The tonal self-noise emission of a vehicle side mirror and the associated flow field is investigated experimentally. The relevant surface on the model includes a region of geometry-induced laminar flow separation close to the trailing edge. In the separated shear layer, high-amplitude instability modes at the acoustic mode frequencies are identified by applying Proper Orthogonal Decomposition on Particle Image Velocimetry snapshots on the one hand and Linear Stability Theory on hotwire data on the other hand. The distinctive pattern of tonal noise emission combined with the shear layer instability modes are characteristic for a self-excited aeroacoustic feedback loop known from investigations of airfoil tonal self-noise emission. The loop establishes by amplified boundary layer instability waves and the acoustic waves they generate via the trailing edge scattering mechanism. The resonance frequencies of the loop are derived from a simplified linear transfer model and adapted to the present conditions. The existence of the feedback mechanism on the side mirror is finally demonstrated by applying the resonance condition to the side mirror model, where the calculated modes are in very good agreement with the modes obtained from the experiments. Even though the feedback mechanism allows for the co-existence of several modes, a special aspect of the side mirror tonal self-noise emission is that the acoustic modes can alternate in time. The selection of a particular mode was successfully triggered by excitation with an external acoustic disturbance at or near the corresponding mode frequency, which emphasizes the sensitivity of the mode selection towards external disturbances.
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