Experimental and numerical acoustic characterization of ultrasonically absorptive porous materials

The paper addresses the experimental and numerical acoustic characterization of ultrasonically absorptive porous materials with random microstructure such as carbon fiber reinforced carbon ceramic C/C or C/C-SiC. The present study builds upon previous efforts by the authors, improving and extending the established experimental method, complemented by a numerical analysis based on linear acoustics. The latter includes a blind-hole porosity approximation, only accounting for the larger cracks in the C/C with complex acoustic impedance given by the inverse Helmholtz Solver approach, and a highly parametrized homogeneous acoustic Absorber model, accounting for the complete volumetric structure of the porous absorber albeit with lower fidelity. The experimental approach is complemented by high-speed Schlieren visualization and Mach-Zehnder Interferometer measurements to qualitatively and quantitatively assess the interaction between an ultrasonic wave packet and a porous surface. It is found that neglecting the smaller pores and only accounting for the surface porosity, as done in the blind-hole porosity approximation, leads to the underestimation of the acoustic energy absorption coefficient. Phase shifts were found to be experimentally assessable, but remain to be corroborated by a numerical analysis. The comparisons carried out in this paper will pave the way for accurate determination of impedance boundary conditions to be applied in direct numerical simulations of hypersonic transition delay over C/C. The main emphasis of the paper is to assess the potential and the limitations of the experimental methods and the comparison of the experimental results to the numerically obtained absorption characteristics.