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Technical Paper

Numerical Prediction of the Exhaust Noise Transmission to the Interior of a Trimmed Vehicle by Using the Finite/Infinite Element Method

During the acceleration of a vehicle, the contribution of the exhaust noise to the interior sound pressure level is significant. The acoustic insulation brought by the trim components must be designed with that consideration in mind. As such, there is an increasing need for developing reliable methods for predicting the airborne noise transmission between the exhaust system and the sound pressure level at the passenger's ears, taking into account the positive impact of various trim components. This paper presents a methodology that has been developed for addressing this need. Based on a finite/infinite element method, the computational procedure is divided in two steps: 1 The first step involves the exterior acoustic field all around the vehicle.
Technical Paper

Vibro-Acoustic Simulation of Intake Air Filter Using a Hybrid Modal Physical Coupling

To assess the acoustic performance of modern automotive air filters, both the air-borne engine noise propagating through the interior air of the system (known as “pipe noise”) and the structure-borne noise radiated by the shell (“shell noise”) should be evaluated. In this paper, these different propagation paths are modeled using the finite element solver Actran on industrial test cases set-up by SOGEFI Air and Cooling Systems. The test-case is designed in such a way that the different propagation paths are assessed separately. First the engine acoustic pulsation that is transmitted through the filter's structure is considered. Second, the noise radiated by the shell excited by mechanical forces at the support location of the filter is evaluated. Finally, the acoustic transmission loss of the filter is predicted. The ingredients of the finite/infinite element models are reviewed in details in the paper.
Technical Paper

Vibro-Acoustic Simulation of Side Windows and Windshield Excited by Realistic CFD Turbulent Flows Including Car Cavity

Nowadays, the interior vehicle noise due to the exterior aerodynamic field is an emerging topic in the acoustic design of a car. In particular, the turbulent aerodynamic pressure generated by the air flow encountering the windshield and the side windows represents an important interior noise source. As a consequence PSA Peugeot Citroën is interested in the numerical prediction of this aerodynamic noise generated by the car windows with the final objective of improving the products design and reducing this noise. In the past, several joint studies have been led by PSA and Free Field Technologies on this topic. In those studies an efficient methodology to predict the noise transmission through the side window has been set up. It relies on a two steps approach: the first step involves the computation of the exterior turbulent field using an unsteady CFD solver (in this case EXA PowerFlow).
Journal Article

Evaluation of Trim Absorption to Exterior Dynamic and Acoustic Excitations Using a Hybrid Physical-Modal Approach

The NVH study of trimmed vehicle body is essential in improving the passenger comfort and optimizing the vehicle weight. Efficient modal finite-element approaches are widely used in the automotive industry for investigating the frequency response of large vibro-acoustic systems involving a body structure coupled to an acoustic cavity. In order to accurately account for the localized and frequency-dependant damping mechanism of the trim components, a direct physical approach is however preferred. Thus, a hybrid modal-physical approach combines both efficiency and accuracy for large trimmed body analysis. Dynamic loads and exterior acoustic loads can then be applied on the trimmed body model in order to evaluate the transfer functions between these loads and the acoustic response in the car compartment.
Technical Paper

From Body in White to Trimmed Body Models in the Low Frequency Range: a New Modeling Approach

This paper describes a new approach for modeling a trimmed vehicle body by blending FEA models of the BIW, the passenger compartment and each individual trim component. The approach bases on the update of modal matrices, transforming the untrimmed body-cavity modal representation into an updated modal model including the effect of the trim configuration on the local and global NVH indicators. Results on simple and more realistic models are presented and show that the methodology fulfills the efficiency and accuracy criteria and is thus to guide the NVH development process.
Technical Paper

Vibro-Acoustic Simulation of Mechanical Components Excited by Distributed Random Loads

The design of automotive mechanical components requires the consideration of various excitations related to physical tests (involved in the validation process) and/or operational conditions. In such a context, random distributed excitations (like diffuse field and turbulent boundary layer) play a particular role. Modeling and simulation of the vibro-acoustic response of systems subjected to such random excitations is the framework of the present contribution. Based on elasto-acoustic assumptions, on one hand, and the assimilation of the excitation to a weakly stationary random process characterized by a reference power spectrum and a particular spatial correlation function, on the other hand, the authors identify various strategies for evaluating the random response. The analysis is performed in a numerical context. The selected discrete models are based on a finite element formulation and exploit a displacement-pressure formulation.
Technical Paper

Reconstruction of a Vibration Field from Acoustical Measurements: A New Inverse Method Based on the Concept of Pellicular Acoustic Modes

