Effects of Mean Flow on Acoustic Luneburg Lenses: Performance, Prediction, and Mitigation

Acoustic Luneburg Lenses are axisymmetric lenses made from acoustic metamaterials with a variable refractive index, capable of omnidirectional wave focusing. Their primary applications include ultrasound devices, waveguides, and directional microphones. In many cases, these lenses operate under mean flow conditions, yet little research has examined how this affects their performance. This study investigates mean flow effects on Acoustic Luneburg Lenses and explores predictive and mitigation techniques. A method is developed to determine the refractive index of the lenses' metamaterial under mean flow. Numerical evaluations using the Lattice Boltzmann Method assess how mean flow alters the refractive index, focusing capability, and focal point position. These results are validated against experimental data collected from a 3D-printed cylindrical lens using microphones and loudspeakers. Findings indicate minimal impact on lens performance for low Mach number mean flows, regardless of the flow's angle relative to the plane wave. However, mean flow has a stronger negative effect on collimation than on focusing performance. This study enhances the understanding of how mean flow influences Acoustic Luneburg Lenses and provides insights into improving their robustness in practical applications.