Direct numerical simulation and analytical modeling of locally reacting, single degree of freedom acoustic liners with turbulent grazing flow

Abstract

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. In this work, detailed interaction of an incident acoustic field with 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 stress measurements, while dominant at low acoustic amplitudes, contribute less at higher acoustic amplitudes. The DNS data further show that the orifice discharge coefficient can be semi-empirically modeled effectively using an acoustic-hydrodynamic scaling. The results indicate that experimental in situ impedance measurements can be contaminated by microphone-orifice interaction. Finally, the time-domain model without grazing flow was extended to include grazing flow by properly modeling the discharge coefficient and the turbulent boundary layer effect. Reasonable agreement of the liner impedance prediction was found with the DNS data. Discrepancies of the prediction suggest the future improvement of the model development.

Analysis of the turbulent boundary layer in the vicinity of a self-excited cylindrical Helmholtz resonator

“…[41,42] Using DNS to solve all characteristics of the flow is the most accurate option, however it requires an intensive computational resource. [11] 3D large-eddy simulation (LES) represents an alternative to DNS, in which small sub-grid scales (SGS) structures are modelled and the large-scale ones are resolved through application of a filtering procedure. [43,44] This filtering reduces the computational complexity, such that the high frequencies are removed from the solutions, but their effects on the resolved scales are still taken into account.…”

“…With increased SPL the structure of the vortices within the shear layer over the orifice is also altered and dissipated more quickly. Using direct numerical simulations (DNS), Zhang et al [10,11]) also analysed the velocity and pressure fluctuations within a Helmholtz resonator in the presence of a grazing flow and an acoustic field. It was concluded that when the grazing flow is laminar the fluctuations within the resonator cavity are limited, while the turbulent boundary layer produces more interactions.…”

A Numerical Investigation on Sound Absorption Mechanism of Micro Resonator with Offset Slits

“…Recently, their numerical and experimental studies 13 of eight slit resonators with grazing flow have been carried out and self-noise has been detected. Zhang and Bodony 14,15 considered 3D hexagonal resonator in calculation with normal incident acoustic wave, and they [16][17][18] have continuously working on 2D and 3D numerical investigation on resonator with grazing flow, which provides detailed flow field revealing the complex interaction between boundary layer and the orifice. These numerical simulations show great advantage in reproducing complex phenomena around resonator slit or orifice.…”