Grid-stretching is often used close to non-reflecting boundaries in order to under-resolve and dissipate outgoing acoustic waves. The resulting buffer zones may be large and computationally expensive, however, since the stretching factor is strongly limited. Two approaches of implementing grid-stretching are considered: Taylor expansion and transformation in curvilinear coordinates. Besides grid-stretching, a new buffer zone technique is introduced, which allows for larger stretching factors and computationally more efficient buffer zones. The speed of sound is varied locally in the buffer zone resulting in spatial under-resolution of outgoing waves similarly to stretched grids. The approach is derived from the concept of acoustic black holes, a method of reducing reflection of bending waves from edges of physical plates, giving the name black hole layer to the new concept. The approach is verified and the performance of stretched grids and black hole layers is evaluated using a linearized Euler equations solver in two space dimensions for several benchmark flows. The most efficient buffer zone is found to be a combination of grid-stretching using Taylor expansion and black hole layer.
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