Acoustic Liners Combined with Fine Perforated Film

Abstract

The presence of grazing flow can alter the noise reduction performance of resonant liner panels. One possible cause is the flow entering the liner opening. In this study, a fine perforated film was applied to the surface of acoustic liners to prevent such flow effects. Three types of liner samples were fabricated and tested in a flow duct rig at the Japan Aerospace Exploration Agency. The first sample was a conventional resonant liner (“baseline”), the second sample had a fine perforated film directly attached to the baseline liner surface (“direct fine perforated film”), and the third sample had a gap fixed between the film and the liner surface (“fine perforated film with a gap”). The impedance eduction results show that the fine perforated film with a gap was effective in separating grazing flow from the liner openings and in reducing the effect of the flow on the impedance. The transmission loss in the duct improved. However, the effectiveness of the direct fine perforated film was limited. The fine perforated film with a gap structure showed potential to facilitate the design of high-efficiency liners under grazing flow conditions.