Resumen de Computational aeroacoustics in the automotive industry
The acoustic field inside a car cabin for low driving speeds is dominated by the engine or the tire noise. However, for mid to high velocities, the noise generated by the interaction of the car with the external air becomes more relevant. The flow separation from the A-pillar or the side mirror generates strong pressure fluctuations which results in acoustic waves propagated to the interior via excitation of the side window or the windshield.
At each point of the flow field domain, the pressure is composed by the hydrodynamic pressure and the acoustic pressure. For typical car speeds, the Mach number is so low that the fluctuations of the compressible part of the flow are much smaller than those of the incompressible part but their characteristic lengths and convection velocities have a higher correlation with the bending waves of the windows of the vehicle. As a consequence, it is usually accepted that the major part of the interior noise in the car cabin is generated by the compressible part of the external flow field arriving to the transmission surfaces. The present thesis is focused on the research of a suitable computational methodology that enables to obtain the acoustic pressure near the transmission surfaces.
To this aim, an extensive review of the most popular methods of Computational Aeroacoustics is carried out in order to understand the physical mechanisms of sound generated by a fluid in motion around a solid body and the mathematical models that describe it based on the up-to-date available literature. Due to its simplicity, efficiency and usefulness, the so-called 'acoustic analogy' proposed by Curle as an extension of the theory developed by James Lighthill is chosen to evaluate the acoustic pressure of the cases of study within this dissertation.
n particular, among all the different components of a vehicle the flow pattern and acoustic performance of very wide open cavities have been deeply analysed due to its common presence in any type of vehicle design for soiling management or manufacturing restrictions. This configuration is known to be the cause of conspicuous acoustic problems such as whistles due to the well defined tonal noise known in the literature as Rossiter modes. The flow nature of this phenomena as well as its radiating pattern and response to different geometrical modifications are addressed in this work. A particular feature of this configuration is the oscillatory mode: shear layer mode (SL) or wake mode (WM). For the parameters considered in the present dissertation it is seen that while in SL the flow shows a two-dimensional behaviour, in WM the flow is three-dimensional, resulting in significantly different sound sources. The computation of the acoustic pressure is done using Curle's formulation evaluated as a post-process of an unsteady incompressible three-dimensional Navier-Stokes solution and compared with the results obtained with Direct Simulation (DS). It is found that DS and Curle's analogy are in good agreement except in the wake area, where quadrupole acoustic sources are present. Regarding the evaluation of the passive noise control techniques, the results show that the modifications on the trailing edge are the most effective to control the flow. They allow to reduce the pressure fluctuations produced by the recirculation confined inside the cavity and the abrupt ejection of the flow at the trailing edge. As a consequence, the overall sound pressure level can be decreased up to 9dB.
Once the model has been validated for an isolated geometry, the application of the same method is extended for a real car geometry. The acoustic radiation to the side window and windshield of a side mirror and A-pillar of a vehicle is shown as an example of the potential of this procedure for aeroacoustic analysis and optimisation of a vehicle straight from the drawing table.
