Measurements of sound-flow interaction at a bias flow liner

Kurzfassung

In order to optimize the noise reduction of perforated liners with bias flow that are applied in jet engines and gas turbines, a better understanding of the inherent aeroacoustic sound damping mechanisms is necessary. For that purpose, the interaction between sound and flow that is responsible for the damping process needs to be localized, analyzed and balanced. To this aim, laser optical measurements are needed that both acquire the complex sound field and flow field contactlessly. Acoustic particle image velocimetry (A-PIV) and Doppler global velocimetry with sinusoidal frequency modulation (FM-DGV) are applied for this measurement task in comparison. The results show that both methods are advantageous due to their ability for multipoint measurement. In particular, A-PIV offers a wide overview of multiple perforation orifices, whereas FM-DGV satisfies with a high measurement rate of up to 100 kHz allowing the spectral analysis of the velocity field. Both methods were applied at a generic bias flow liner, showing acoustically induced flow velocity oscillations near the rim of the perforation orifices. The spatially resolved oscillation magnitude shows a correlation to the dissipation coefficient and indicates an acoustically induced flow vortex generation and collapse in the vicinity of the liner perforation, respectively, which depends on the relative position to the orifice. As shown in this paper, the measurement methods have the potential to enhance current knowledge about the aeroacoustic interaction at liners, especially in the scenario with both grazing and bias flow. In order to fully understand these interaction phenomena an expansion to volumetric measurements is needed in the future.