Use the Asymptotic Far-Field Radiation node to prescribe open non-reflecting radiation conditions. The boundary condition is based on the asymptotic behavior of the acoustic waves in the far field. The condition is also known as the condition of Tam and Dong (see Ref. 17). The condition applies at boundaries far from a compact acoustic source. The waves must be assumed spherical in 3D (and 2D axisymmetric) or cylindrical in 2D. This means that the condition is valid in the limit where

where r is the distance from the source located at r0 and k is the wavenumber. The condition is formulated for the scattered field only, in problems involving an Background Acoustic Fields feature. The condition of Tam and Dong reads:

where η = 1 in the 3D (and 2D axisymmetric) spherical case and η = 2 in the 2D cylindrical case.

The acoustic perturbations behave differently than the entropy and vorticity waves. The acoustic waves propagate in all directions at the speed of sound (correct by the background flow) while the entropy and vorticity waves are only convected by the background flow. The linearized Euler equations support both these waves so both have to be taken care off. At boundaries where the background flow leaves the computational domain, add the Outflow Boundary sub-feature. A default Outflow Boundary node is added with a cleared selections.

Enter a Source location r0 (SI unit: m).