In this study, we investigate the drag regimes of acoustic liners in the absence of external acoustic excitation. Friction coefficients $C_{\text{f}}$ are estimated in a turbulent channel flow over a wide range of Reynolds numbers using mean pressure-drop measurements. Two main drag regimes are identified: a viscous regime and a transitional regime in which pressure drag becomes significant. Within the transitional regime, two distinct behaviors are observed. The first exhibits low to moderate $C_{\text{f}}$ increments relative to the smooth-wall reference, consistent with $d$-type roughness trends. The second exhibits large drag increases associated with the onset of aeroacoustic resonance at the liner resonance frequency, which significantly distorts the mean velocity profiles. Despite the one-decade range of Reynolds numbers investigated, no fully rough regime is reached, questioning current approaches to modeling acoustic liner drag. Finally, liner hierarchy in terms of $C_{\text{f}}$ within the transitional regime correlates satisfactorily with a geometric dimensionless parameter ${\lambda }_{\text{f}}$, analogous to the frontal solidity used in roughness studies, offering insight into the role of orifice geometry in liner drag.

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• Acoustic measurements
• Aeroacoustics
• Aerodynamic noise
• Flow instability
• Flow-structure interactions
• Structure & turbulence of boundary layers