Variable-Depth Liner Evaluation Using Two NASA Flow Ducts

Abstract

Four liners are investigated experimentally via tests in the NASA Langley Grazing Flow Impedance Tube. These include an axially-segmented liner and three liners that use reordering of the chambers. Chamber reordering is shown to have a strong effect on the axial sound pressure level profiles, but a limited effect on the overall attenuation. It is also shown that bent chambers can be used to reduce the liner depth with minimal effects on the attenuation. A numerical study is also conducted to explore the effects of a planar and three higher-order mode sources based on the NASA Langley Curved Duct Test Rig geometry. A four-segment liner is designed using the NASA Langley CDL code with a Python-based optimizer. Five additional liner designs, four with rearrangements of the first liner segments and one with a redistribution of the individual chambers, are evaluated for each of the four sources. The liner configuration affects the sound pressure level profile much more than the attenuation spectra for the planar and first two higher-order mode sources, but has a much larger effect on the SPL profiles and attenuation spectra for the last higher-order mode source. Overall, axially variable-depth liners offer the potential to provide improved fan noise reduction, regardless of whether the axially variable depths are achieved via a distributed array of chambers (depths vary from chamber to chamber) or a group of zones (groups of chambers for which the depth is constant).

Reducing aerofoil–turbulence interaction noise through chordwise-varying porosity

“…This, therefore, provides immediate instruction as to how one would practically manufacture a plate corresponding to our model for testing. It may be helpful to the reader to consider our variation of porosity through a change in hole spacing as similar to the variation of acoustic impedance in traditional liners through a change of resonator depth (Jones et al 2017). We note, however, this choice may lead to unphysical values of α H > 1 and in such a case, to design a corresponding practical experiment, variations of r and K R would be necessary to achieve the same μ(x) values.…”

Novel liner concepts

“…Recent developments try to address a larger frequency range by combining cells of different volumes in one single structure [11,12]. By placing larger and smaller cells next to each other and folding the cavities for the larger cells, the effective damping range can be increased with only moderate increase in overall liner-depth [10,25]. A virtual increase of cell depth is also used in the special acoustic absorber concept aiming to address multiple frequencies with a special emphasis on the low-frequency region [19].…”

Initial Developments of a Low-Drag, Variable-Depth Acoustic Liner

“…Therefore, broadband concepts, such as variable depth liners, are also being developed. As the name implies, variable depth liners contain chambers with different depths tuned for different frequencies [4,5]. While this offers acoustic benefits, variable depth liners still require perforated facesheets and, therefore, have a similar drag penalty as conventional liners.…”