This paper describes a new approach for performing indirect vibration measurements using acoustic measurements and an inverse calculation procedure based on a finite element model and a new type of hierarchical basis (so-called pellicular modes) for representing the unknown vibration field. The paper first presents the theory of the method. A parametric study on a simple test case then highlights the effect of the number of microphones and of the background noise level on the quality of the reconstruction of the vibration pattern. The application to a real set of acoustic data on a section of an ATR regional aircraft is then provided.
Technical Paper

Aero-Acoustic Predictions of Automotive Instrument Panel Ducts

The air noise generated by automotive climate control systems is today emerging as one of the main noise sources in a vehicle interior. In the confined instrument panel (I.P.) ducts, that lead the air flow from the HVAC outlets to the cabin, the highly constrained geometry generally leads to flow separation and to complex flow structures that contribute to the noise perceived in the car. Numerical simulation offers a good way to analyze these mechanisms and to identify the aerodynamic noise sources, in an industrial context driven by permanent reduction of programs timing and development costs, implying no physical prototype of ducts before serial tooling. This paper presents an example of aero-acoustic study of simple I.P. ducts performed with the finite element code ACTRAN to estimate the sound produced by the turbulent flow. For this type of configuration, the acoustic propagation is decoupled from the noise generation mechanism that is essentially of aerodynamic nature.
Technical Paper

Numerical Simulation of Noise Transmission from A-pillar Induced Turbulence into a Simplified Car Cabin

At high cruising speed, the car A-pillars generate turbulent air flow around the vehicle. The resulting aerodynamic pressure applied on the windows significantly contributes to the total cabin noise. In order to predict this particular noise contribution, the physic of both the flow and the cabin needs to be accurately modeled. This paper presents an efficient methodology to predict the turbulent noise transmission through the car windows. The method relies on a two-step approach: the first step is the computation of the exterior aero-dynamic field using an unsteady CFD solver (PowerFLOW); the second step consists in the computation of the acoustic propagation inside the cabin using a finite element vibro-acoustic solver (ACTRAN). The simplified car cabin of Hyundai Motor Company, studied in this paper, involves aluminum skin, windows, sealant, inner air cavity and acoustic treatment inside the passenger compartment (porous material, damping layer).
Technical Paper

Robust Design of Acoustic Treatments for Powertrain Noise Radiation

The reduction of the emitted noise from vehicles is a primary issue for automotive OEM’s due to the constant evolution of the noise regulations. As the noise generated by the powertrain remains one of the major noise sources at low/mid vehicle velocities, focus is set on efficient methods to control this source. Acoustic treatments and covers, made of multi-layered trimmed panels, are frequently selected to control the radiated sound and its directivity. In this context, numerical acoustic simulation is an attractive approach as efficient methodologies are available to study the acoustic radiation of powertrain units in working conditions (up to 6500 RPM nd frequencies up to 4 kHz). Moreover, handling acoustically-treated covers in such simulations has a low impact on the computational cost.
Technical Paper

Optimization of Trim Component and Reduction of the Road Noise Transmission Based on Finite Element Methods

The acoustic trim components play an essential role in NVH behavior by reducing both the structure borne and airborne noise transmission while participating to the absorption inside the car. Over the past years, the interest for numerical solutions to predict the noise transmission through trim packages has grown, leading to the development of dedicated CAE tools. The incrementally restrictive weight and space constraints force today CAE engineers to seek for optimized trim package solution. This paper presents a two-steps process which aims to reduce the structure borne road noise due to floor panel using a coupled simulation with MSC NASTRAN and Actran. The embossment of the supporting steel structure, the material properties of porous layers and the thickness of visco-elastic patches are the design variables of the optimization process.
Technical Paper

Extensive Correlation Study of Acoustic Trim Packages in Trimmed Body Modeling of an Automotive Vehicle

In the automotive sector, the structure borne noise generated by the engine and road-tire interactions is a major source of noise inside the passenger cavity. In order to increase the global acoustic comfort, predictive simulation models must be available in the design phase. The acoustic trims have a major impact on the noise level inside the car cavity. Although several publications for this kind of simulations can be found, an extensive correlation study with measurement is needed, in order to validate the modeling approaches. In this article, a detailed correlation study for a complete car is performed. The acoustic trim package of the measured car includes all acoustic trims, such as carpet, headliner, seats and firewall covers. The simulation methodology relies on the influence of the acoustic trim package on the car structure and acoustic cavities. The challenge lies in the definition of an efficient and accurate framework for acoustic trimmed bodies.