Combined CFD and CAA Simulations with Impedance Boundary Conditions 2021-01-1048

In computational fluid dynamic (CFD) and computational aeroacoustics (CAA) simulations, the wall surface is normally treated as a purely reflective wall. However, some surface treatments are usually applied in experiments. Thus, the acoustic simulations cannot be validated by experimental results. One of the major challenges is how to define acoustically boundary conditions in a well-posed way. In aeroacoustics analysis, impedance is a quantity to characterize reflectivity and absorption of an acoustically treated surface, which may be introduced into the numerical models as a frequency-domain boundary condition. However, CFD and CAA simulations are time-domain computations, meaning the frequency-domain impedance boundary condition cannot be adopted directly. Several methods, including the three-parameter model, the z-transform method and the reflection coefficient model, were developed. In the present study, a coupling method that combines the time-domain impedance boundary condition and Large Eddy Simulations (LES) is proposed. A cavity flow with wall-impedance is simulated. The results of flow field are validated by the case with purely reflective wall-impedance. And the effects of the impedance boundary condition on flow field and acoustic field are investigated. The results indicate that the impedance boundary condition does not affect the flow field significantly, but contributes to noise reduction within the cavity. The method developed in this study is expected to be applied to a variety of passive noise-control problems.