A new FEA tool for the optimization of a baffles kit to enhance the car hollow bodies acoustics

摘要

The stiff parts of an automotive frame (side rails, A-pillars, B-pillars…) form a hollow body network. Baffles are used to seal the cavities. The objective is to avoid water intrusion (cause of corrosion) air flow and noise. These baffles are inserted at the body shop. Usually they are made of a nylon carrier surrounded by a foam that inflates with heat. This foam blows in the e-coat oven up to the walls and thus seals the cavity. In the past, Henkel has carried out studies on the modelling of the acoustic behaviour of its foams. The proposed model predicts with accuracy the efficiency of pillar fillers in term of acoustic transmission and absorption. The objective of the presentation is to explain the current work which deals with the development of an optimisation process of the baffles inside the hollow body network. A dedicated FEA tool has been developed to simulate the air borne transmission. The modelling principle consists in the use of 1D elements for the fluid inside the hollow bodies with simple section, 3D elements are used for complex sections and nodes of the body frame, finally the baffles are taken into account using a coupling transfer matrix method. The objective is to generate light hollow bodies and baffles finite element models in order to have small computation times, which is required in an optimization process. This FEA tool has been validated using the model of a 3 doors sedan car without and with baffles. The frequency range was from 10 Hz to 1600 Hz. At these frequencies, the acoustic pressure field is constant over the section; this justifies the use of 1D elements where it was possible. Moreover, a modal behaviour of the acoustic waves has been observed. It is controlled by the hollow body network natural boundary conditions but also by the added baffles. The baffles have also a (vibratory) modal behaviour that makes them almost transparent at their first resonance. Thus one must pay attention to avoid coincidence between the cavities and baffles modes. Finally, some correlations with experimental results are presented.