Thomas Herdtle

Transfer impedance of microperforated materials with tapered holes

It has been previously shown that relatively simple computational fluid dynamics (CFD) models can be used to calculate the transfer impedances, including the associated end corrections, of microperforated panels. The impedance is estimated by first calculating the pressure difference across a single hole when a transient input velocity is imposed, and then Fourier transforming the result to obtain the impedance as a function of frequency. Since the size of the hole and the dimensions of the inlet and outlet channels are very small compared to a wavelength, the flow through the hole can be modeled as incompressible. By using those procedures, Bolton and Kim extended Maas classical theory to include a resistive end correction for sharp-edged cylindrical holes which differs from those previously proposed by the inclusion of a static component. Here it is shown that CFD models can also be used to compute end corrections for tapered holes. Since practical experimental characterization of perforated materials often involves measurement of the static flow resistance, a closed form empirical equation for that quantity has been developed. Finally, it is shown that configurations having equivalent static flow resistances can yield different acoustic absorptions.

Computational investigation of microperforated materials: End corrections, thermal effects, and fluid-structure interaction

The concept of microperforated noise control treatments was introduced by Maa 1975; in that theory, the transfer impedance of the microperforated layer was calculated based on oscillatory viscous flow within a small cylinder combined with resistive and reactive end corrections. Recently, new manufacturing procedures have dramatically lowered the cost of these materials, and as a result, there has been renewed interest in studying their properties. Since 1975, Maa’s original theory has been widely used. However, in principle, that theory can only be used to describe cylindrical perforation, while in practice, perforations are rarely cylindrical. In addition, there have been questions about the dependence of end corrections on frequency, and on the effect of coupling between the motion of the fluid in the perforations and the solid sheet in which they are formed. Additionally, in his original paper, Maa drew a distinction between the dissipative properties of thermally conducting and adiabatic materials. Th...