The choice of a computational domain should not be taken lightly. A computational domain is, inevitably, finite in size. A natural tendency is to make the domain as small as possible. However, the domain should not be too small so as to leave some important noise sources outside. In some cases, the simulation involves flows with energetic disturbances. For this type of problem, it is advisable to choose a larger computation domain to allow the disturbances to decay to a less energetic state before leaving the computational domain.
CAA problems often involve the generation of tones by flow resonances. For example, the flow over a cavity or a resonator mounted flush to a flat wall often leads to the generation of strong tones. Since the tones are radiated to infinite space outside, the problem to simulate is an external radiation problem. The question often asked is, “How large should the external (external to the cavity) computational domain be?” Experience indicates that, if the domain is too small, it would lead to
a higher computed frequency. This could be due to partial reflection because the external computational boundary is too close to the sources. To prevent this from happening, a rule of thumb is to use an external computational domain no smaller than 1.5 times the acoustic wavelength. Apparently no external radiation boundary condition is perfect. The reflections from the external boundary, even though small, could affect the noise generation processes resulting in a shift in tone frequency.