Verification and Application of Time-domain Impedance Boundary Condition in CFD-CAA Method

In engineering applications, the surface structure plays a crucial role in influencing flow fields and noise generation. The CFD-CAA method commonly assumes smooth and purely reflective wall surfaces; however, experiments often involve surface treatments to explore noise control techniques. Consequently, there is a growing interest in incorporating impedance boundary conditions into CFD-CAA simulations. Since impedance boundary conditions are defined in the frequency domain, while CFD-CAA simulations operate in the time domain, direct implementation is not possible. To address this issue, several methods have been proposed to define time-domain impedance boundary conditions in simulations. The present study employed the wall softness model to examine a vortex whistle featuring an acoustically permeable surface. In the simulations, an impedance boundary condition representing the properties of melamine foam was defined over the surface of a cylindrical cavity. LES was used for CFD simulations, while FW-H analogy was employed for CAA simulations. The simulation results were validated against experimental data obtained from a vortex whistle with melamine foam. The findings revealed that the impedance of the melamine foam contributed to noise reduction at high frequencies. Additionally, at low airflow rates, the impedance boundary condition demonstrated the ability to enhance the signal-to-noise ratio for the low-frequency peak, which is advantageous in clinical applications.