In recent years, automotive engine manufacturers are increasingly focusing their attention on noise generated by plastic air intake manifolds (AIMs). Due to their lower density and stiffness, some deficiencies in terms of acoustical properties have been observed for plastic intake systems compared to metallic manifolds. In this framework, it seems to be very important to address not only the issue of reducing inlet noise, but also noise radiated via the coupled fluid-structure interaction.
In this work three AIMs, a baseline and two modified models, nominally having equal breathing performance, have been analyzed and compared. The modified ones presented ribs and stays for strengthening the structure. The analyses were performed with concurrent experimental and numerical validated procedures. Accelerometer data on the engine head coupling flange, inner pressure data at the inlet ports and sound intensity measurements on a surrounding grid were collected on the AIMs mounted on a dynamic flow test bench. A structural FEM approach coupled with an acoustic indirect BEM (IBEM) procedure was used to calculate the radiated sound power. Attempts were made at identifying the main noise sources by comparing the results obtained from the coupled simulations, with those achieved with other approaches: uncoupled acoustic IBEM, modal analysis (fluid and structural calculations) and coupled simulations including structural damping.
Due to their equal breathing performance, the differences calculated for the three manifolds could be ascribed to the structure-borne sound. Apart from the first few bands, the latter appeared to be significant throughout the whole examined frequency range and it showed a complex behavior (i.e. shift of resonance frequencies, stiffening the structure). Depending on the design targets (e.g. overall noise reduction, interior sound quality), the optimization of the NVH performance should be rather different and also the structural design of the component should follow different criteria.