Reducing the noise in a duct in the presence of a flow turns out to be a current industrial problem in many engineering applications, such as aircraft engine nacelles, ventilation systems and exhaust pipes. This study aims to analyze nonlinear behavior of absorbent materials used on the duct walls. This research is conducted experimentally through the use of an acoustic wind tunnel (Caïman wind-tunnel of the Centre Acoustique du LMFA, Ecole Centrale de Lyon). As an initial approach, the study is limited to a plane wave analysis. The two-source method is used to characterize the transmission loss of the liners. Different configurations are investigated at different flow regimes up to a Mach number of 0.3. Regarding the treated wall case, a perforated metal liner is considered. A sample with the same geometrical properties of the liner is initially tested on a Kundt tube, in order to characterize the absorption coefficient and the impedance. The perforated liner is thereafter tested in the wind tunnel for a transmission loss analysis with and without flow, first in linear and then in nonlinear regime. A comparison of the TL obtained experimentally with the one determined by calculation using a multi-modal approach, and a numerical time-domain code, based on finite-difference techniques, are presented.