Experimental evidence of an instability over an impedance wall in a duct with flow

An experimental investigation of the acoustical behaviour of a liner in a rectangular channel with grazing flow has been conducted. The liner consists of a ceramic structure of parallel square channels: 1 mm × 1 mm in cross section, 65 mm in length and a surface density of 400 channels / in 2 . The channels are rigidly terminated, thus constituting a locally reacting structure. In the absence of flow the liner reacts classically: there is a significant decrease in the transmission coefficient around the frequency of minimal impedance. When the wall is exposed to a grazing flow this behaviour is changed: there is an increase in the transmission coefficient at this resonance frequency. The transmission coefficient can even rise above 1 (up to 3 for a Mach number of 0.3). This behaviour is caused by the appearance of a hydrodynamic instability above the liner. Furthermore, the stationary pressure drop induced by this liner is deeply affected by its acoustic behaviour. When a sound wave is added, at the resonance frequency of the liner, the pressure drop can increase by a factor 3 when the Mach number is 0.3. This effect is attributed to a modification of the turbulent boundary layer induced by the acoustic wave.