Noise dissipation mechanisms of an acoustic liner under grazing flow

This study utilizes high-fidelity lattice-Boltzmann very-large-eddy simulations to reveal that grazing flow fundamentally alters the noise dissipation mechanisms of an acoustic liner by modifying the near-wall flow topology, which increases viscous losses at low sound pressure levels while introducing phase-dependent vortex shedding that generates energy during outflow, ultimately reducing the liner's net acoustic dissipation.

The Gist

Imagine an airplane engine as a very loud, angry beast. To keep it from roaring too loudly, engineers line the inside of the engine with a special "noise sponge" called an acoustic liner. This liner is basically a wall covered in tiny holes (like a honeycomb) leading into small chambers. When sound waves hit these holes, they get sucked in, swirl around, and lose their energy, turning into harmless heat.

This paper is a deep dive into how that noise sponge actually works when the engine is running. Specifically, the researchers wanted to understand what happens when two things happen at once:

1. Loud sound waves are trying to get into the holes.
2. Fast-moving air (like a strong wind) is blowing across the top of the holes.

Here is the story of their findings, explained simply:

The "No-Wind" Scenario: A Perfect Dance

First, imagine the engine is off, but a loud speaker is playing a tone right next to the liner.

• The Dance: The air in the tiny holes breathes in and out perfectly in sync with the sound.
• The Noise Killers: There are two ways this air loses its energy:
  1. Friction (Viscous Loss): The air rubs against the rough walls of the tiny holes, like your hands rubbing together to make heat. This happens mostly when the sound is quiet.
  2. Whirlpools (Vortex Shedding): When the sound is very loud, the air doesn't just slide in; it gets chaotic. It forms little whirlpools (vortices) at the mouth of the hole. These whirlpools spin and break apart, turning the sound energy into heat. This is the main noise killer when the sound is loud.
• The Result: In this calm, no-wind scenario, the liner is a great noise sponge. It absorbs sound equally well when the air is breathing in and when it's breathing out.

The "Wind" Scenario: The Traffic Jam

Now, turn on the engine. A fast stream of air (the "grazing flow") blows across the top of the liner. This changes everything.

1. The "One-Way Street" Effect
The fast wind acts like a traffic jam at the entrance of the holes.

• The Blockage: The wind pushes a giant, lazy whirlpool (a "quasi-steady vortex") right at the front edge of the hole. This vortex acts like a bouncer, blocking the entrance.
• The Shift: Because of this bouncer, the air can't breathe in and out evenly anymore. It gets squeezed into the back half of the hole. The front half is effectively closed off.

2. The "Bad Neighbor" Effect (Why it gets worse)
This is the most surprising part. The wind changes the rules of the game for the two noise killers:

• Friction gets a boost (at low volume): Because the wind pushes the air hard against the back wall of the hole, the friction increases. The liner actually gets better at absorbing sound via friction when the wind is blowing, but only if the sound isn't too loud.
• Whirlpools get confused: This is the problem.
  • When breathing IN: The wind helps create whirlpools that eat up sound energy (good!).
  • When breathing OUT: The wind fights the air trying to leave the hole. Instead of just dissipating energy, this struggle creates new sound waves. It's like blowing across the top of a bottle to make a whistle; the liner starts acting like a whistle generator instead of a sponge.

The Net Result: Because the liner starts making noise when the air breathes out, the total amount of noise it absorbs drops significantly. The wind turns a good noise sponge into a less efficient one.

What the Researchers Found Out

The team used super-powerful computer simulations (like a virtual wind tunnel) to watch these tiny holes in extreme detail. They tested different volumes (from a shout to a jet engine roar) and different frequencies.

• Volume Matters: When the sound is very loud, the sound waves are so strong they push the "bouncer" vortex out of the way. The hole opens up, and the liner starts working better again, though it still isn't as good as it would be without the wind.
• Frequency Matters: The wind changes the "tuning" of the liner. A hole that is perfectly tuned to absorb a specific sound frequency when the engine is off might need a different frequency to work well when the engine is running.
• Direction Matters: They checked if it mattered if the sound traveled with the wind or against it. It turned out to make very little difference; the wind's speed and the hole's shape were the real bosses.

The Big Picture

The main takeaway is that flow topology (the shape and path of the air) is everything. You can't just look at the hole and the sound; you have to look at how the wind reshapes the air inside the hole.

The wind creates a "traffic jam" that blocks the hole, forces the air to rub harder against one side, and turns the "breathing out" phase into a noise generator. This explains why acoustic liners sometimes struggle to work as well as predicted when installed in real, running engines. To make better liners, engineers need to design them to handle these specific "traffic jams" caused by the wind.

Technical Summary

Technical Summary: Noise Dissipation Mechanisms of an Acoustic Liner Under Grazing Flow

Problem Statement
Acoustic liners, typically single-degree-of-freedom (SDOF) configurations comprising a perforated face-sheet and backing cavity, are critical for noise attenuation in aircraft engines. While the dissipation mechanisms of liners under pure acoustic excitation are well-established (dominated by viscous losses at low sound pressure levels (SPL) and vortex shedding at high SPL), a significant gap exists in understanding their behavior under realistic operating conditions. Specifically, the coupling between high-intensity acoustic waves and grazing turbulent flow at high Mach numbers remains poorly characterized. Previous studies indicate that grazing flow alters the liner's impedance and that the relationship between impedance and absorption coefficient breaks down in these regimes. The precise flow topology changes within the orifice and their specific impact on the balance between viscous dissipation and vortex shedding under combined acoustic and flow forcing are not fully quantified.

