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

TL;DR

This study experimentally demonstrates that a ceramic liner in a duct with flow can induce hydrodynamic instabilities, significantly altering acoustic transmission and pressure drop, especially near resonance frequencies.

Contribution

It provides the first experimental evidence of flow-induced instability over an impedance wall in a duct, revealing new acoustic and hydrodynamic interactions.

Findings

Transmission coefficient increases above 1 at resonance with flow

Flow induces hydrodynamic instability causing abnormal acoustic behaviour

Pressure drop can triple at resonance with flow and sound interaction

Abstract

An experimental investigation of the acoustical behaviour of a liner in a rectangu- lar channel with grazing flow has been conducted. The liner consists of a ceramic structure of parallel square channels: 1mm by 1 mm in cross section, 65 mm in length, and a surface density of 400 channels/inch square. 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 transmission coefficient around the frequency of minimal impedance. When the wall is exposed to a grazing flow this behaviour is changed: an increase in transmission coefficient appears at this resonance frequency. The transmission coefficient can be 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.