Quantitative visualization of flows upstream and downstream of an obstacle impinged by a jet using two synchronized S-PIV: application to aeroacoustics

The results presented here concern studies related to air treatment and ventilation flows in habitable enclosures. These studies fall within a broader context involving human health and well-being, comfort, indoor environmental quality, and energy savings. In certain configurations, impinging jets used in such enclosures can generate whistling noises, which are perceived as acoustic nuisances. To address this issue, the characteristics of the vortex structures generated by jets interacting with ventilation openings and the noise produced by a jet impinging on a slotted surface were studied experimentally. Experiments were conducted for an impingement distance of 4 cm and two Reynolds numbers, 4700 and 4800. A dedicated experimental setup was designed for this study to enable a quantitative analysis of the correlations between vortex dynamics upstream and downstream of the slotted plate and the radiated acoustic field, using a dual stereo-PIV configuration combined with microphones. The results show that, when transitioning from Re = 4700 to Re = 4800, the acoustic pressure level drops by 8 dB. Spectral analysis reveals a shift from a self-sustained tonal feedback loop at a single frequency (204 Hz) to a dual-frequency loop (129 and 270 Hz). The analysis of vortex dynamics, using the Lambda-2 criterion, indicates that this drop is associated with a transition from a symmetric to an antisymmetric vortex organization. The spectral analysis of velocity signals extracted from the S-PIV measurements makes it possible to interpret all acoustic frequencies and identify the sources of the acoustic noise. The use of correlation functions between acoustic and velocity signals confirms the presence of aeroacoustic coupling.