Unveiling noise reduction mechanism of serrated orifices from hydrodynamic-acoustic decoupling perspective

This study experimentally unveils the noise reduction capabilities of serrated orifices, focusing on the hydrodynamic-acoustic pressure fluctuations decoupling perspective during the noise reduction process. Acoustic measurements were performed on orifices with varying percent open areas (φ = 11%, 20%, and 31%) and serration counts. Two distinct pressure fluctuation reduction regions were identified: a rapid growth region, characterized by efficient pressure fluctuation reduction, and a saturated region, where the benefits diminish. A pressure decomposition framework, including spectral proper orthogonal decomposition and wavenumber-frequency spectral analysis, was employed to identify hydrodynamic and acoustic pressure fluctuations from sparse experimental data. It was unveiled that the noise reduction mechanism involved a reduced acoustic energy contribution, effectively suppressing low-dissipation acoustic waves. In the vicinity of characteristic acoustic frequency, the serrated structure disturbed the confined flow, increasing hydrodynamic pressure fluctuations. A supplementary experiment on the frequency sensitivity of flow-acoustic power conversion showed that the self-sustained power conversion at acoustic characteristic peaks led to the saturation of noise reduction, and the core noise reduction trends were confirmed to be robust across the investigated Reynolds number range.