Mechanism for Increased Viscous Drag over Porous Sheet Acoustic Liners

Abstract

Traditional acoustic liners used in damping sound in aircraft applications consist of a porous face sheet, honeycomb core, and solid backing. Experiments have observed that, under conditions of high tonal sound levels, there occurs a dramatic increase in the viscous drag in the turbulent boundary layers over these surfaces. The mechanism for this increase in drag is the topic of interest. This involves experiments on a series of acoustic liners with fabricated porous sheets having a range of hole diameters, spacing, and patterns. The hole spacing spanned a range of viscous spanwise length scales from . The boundary-layer measurements include mean velocity and turbulence intensity profiles, velocity spectra, and turbulence burst, ejection, and sweep statistics based on the variable interval time-averaging and -quadrant methods. Coupled with this are direct drag measurements of the acoustic liners. The results show a dramatic increase in drag at frequencies near acoustic resonance and at high sound levels. This is presumed to be driven by net zero mass jets emanating from the porous sheets. For the experimental data available, the largest increase in drag is observed for the smaller spanwise distance between perforations, which is comparable to the typical spanwise wavelength of wall streak structures in turbulent boundary layers. The increase in drag is correlated with an increase in the turbulence wall layer burst and ejection events.