Experimental Investigation of Tonal Self-Noise Emission of a Vehicle Side Mirror

Abstract

The tonal self-noise emission of a vehicle side mirror and the associated flowfield are 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. 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 resonance frequencies of the loop are derived from a simplified 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, in which the calculated modes are in very good agreement with the measured modes. Even though the feedback mechanism allows for the coexistence 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 is successfully triggered by excitation with an external acoustic disturbance at or near the corresponding mode frequency, which emphasizes the sensitivity of the mode selection toward external disturbances.