Optimization of Variable-Depth Liner Configurations for Increased Broadband Noise Reduction

Abstract

This paper employs three acoustic propagation codes to explore variable-depth liner configurations for the NASA Langley Grazing Flow Impedance Tube (GFIT). The initial study demonstrates that a variable impedance can acceptably be treated as a uniform impedance if the spatial extent over which this variable impedance occurs is less than one-third of a wavelength of the incident sound. A constrained optimization study is used to design a variable-depth liner and to select an optimization metric. It also provides insight regarding how much attenuation can be achieved with variable-depth liners. Another optimization study is used to design a liner with much finer chamber depth resolution for the Mach 0.0 and 0.3 test conditions. Two liners are designed based on spatial rearrangement of chambers from this liner to determine whether the order is critical. Propagation code predictions suggest this is not the case. Both liners are fabricated via additive manufacturing and tested in the GFIT for the Mach 0.0 condition. Predicted and measured attenuations compare favorably across the full frequency range. These results clearly suggest that the chambers can be arranged in any order, thus offering the potential for innovative liner designs to minimize depth and weight.

Variable-Depth Liner Evaluation Using Two NASA Flow Ducts

“…As mentioned earlier, the C1 and C2 liners were designed as part of an earlier study. 9 The CDL code was used with an optimizer developed using the Python programming language 12, 13 to design the C1 liner. For this design optimization, the axial length of the liner was assumed to be divided into eight equal-length axial segments, and each segment consisted of an array of 3×3 identical chambers.…”

“…The design was optimized a Facesheet normalized resistance was erroneously reported as 0.5 in earlier report. 9 b An e iωt time convention is assumed. Also, all impedances in this paper are assumed normalized by ρc, where ρ and c represent the density and sound speed of air, respectively.…”

“…The purpose of the current study is to extend the evaluation of the two variable-depth liners 9 to include the effects of Mach 0.3 grazing flow. Two additional variable-depth liners are explored, with and without grazing flow.…”

Impedance Eduction for Multisegment Liners

“…One option that has been the focus of study at NASA Langley is the multisegment (multizone) liner. For fabrication convenience, these are often variable-depth liners, 1,2 where each axial segment is comprised of chambers with different depths. Whereas these multisegment liners provide the capability of increased broadband sound absorption, they also present unique challenges.…”

Applications of Parallel-Element, Embedded Mesh-Cap Acoustic Liner Concepts

“…This depth variation may be continual [8][9][10] or segmented (e.g., multisegment liner). 11,12 Three other configurations considered by NASA are porous honeycomb, 13,14 dual-resonance, 15 and embedded-membrane 16 liners. The porous honeycomb liner is a conventional perforate-over-honeycomb liner for which the honeycomb walls are sufficiently porous to support sound transmission between adjacent core chambers.…”