and, more importantly, the effect of the limited volume of the measurement environment. Both these latter effects cannot be taken into account in any way by means of the TMM. In the second part of the paper, a further degree of complexity is added: the same type of simulations are carried out by means of a finite element model of the widely used Alpha Cabin (whose volume is about 6.5m 3), coupled to a finite element model of the absorbing sample. Also in this case, simulation results are compared to results coming from testing. The use of a finite element model allows taking into account also the effect of the diffraction around the sample's edges. Also this effect is known to have, in special cases, some influence on the results of DFAC measurements and, of course, cannot be taken into account within the analytical model based on a rectangular cavity. © 2011 SAE International.

Guj L.,Rieter Automotive Management AG | Courtois T.,Rieter Automotive Management AG | Bertolini C.,Rieter Automotive Management AG
SAE International Journal of Passenger Cars - Mechanical Systems | Year: 2011

Typically, in the automotive industry, the design of the body damping treatment package with respect to NVH targets is carried out in such a way to achieve panel mobility targets, within given weight and cost constraints. Vibration mobility reduction can be efficiently achieved thanks to dedicated CAE FE tools, which can take into account the properties of damping composites, and also, which can provide their optimal location on the body structure, for a minimal added mass and a maximized efficiency. This need has led to the development of different numerical design and optimization strategies, all based on the modeling of the damping composites by mean of equivalent shell representations, which is a versatile solution for the full vehicle simulation with various damping layouts. However, these approaches, which can estimate correctly the beneficial vibration effect of damping pads application on the vehicle body, address the body NVH target with no consideration of the impact that the presence of the insulation on body panels can have on the final vibration result. On the other hand, the efforts carried out in the last years for FE implementations of Biot's system of equations have led to simulation methods at vehicle level, which can take into consideration the dynamical behaviour of porous materials and which allow including in an efficient and flexible way sound package parts into vehicle FE models used for NVH analyses. This paper presents a FE-based procedure, thanks to which it is possible to design the optimal damping lay-out with respect of panel mobility targets, while taking into account the presence of the insulation part on body panels. In a first section a design methodology for damping layout is presented. This method, that is completely integrated in Nastran, is able to provides the ranking and vibration pattern of the vehicle panels with highest mobility for a given frequency range and set of loads in order to maximize the effect of the damping treatment. Then the problem of the influence of the acoustic treatment on the panel vibrations has been addressed. The proposed solution is represented by an implementation in MSC/Nastran of the Biot-Allard theory for porous media. This procedure allows a smart coupling of structural FE model with a FE boundary representation of the acoustic part. In the last section, the benefits of the joined use of the two techniques are highlighted by mean of their application on a simple test case as well as on a full-vehicle. © 2011 SAE International.

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