Noise dissipation mechanisms of an acoustic liner under grazing flow

TL;DR

The paper addresses how grazing turbulent flow alters noise-dissipation mechanisms in an acoustic liner by deploying high-fidelity LB/VLES simulations of a single-cavity, multi-orifice liner under plane acoustic forcing. It introduces a data-reduction framework combining viscous dissipation, Howe's energy corollary for vortex shedding, and SPOD-based estimation of the acoustic-induced velocity to quantify energy exchange between acoustics and fluid motion. The key findings show that grazing flow creates a quasi-steady upstream vortex that concentrates acoustic-induced flow in the downstream half of the orifice, shifts the resonant frequency, and reduces net acoustic dissipation by making vortex shedding phase-dependent and, at moderate SPL, energy-generating during outflow. These insights reveal that near-wall flow topology critically governs liner performance and suggest design and flow-control strategies to mitigate outflow-induced generation, with implications for multi-orifice liners and advanced edge geometries. The work provides a rigorous energy-budget framework and validates grid convergence and impedance against GFIT experiments, offering practical guidance for predicting liner behavior in engine-like grazing-flow environments.

Abstract

High-fidelity lattice-Boltzmann very-large-eddy simulations are performed to describe the noise dissipation mechanisms in an acoustic liner subjected to grazing turbulent flow at a Mach number of 0.3 and plane acoustic waves. The study examines the effects of sound pressure level (ranging from 130 to 160 dB) and frequency, as well as the direction of acoustic-wave propagation relative to the grazing flow. The considered mechanisms of acoustic energy dissipation are the viscous losses along the internal walls of the orifice and the vortex shedding. The latter is quantified through Howe's energy corollary. In the absence of grazing flow, acoustic energy is dissipated almost equally during both inflow and outflow phases, with vortex shedding dominating at high SPL and viscous losses at low SPL. The introduction of a grazing flow alters the flow topology; in particular, the shear layer past the orifice generates a quasi-steady vortex that confines the acoustic-induced flow to the downstream half of the orifice. This topological change modifies the two noise dissipation mechanisms: viscous losses increase at low SPL because the grazing flow pushes the fluid toward the downstream lip of the orifice; vortex shedding becomes phase dependent, dissipating acoustic energy during the inflow phase and generating acoustic energy during the outflow phase. This explains why the net acoustic dissipation decreases in the presence of grazing flow, highlighting the crucial role of near-wall flow topology on liner performances.

Figures (21)

• Figure 1: (a) Schematic of the cavity with representation of the coordinate reference system. The $\textit{y}$ axis is oriented towards the inside of the cavity. (b) Schematic of the computational setup with the grid in a plane crossing the central orifice.
• Figure 2: Contour of the wall-normal acoustic-induced velocity at the inflow ($\phi=\pi/2$) and outflow ($\phi=3\pi/2$) phases, effect of the SPL (left to right column), no flow condition.
• Figure 3: Contour of the wall-normal acoustic-induced velocity at the inflow ($\phi=\pi/2$) and outflow ($\phi=3\pi/2$) phases, effect of the SPL (left to right column), $M=0.3$.
• Figure 4: Spatial distribution along the diameter of the non-dimensional acoustic-induced vertical velocity $v_{ac}$ as a function of the SPL at half orifice thickness. Various phases are reported, dashed line is the no flow case and solid line is $M=0.3$ case.
• Figure 5: Contour of the friction velocity [m/s] to represent the shear layer forming at the mouth of the orifice in the presence of grazing flow at $M=0.3$, effect of the SPL.