An improved method for mean flow boundary conditions for computational aeroacoustics

Abstract

An accelerated method of specifying Mean Flow Boundary Conditions for an unsteady flow is developed. Computation domains are inevitably finite in size, which means that its boundaries have to be located within the flow itself. This requires special boundary conditions which do not reflect outgoing waves while still being able to impose any incoming disturbances into the computation domain. As is well known, the Euler equations contain acoustic, vortical, and entropy waves, and all possible flows can be made by combining them. Because these waves are coupled, it is difficult to separate them into individual waves that the solver can modify. Moreover, it is desirable to set not just the waves but the mean of the combination of these waves at the boundaries and this takes a very long time. In this work, a new method for imposing the desired time-averaged mean flow at these computational boundaries is introduced. To reduce the complexity of the problem, but retaining the basic physics and difficulties, one-dimensional acoustic wave transmission is used to model wave propagation through a nearly choked nozzle. Mean and unsteady flow equations solved, simultaneously; and integrated with the existing non reflecting boundary conditions, and is able to quickly and accurately achieve the desired mean flow without spurious reflections of the outgoing disturbances, not only for deterministic, but chaotic flows.