Advanced Numerical Boundary Treatments

High-quality boundary conditions are an essential part of computational aeroacous – tics (CAA). Because a computational domain is necessarily finite in size, numerical boundary conditions play several diverse roles in numerical simulation. First and foremost, they must assist any outgoing disturbances to leave the computational domain with little or no reflection. The alternative is to use a perfectly absorbing layer as a numerical boundary treatment. Such a layer absorbs all outgoing distur­bances without reflection as in the case of an anechoic chamber. In addition, if the problem to be simulated involves incoming disturbances, then these disturbances must be generated by the boundary conditions prescribed at the outer boundary of the computational domain. Furthermore, if there are flows that are originated from outside the computational domain, they must be reproduced by the boundary treat­ment as well. In this chapter, methods to construct numerical boundary conditions that perform these various functions are discussed.