Effect of boundary slip on the acoustical properties of microfibrous materials

Abstract

A variety of new porous materials with unusually small pores have been manufactured in the past decades. To predict their acoustical properties, the conventional models need to be modified. When pore size becomes comparable to the molecular mean free path of a saturating fluid, the no-slip conditions on the pore surface are no longer accurate and hence the slip effects have to be taken into account. In this paper, sound propagation in microfibrous materials is modeled analytically, approximating the geometry by a regular array of rigid parallel cylinders. It has been shown that velocity and thermal slip on a cylinder surface significantly changes the model predictions leading to lower attenuation coefficient and higher sound speed values. The influence of material porosity, fiber orientation, and size on these effects is investigated. Finite element method is used to numerically solve the oscillatory flow and heat transfer problems in a square array of cylindrical fibres. Numerical results are compared with predictions of the analytical model and the range of its validity is identified.

Low frequency sound propagation in activated carbon

“…The results gathered are consistent with this hypothesis, but the conclusive proof of what causes the change in surface reactance remains elusive. It is still conceivable that other phenomena, such as multi-scale porosity, slip flow, 20 or gas diffusion, might play a part. Whatever the cause, this study has confirmed that activated carbon can be used to change the compliance of enclosures, and to enable higher absorption at a lower frequency than would otherwise be expected from conventional porous absorbers.…”

Acoustical properties of double porosity granular materials

“…Thermal slip effects modify the dynamic bulk modulus but not its static value 46,47 Mass transfer processes also alter the dynamic bulk modulus. For example, Raspet et al 49 have studied the sound attenuation in rigid cylindrical pores filled with air and saturated water vapor accounting for the mass transfer of vapor from the wet tube wall.…”

“…Considering that the molecular mean free path l mean is approximately 60 nm at normal conditions, 47 the calculation of the static viscous . The agreement between the double porosity model and the data is better than that for the single porosity model.…”

Omnidirectional acoustic absorber with a porous core and a metamaterial matching layer

“…(44), (46), and (55)-(58) from Ref. 21 and assuming than Knudsen number K ¼ 0 (i.e., that the cylinders and the distances between them are large compared to the molecular mean free path). This results in the following expressions:…”