Evidence of hydrodynamic instability over a liner in a duct with flow

Lined ducts are widely used to reduce noise radiation from ducts, from air conditioning systems to turbofan engines. Liners may sustain some instabilities although there is no definite knowledge on that subject. Some analytical studies based on simple flow models have predicted the existence of unstable hydrodynamic modes, but their prediction capa-bility for realistic conditions is difficult to assess. Experimental evidence of the existence of instable modes in realistic flow conditions exist but is rare. While large increase in acoustic transmission across a lined part of ducts have been measured in the past, the flow itself over the liner has never been investigated. In the present paper an experimental study of the flow over a liner is made in such a configuration where the acoustic transmission is large, and evidence of an underlying hydrodynamic instability is provided, based on optical flow measurements. I