Whistling side-view mirrors: Modelling ladder-type structure tonal noise from flow intermittency

Abstract

Whistling side-view mirror flows are investigated experimentally. Two designs are considered: the baseline, and a controlled one, whose cap is equipped with a protruding step, used as a vortex generator to turn the boundary layer turbulent upstream of the trailing edge. The acoustic field is measured with microphones while the inflow velocity is decreasing from 50 to 15 m s${}^{\mathrm{-1}}$. Hot-wire anemometry and time-resolved particle image velocimetry are performed at 26 and 34 m s${}^{\mathrm{-1}}$, synchronised with microphones, allowing whistling source localisation. Although the feedback loop that generates the tonal noise emission is deactivated in the controlled case in the step region, a switch from single tone to ladder-type structure tones is noticed on the outer side of this side-view mirror. This is associated with a flow intermittency in that region, at a low frequency corresponding to the peak distance. Fluctuation maps at the whistling frequencies are qualitatively different too, suggesting that, in the controlled case, the feedback loop is only active during the phase of the intermittent process when the shear layer deviates outward. This interpretation is supported by the estimation of a higher convection velocity of the fluctuations in the shear layer. Introducing the intermittency into time series models allows to reproduce the ladder-type structure, that is the harmonic jumps when the velocity increases. A Fourier analysis shows that whatever the vortex passing frequency over the trailing edge (e.g. as fixed by the feedback loop), the spectrum is made of harmonics of the intermittency frequency only. However, the specific velocities of rung changes on the ladder results from a higher growth rate of the passing frequency with velocity.