This paper investigates effects of mean flow and temperature on acoustic liner performance in dual stream flow ducts using solutions from a linearized Euler solver, which are verified with and an approximate eigenmode/mode matching technique. A simple, uniform, long cowl duct configuration with partial liner treatment on the outer wall exposed to the cold stream is studied in the present investigation. While the linearized Euler solver uses a Reynolds-Averaged Navier-Stokes solution as the mean flow, the eigenmode approach assumes uniform plug flow for the cold and hot streams separated by a vortex sheet. Results are obtained for incident modes from the core duct by both methods and compared. Results indicate that the liner effectiveness is a strong function of the mode cut-on ratio as well as the refraction mechanisms of the mean flow. This type of liner configuration could therefore provide effective noise suppression over a useful range of mode orders generated within the core. Copyright © 2009 by Y. Ozyoruk and B.J. Tester.