Comparison of outflow boundary conditions for subsonic aeroacoustic simulations

Aeroacoustics simulations require much more precise boundary conditions than classical aerodynamics. Two classes of non‐reflecting boundary conditions for aeroacoustics are compared in the present work: the characteristic analysis‐based methods and the Tam and Dong approach. In the characteristic methods, waves are identified and manipulated at the boundaries, whereas the Tam and Dong approach use modified linearized Euler equations in a buffer zone near outlets to mimic a non‐reflecting boundary. The principles of both approaches are recalled, and recent characteristic methods incorporating the treatment of transverse terms are discussed. Three characteristic techniques—the original Navier–Stokes characteristic boundary conditions (NSCBC) of Poinsot and Lele and two versions of the modified method of Yoo and Im—are compared with the Tam and Dong method for four typical aeroacoustics problems: vortex convection on a uniform flow, vortex convection on a shear flow, acoustic propagation from a monopole, and acoustic propagation from a dipole. Results demonstrate that the Tam and Dong method generally provides the best results and is a serious alternative solution to characteristic methods even though its implementation might require more care than the usual NSCBC approaches. Copyright © 2011 John Wiley & Sons, Ltd.