Development of an Evaluation Methodology for PIV Measurements of Low-Frequency Flow Phenomena on the Vehicle Underbody

Aeroacoustics is important in the automotive industry, as it significantly influences driving comfort. Particularly in the case of battery electric vehicles (BEVs), the flow noise is already crucial at lower driving speeds, because they generate barely any drive noise. In addition, the masking effects from engine noise are missing. Due to the increasing importance of drag minimization and elimination of the exhaust system, the underbody of BEVs is typically very streamlined and exhibits a low acoustic interference potential. However, even small geometric modifications of the vehicle can lead to changes in the flow around the vehicle and consequently to significant noise sources. Considerable flow resonances in the low frequency range below 30 Hz have been detected on certain prototype configurations. Initial investigations have shown that the flow around the front wheel spoilers is relevant for the development of the flow phenomenon. Nevertheless, the exact and complex mechanism of formation of the low-frequency flow resonances is still largely unknown. The aim of the study is to identify their exact origin. Particle Image Velocimetry (PIV) measurements were carried out in the wind tunnel. The flow field was measured in a plane parallel to and below the vehicle underbody. Acoustic quantities were measured using surface microphones. An evaluation method was developed in order to investigate the cause of the low-frequency pressure fluctuations in detail. The evaluation method developed is presented in this paper. The results are then described and discussed. Using these results, the origin of the low-frequency flow resonances is finally explained.