Abstract
The noise which is emitted by a truck to the environment is an important design criterion. Society does not accept the annoyance of high noise levels anymore, especially in urban areas. Therefore, legislation is imposed to reduce the emitted noise. All trucks must comply with a maximum noise pressure level at 7.5 [m] distance during a pass by manoeuvre. The main noise source is the engine gearbox combination, secondary noise sources are exhaust, tyres, air intake system, rear axle and such. Because design changes can be done easier and more cost effective in the early stages of the project, it is advantageous to have a capability to predict the pass by noise numerically. During the definition and concept phases of a project no prototypes are available to measure pass by noise.
In this paper an analysis is presented where the contribution of the air intake to the pass by noise is calculated. The air inside the intake system from turbo compressor to orifice is modelled with finite elements. The exterior air around the truck within a radius of 8 [m] is also represented by finite elements and it is coupled to the air inside the system at the orifice. A free field radiation boundary condition is imposed at the surface of the radiation sphere. Furthermore, the relatively flexible structure of the air intake system itself is included in the model by means of strong fluid structure interaction. The excitation of the model at the turbo compressor is identified from a series of test-cell measurements via the multi-load noise source identification method. The model is limited to a frequency range up to 300 [Hz].
Because validation at pass-by distance is problematic due to the presence of the other noise sources, validation of the model has been carried out at the orifice of the air intake. A good agreement is found when the predictions are compared to the measurements.
During the design phase several design changes have been analysed and compared. It has been concluded that this analysis method is an effective tool to predict the effect of design changes on the exterior noise and that it is efficient to support the process to arrive at an optimal design. It has also been concluded that the identification of the noise source by means of the multi load method and the use of such identified noise sources on finite elements is an accurate and efficient method to split acoustic systems.
In this paper an investigation is presented into the effect of the duct flexibility on the sound pressure level at the pass-by microphone location. It is well known that a high stiffness reduces the vibrations of the tube, thereby reducing the shell noise radiated by the tube. On the other hand, the orifice noise level increases with stiffness. From the analysis results it follows that the radiated noise level at the location of the pass-by microphone is reduced significantly by the flexible duct.
KEYWORDS – Air intake systems for trucks, acoustics, noise radiation, multi-load method