Acoustic impedance and energy dissipation mechanism of a slit resonator under combined sound pressure, grazing flow, and bias flow

Acoustic liners are widely used to damp combustion oscillations in gas turbines for power generation and aviation. Their acoustic resistance depends strongly on the sound pressure level (SPL), the grazing flow Mach number M g , and bias flow Mach number M b , but the underlying dissipation mechanism—and its evolution across the regime transition under combined SPL, grazing, and bias flows—remains incompletely understood. Using a slit-type resonator, SPL, M g , and M b are independently varied across the full regime transition, with simultaneous measurement of the acoustic impedance and the flow field structure. Building on the regime transition revealed by the combined measurement, a semi-empirical regime-selection model θ ′ = max ⁡ { θ a ′ , θ f ′ } is proposed, where θ a ′ and θ f ′ are the acoustic resistances in the SPL-dominated and flow-velocity-dominated regimes, respectively, incorporating M 0 (the acoustic particle velocity Mach number corresponding to SPL), M g , and M b simultaneously across the full transition. The combined measurement also resolves how the vortex–sound interaction at the orifice corners evolves across the regime transition: the quantitative agreement of div Ī and Howe's vortex-sound dissipation rate density ε ¯ obtained from CFD identifies vortex–sound interaction as the dominant dissipation mechanism, and the agreement of ε ¯ between CFD and PIV provides experimental confirmation. The model is applicable to acoustic liner design in hydrogen and lean-premixed combustors.