Modelling boundary layer effects in ActranTM for tonal sound propagation in turbofan intakes

ACTRAN TM has been successfully used for predicting fan noise propagation and radiation for turbofan engine intakes and bypass ducts over many years at ISVR. ACTRAN TM predictions generally achieve good agreement with test data, particularly for hard-walled cases, when non-linear propagation effect is not significant. The predictions for lined cases are, however, often lower than measurements, in other words, noise attenuation by acoustic liners tends to be over-predicted.Acoustic liners in the presence of flow are modelled through Myers impedance boundary condition in ACTRAN TM. An infinitely thin boundary layer assumed in this condition causes instability problem in time domain analysis, but it is not an issue for frequency-domain analysis such as ACTRAN TM. The ease of using impedance boundary condition is one of advantages of frequency-domain analysis, but it has been suspected that not having finite thickness boundary layer might be contributing to the over-prediction of the attenuation by the liners. The aim of this study is to investigate the impact of a finite thickness boundary layer on the noise attenuation by acoustic liners predicted by using ACTRAN TM models. A uniform duct model with simplified flow is used in the current work.