The evaluation of non-reflecting boundary conditions for duct acoustic computation

A simple method is used to quantify the performance of non-reflecting boundary conditions for duct acoustic applications. The method uses a two-dimensional wavesplitting technique to decompose the linearized Euler equations into its constitutive modes, allowing the magnitude of reflected waves from outflow boundaries to be accurately determined. Realistic conditions are simulated by conducting the boundary condition analysis using acoustic waves with characteristics often found in duct and turbomachinery problems. For this paper, the method is used to investigate the performance of three different buffer zone implementations and the Perfectly Matched Layer as non-reflecting boundary conditions. The effect of the damping properties of these boundary conditions and the incident acoustic wave characteristics on performance are considered. Results indicate that the buffer zone boundary condition using explicit damping of the solution vector after each timestep produces the best non-reflecting performance. A deterioration in performance was observed for incident waves at high angles relative to the boundary normal for all implementations.