Acoustic quasi-steady response of thin walled perforated liners with bias and grazing flows

This paper considers the acoustic performance of a passive damper in which acoustic energy is absorbed by orifices located within a thin plate (i.e. a perforated liner). The perforated liner, which incorporates orifices of length to diameter ratios of ~0.2, is supplied with flow from a passage. This enables the liner to be subject to a flow that grazes the upstream side of each liner orifice. Flow can also pass through each orifice to create a bias flow. Hence the liner can be subjected to a range of grazing and bias flow combinations. Two types of liners were investigated which incorporated either simple plain or ‘skewed’ orifices. For the mean flow field, data is presented which shows that the mean discharge coefficient of each liner is determined by the grazing to bias flow velocity ratio. In addition, measurements of the unsteady flow field through each liner were also undertaken and mainly presented in terms of the measured admittance. For a given liner geometry, the admittance values were found to be comparable for a given Strouhal number (with the exception of the lowest bias to grazing flow velocity ratio tested) which has also been noted by other authors. The paper shows that this is consistent with the unsteady orifice flow being associated with variations in both the velocity and the area of the vena contracta downstream of each orifice. These same basic characteristics were observed for both of the liner geometries tested. This provides a relatively simple means of predicting the acoustic liner characteristics over the specified operating range.