Investigating the Aeroacoustic Properties of Kevlar Fabrics

Abstract

The aeroacoustic properties of porous fabrics are investigated experimentally in an effort to find a porous fabric as an ideal interface between wind tunnel flow and quiescent conditions. Currently, the commercially available Kevlar type 120 fabric is widely used for similar applications, such as side-walls in hybrid anechoic wind tunnels or as a cover of phased microphone arrays. A total number of 8 fabrics were investigated, namely, four glass fiber fabrics, two plain weave Kevlar fabrics, and two modified plain Kevlar fabrics with their pores clogged. Two, custom-made rigs were used to quantify the transmission loss and self-noise of all eight fabrics. It was found that the pores serve as a low-resistance gateway for sound waves to pass through, hence enabling a low transmission loss. The transmission loss was found to increase with decreasing open area ratio while other fabric properties had a minor impact on transmission loss. The self-noise of the fabrics has also been evaluated and it was found that the thread density (thread per inch) is a primary factor of determining the frequency range of self-noise with the open area ratio potentially playing a secondary role in the self-noise levels. For both metrics, the mass per unit area seemed to play a minor role in the aeroacoustic performances of the fabrics. Finally, surface pressure measurements revealed that the commercially available plain Kevlar (type 120) has no quantifiable effect on the hydrodynamic pressure field passing over the fabric, suggesting that Kevlar behaves as a no-slip wall from the flow's perspective when no pressure difference is present on the two sides of the fabric.

Acoustic transmission loss and noise from Kevlar wind tunnel walls

“…For the case of no flow we can compare the predictions using the formulas outlined above to the empirical formulas for transmission loss given in Glegg and Devenport. 13 The calculations are carried out for a weave of 34 threads per inch, with a mass per unit area of 58 gm/m 2 , with a tension of 1500 N /m. Figure 4 shows transmission loss calculations based on a viscous model defined by equation (30), for different values of the porosity and an inviscid model based on equation (26).…”

“…To analyze this problem, we must specifically take into account the motion of the surface and so, rather than start with Curle’s equation as was done in 13 we will base the analysis on the Ffowcs Williams and Hawkings Equation using the notation given in. 14 where p ij is the compressive stress tensor, v i is the velocity of the fluid, V j is the velocity of the surface, G is a tailored Greens function for an observer at x and time t for a source at y and time τ.…”

“…An important simplification is obtained by making use of a tailored Greens function that matches the non-penetration boundary condition on the surface y 2 =0 . To formalize this, we first note that the surface integral may be projected onto the plane y 2 =0 by using the transformation, 13,14 thatwhere the moving surface is defined by f=0 . Then…”

“…As discussed in, 13,14 there is some flexibility in choosing the Greens function in this equation. For example, the free field Greens function could be used so both terms would contribute.…”

“…Since the surface is not flat there will be additional sound caused by the pressure fluctuations on the roughness, which is referred to as roughness noise, Glegg and Devenport. 13 So, to include the roughness noise we introduce the tailored Greens function for which ∂trueG˜/∂y2=0 on y 2 =0, and using the far field approximation, we can define this as…”