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

The purpose of this study is to assess the acoustic performance and relative drag of a new type of variable depth liner containing pairs of resonators coupled together by shared inlet volumes just below the facesheet.This type of liner has the potential to achieve a targeted impedance with fewer openings in the facesheet, and therefore less drag, than previous designs.To better understand the limitations of the concept, three sets of samples were designed and tested in the NASA Langley normal incidence and grazing flow impedance tubes.Each set of samples consisted of a baseline variable depth liner with straight chambers, and a prototype liner with shared inlet volumes.Measurements conducted in both the normal incidence and grazing flow impedance tubes confirm that the prototype liner can achieve the same impedance as the baseline variable depth liner at discrete frequencies with 50% less open area.This results in a 75% reduction in the drag penalty relative to the baseline design.The proposed concept is not, however, able to achieve a flat, broadband impedance as effectively as the baseline liner.So, while the proposed concept is well suited for multitonal design metrics, tradeoffs must be made between liner drag and acoustic performance when broadband attenuation is required.𝛿 end correction 𝛿 * , 𝜀 boundary layer displacement thickness and hole length end correction multiplier, respectively 𝜆 L , 𝜆 SW resistance factors for the liner and smooth wall sample, respectively ∆𝜆 L relative liner drag 𝜇 dynamic viscosity of air 𝜎 open area ratio 𝜃 lin , 𝜃 nonlin , 𝜃 gf linear, nonlinear, and grazing flow components of the normalized resistance, respectively 𝜒 fs , 𝜒 rad normalized facesheet reactance and radiation reactance, respectively 𝜔 circular frequency 𝜁 𝑠 , 𝜁 opt , 𝜁 target normalized smeared surface impedance, optimal impedance, and target impedance, respectively 𝜁 𝐴 ′ normalized total resonator impedance