Numerical dispersion and absorbing boundary conditions

Predictions of performance of exact and approximate absorbing boundary cond itions (ABCs) do not take into account the fact that in an actual simulatio n it is numerical waves that are incident on the computational domain bound ary where they are imposed. Via a model problem in rectangular co-ordinates we identify and examine this issue. Then, we study the reflection produced by discrete local ABCs in cylindrical co-ordinates using the first-order B ayliss-Turkel operator as a model. We find the analytical reflection coeffi cient of this ABC significantly underestimates the actual reflection on the grid. Also, we identify the source of the additional error and show it dec ays slowly with increasing resolution. Implications for other ABCs are disc ussed. Copyright (C) 2000 John Wiley & Sons, Ltd.