PIV and LDV evidence of hydrodynamic instability over a liner in a duct with flow

The acoustical behavior and the flow in a rectangular lined channel with grazing flow have been investigated. The liner consists of a ceramic structure of parallel square channels and is locally reacting. In the absence of flow, the liner has a classical behavior: the acoustic transmission coefficient has a minimum at the resonance frequency of the resonators. When the Mach number of the grazing flow increases, the material behavior becomes unclassical in the sense that its acoustic transmission increases strongly around the resonance frequency. To connect this behavior with flow features, the flow itself in the vicinity of a liner has been measured by means of laser velocimetry. Periodic structures have been observed along the liner that are phase-locked with the incident sound wave. The axial and transverse velocity of these structures bear the typical features of an instability. In particular, the wavelength, convection speed, and growth rate are given. This is the first time that an aeroacoustic instability resulting from the interaction of flow and sound over a liner is measured.