AbstractsBiology & Animal Science

Single degree-of-freedom conventional acoustic liners are widely installed in jet engines to reduce internal engine noise. They work by converting acoustic energy into vorticity-bound fluctuations. Despite being widely used, effective design-stage models of acoustic liners placed in high sound amplitude conditions, possibly with a turbulent grazing flow, are not available due to the near-liner flow complexity and diagnostic challenges. The work presented in this thesis uses direct numerical simulations (DNS) of a compressible, viscous fluid to understand the inherent fluid mechanics and guide reduced-order-model development. While there are numerous orifices and cavities in a general conventional acoustic liner sample, only the one orifice and one cavity is investigated in this work. Resolved simulations of the sound-induced flow through a circular orifice with a 0.99 mm diameter are examined. The detailed investigations are split into two steps: the first step neglects any grazing flow. The no-flow simulation data identify the role the orifice wall boundary layers play in determining the orifice discharge coefficient which is an important indicator of liner non-linearity. It is observed that when the liner behavior is not well described by linear models, the orifice boundary layers contain secondary vorticity generated from its separation from the corner on the high-pressure side of the orifice. Quantitative comparisons of the simulation-predicted impedance match available data for incident sound amplitude of 130 dB at frequencies from 1.5 kHz to 3.0 kHz. At amplitudes of 140  – 160 dB the simulation impedance are in agreement with analytical predictions when using simulation-measured quantities, including the discharge coefficient and root-mean-square velocity through the orifice, although no experimental data for this liner exist at these conditions. The simulation data are also used to develop two time-domain models for the acoustic impedance wherein the velocity profile through the orifice is modeled as the product of the fluid velocity and a presumed radial shape, ??V(r). The models perform well, predicting the in-orifice velocity and pressure, and the impedance, except at the most non-linear cases where it is seen that the assumed shape V(r) can affect the back-plate pressure predictions. These results suggest that future time-domain models that take the velocity profile into account, by modeling the boundary layer thickness and assuming a velocity profile shape, may be successful in predicting the non-linear response of the liner. The second step introduces a grazing flow where the detailed interaction of an incident acoustic field and a Mach 0.5 laminar and turbulent grazing flow with a cavity-backed circular orifice is studied. All results are for tonal excitation at 130 dB from 2.2  – 3.0 kHz, or at 3 kHz with 130  – 160 dB acoustic amplitude. The results suggest that the liner experiences a drag increase over the baseline geometry with acoustic excitation and that facesheet shear…