Methodology
The study employs high-fidelity Lattice-Boltzmann Very-Large-Eddy Simulations (LB/VLES) using the commercial solver PowerFLOW. The numerical approach utilizes a regularized collision operator and a turbulence transport model (based on the RNG $k-\epsilon$ model) to dynamically recalibrate relaxation times, capturing non-linear flow-acoustic interactions without the prohibitive cost of Direct Numerical Simulation (DNS).

The computational domain models a single cavity with seven orifices, replicating the geometry of the Grazing Flow Impedance Tube (GFIT) facility at NASA Langley. The simulations cover a centerline Mach number of $M=0.3$ and a range of SPLs from 130 to 160 dB. The study investigates:

1. Flow regimes: Cases with and without grazing flow.
2. Parametric variations: Source frequencies (1200–3000 Hz), SPLs, and acoustic propagation directions (with and against the flow).
3. Dissipation quantification:
   • Viscous losses: Evaluated by integrating the viscous dissipation density ($\Phi$) within a near-wall control volume (approx. 30 wall units) along the orifice internal walls.
   • Vortex shedding: Quantified using Howe's energy corollary ($\Pi_g = \rho(\boldsymbol{\omega} \times \mathbf{u}') \cdot \mathbf{u}_{ac}$), which measures the transfer of acoustic energy to vortical motion. The acoustic-induced velocity ($\mathbf{u}_{ac}$) is isolated from turbulent fluctuations using Spectral Proper Orthogonal Decomposition (SPOD).

Both 2D streamwise slices (for parametric sweeps) and full 3D volumes (for validation) are analyzed.

Key Results

• Flow Topology Alteration: In the absence of grazing flow, the acoustic-induced velocity is symmetric across the orifice. The introduction of grazing flow creates a near-wall shear layer that generates a quasi-steady vortex at the upstream lip. This vortex acts as a barrier, confining the acoustic-induced flow to the downstream half of the orifice and reducing the effective porosity. This topological change shifts the liner's resonant frequency from approximately 1600 Hz (no flow) to 2200 Hz (with flow).
• Dissipation Mechanisms without Flow: In quiescent conditions, viscous dissipation dominates at low SPLs (linear regime), while vortex shedding becomes the primary dissipation mechanism at high SPLs (non-linear regime). Both inflow and outflow phases contribute positively to energy dissipation.
• Dissipation Mechanisms with Grazing Flow:
  • Viscous Losses: At low SPLs, viscous dissipation increases compared to the no-flow case because the grazing flow pushes fluid toward the downstream lip, intensifying wall shear. However, at high SPLs, the acoustic forcing overcomes the grazing flow, and viscous losses approach no-flow levels.
  • Vortex Shedding: The presence of grazing flow fundamentally alters the phase dependence of vortex shedding. During the inflow phase, energy is dissipated as turbulent structures are entrained. However, during the outflow phase, the interaction between the acoustic jet and the grazing flow generates vorticity that radiates acoustic energy (acting as a noise source). Consequently, the net vortex shedding contribution over a full cycle is significantly reduced and can become negative (energy generation) at moderate SPLs.
• Net Effect: The transformation of the outflow phase from a dissipative to a generative mechanism results in a net reduction of acoustic energy dissipation. The total energy dissipated by a single orifice in the presence of grazing flow is approximately 15% of that observed in the no-flow configuration across the tested SPL range.
• Parametric Sensitivity:
  • Frequency: The shift in resonant frequency due to grazing flow is confirmed; maximum dissipation occurs at the shifted frequency (2200 Hz) rather than the quiescent resonance.
  • Propagation Direction: Reversing the acoustic propagation direction (against the flow) produces only minor modifications to the shear layer and dissipation rates, suggesting that liner performance is governed more by boundary layer properties and acoustic amplitude than by propagation direction.
  • Orifice Interference: Analysis of two adjacent orifices shows that the upstream orifice slightly modifies the boundary layer (increasing displacement thickness) for the downstream orifice, leading to a modest (~5%) increase in viscous dissipation for the second orifice.

Significance and Conclusions
The paper provides a detailed physical explanation for the reduced noise absorption efficiency of acoustic liners under grazing flow. The primary finding is that the grazing flow does not merely add resistance but fundamentally alters the flow topology, creating a quasi-steady vortex that restricts acoustic motion and, crucially, turns the outflow phase into a source of acoustic energy via vortex shedding. This mechanism explains why net dissipation decreases despite increased viscous losses at the downstream lip.

The study validates that high-fidelity LB/VLES simulations can resolve these complex, non-linear interactions. The authors conclude that the near-wall flow topology is the critical factor governing liner performance, and that the detrimental effect of the outflow phase in generating acoustic energy is the dominant factor reducing liner effectiveness in grazing flow conditions. Future work is suggested to investigate streamwise evolution in multi-orifice liners and the impact of orifice geometry modifications to mitigate outflow-induced acoustic generation.