Application and Verification of Time-Domain Impedance Boundary Conditions in CAA Simulations

Abstract

Abstract 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 simulation results cannot be validated by experimental results. In aeroacoustics analysis, impedance is a quantity to characterize reflectivity and absorption of an acoustically treated surface. One of the major numerical challenges in CAA simulations is to define acoustically well-posed boundary conditions. The impedance boundary condition is a frequency-domain boundary condition. However, CFD and CAA simulations are time-domain computations, which means 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 channel flow with wall impedance is simulated at different acoustic resistance and reactance. The approach is verified by the case with purely reflective wall impedance. For the flow with wall impedance. The effects of acoustic resistance and reactance are investigated. It is found that the wall impedance contributes to the noise reduction in the near-wall region, and with the decrease of the resistance or reactance, the sound pressure level is decreased. The method developed in this study is expected to be applied to a variety of noise-control problems.