The acoustic problems arising in these days are closely related to flow phenomena. The methods for solving the problems of classical acoustics and theoretical aeroacoustics have limitations to apply to those phenomena. Dramatic increases in computer power, a confluence of aeroacoustics and CFD, and a maturer understanding of numerical methods produce a new field of computational aeroacoutics i.e. CAA. Two approaches have been pursued for the CAA; the direct simulation of acoustic fields generated by flow and of the interaction of acoustic fields with flows. One employs the linearized Euler equations with source terms and the other makes use of flow equations such as Navier-Stokes equations directly. Efficient schemes for the CAA applications are currently developed by several researchers.
In this study, Euler equations are employed for the stepping stone with Roe-MUSCL-TVD scheme, which is one of the state-of-the-art schemes for the CFD. Thompson's nonreflecting boundary conditions are employed and new formulae of source conditions are developed. The main objective of this paper is to know the limitations and the capabilities of the state-of-the-art CFD scheme solving Euler equations. The first part of these researches is that the fundamental tests are carried out for the case of a uniform duct, a nozzle, a muffler and a pulsating/oscillating cylinder with a periodic excitation. Especially, the muffler which has been one of the representative problems of a classical acoustics is calculated for the case of the conventional linear wave excitation with or without mean flow gradient and the nonlinear wave excitation. The second part is that transient phenomena such as a moving piston with or without a radiation field, a complex wave mechanism of train-tunnel interaction and a reduction of the radiated noise by an expansion chamber are studied by the present tool with an unsteady deforming mesh algorithm. Especially, the study of the train-tunnel interaction include the generation, the propagation and the radiation process. Impulsive noise reduction using an expansion chamber based on the results of a periodic excitation is also studied.
The summaries of main results are as below. First, in order to simulate an acoustic wave propagation, 30~40PPW(points per wavelength) is required for the present CFD scheme. Second, the performance of muffler for the linear acoustic waves with or without mean flow are well predicted and verified. Third, for the case of the nonlinear wave excitation in a muffler, the impulsive part of a wave is not transmitted due to the muffler characteristics. Fourth, the moving body problem such as a moving piston and a pulsating/oscillating cylinder can be simulated and verified. Fifth, the amplitude of a compression wave generated by a train-tunnel interaction is proportional to the square of the train speed. Sixth, the emitted impulsive noise decays proportional to 1/r from near the exit not from the exit due to the complex wave field near the exit. Seventh, the reduction of the impulsive noise is accomplished by using an expansion chamber in a tunnel.
