Influence of Duct Open-End Reflection on Liner Performance

In a finite lined duct, strong reflections from the duct open end are observed when the sound frequency is near the cut-on/off boundary. Under such condition, in the rigid-wall segment upstream of the liner, the interference happens between the upstream-propagating transmitted wave and its copropagating coherent wave, which is formed after being reflected twice at the duct open end and the liner’s upstream end. When the two waves are out of/in phase, the interference is destructive/constructive. To validate this theory, three dissipative liners are designed and tested on the Fan-200 test rig at Beihang University. A modified exact transmission loss is proposed to quantify the effect of interference. Both numerical analyses and experimental measurements consistently demonstrate that the interference affects the liner performance by approximately 10 dB ([Formula: see text]), which is relative to the length of the rigid-wall segment upstream of the liner and the reflection coefficients at the liner upstream end and duct open end. The comprehensive error analysis validated the quality of the experiments. These findings provide new insights for optimizing the installation position of liners, a crucial yet often overlooked aspect in engineering practice.