Investigations of Sunroof Buffeting in an idealised Generic Vehicle Model - Part I: Experimental Results

Abstract

This paper is the first of a two-paper series that describes the experimental portion of a long-term, ongoing investigation - carried out by a consortium of the German automotive manufacturers Audi, BMW, Daimler, Porsche and Volkswagen - of the feasibility of predicting buffeting phenomena by means of computational fluid dynamics (CFD). The numerical simulations carried out as part of the study are reported on in the second paper of the series. Sunroof and side-window buffeting is a common phenomenon in passenger cars, and can cause considerable discomfort to the passengers due to the high sound-pressure levels (SPL) that are generated. The general physical mechanism, whereby an unsteady shear layer in the sunroof or window opening induces an acoustic resonance in the passenger compartment, is well-understood. However, experience has shown that making a priori predictions about the tendency of a given configuration to experience buffeting - essential in the early stages of the vehicle design process - with the required degree of reliability and accuracy is not possible. In the vehicle development process, pragmatic design solutions for suppressing buffeting at various wind speeds and geometric configurations are found experimentally. Previous work carried out by members of the consortium has shown that clearly identifying and isolating the individual factors that influence buffeting in real vehicles is very diffcult. They range from the geometry of the sunroof or side window, including small geometric details of the surrounding vehicle parts, to the quality of the oncoming flow, to the arrangement of the internal volume of the vehicle. Moreover, it has been found that the interaction of the fluid flow with the vehicle structure has a particularly large influence on buffeting behaviour and makes the analysis of the individual physical phenomena that contribute to the overall effect very diffcult. In order carry out a systematic analysis of these issues, the consortium devised a long-term project in which the first step, presented in this two-part publication, isolates and investigates only the issues related to fluid dynamics and acoustics, and ensures that the other aforementioned factors influencing buffeting play no role. Rigorous studies of this type which are also directly relevant to vehicle design are not found in the present literature. Only sunroof buffeting was considered in the present study, as the physical mechanism is essentially identical to that of side-window buffeting.