Mechanism for Increased Viscous Drag over Porous Sheet Acoustic Liners

High tonal sound levels have been observed to produce a dramatic increase in the viscous drag of turbulent boundary layers over porous face-sheet acoustic liners. The mechanism for this increase in drag is the topic of this presentation. This involves a series of acoustic liners with fabricated porous sheets having a range of hole diameters, spacing and patterns. These spanned a range of viscous length scales from 22 < z⁺ < 178. Boundary layer measurements include mean velocity and turbulence intensity profiles, velocity spectra, and ``burst" statistics based on VITA and uv-quadrant methods. Coupled with this are direct drag measurements. The results show a dramatic increase in drag at critical sound levels and frequencies. This is presumed to be driven by net zero mass jets emanating from the porous sheets, but only occurs when the hole spacing is comparable to the spanwise wavelength of the wall ``streak" structure. The increase in drag is also correlated with an increase in the turbulent ``burst" events. The details of this are presented.