What the study found
The study found that a spatially evolving turbulent grazing flow can change how an acoustic liner responds to sound and how its impedance is measured. The authors report that the asymmetry of the flow field depends on wave propagation direction, and that this needs to be considered when studying acoustic–flow interaction and measuring impedance.
Why the authors say this matters
The authors conclude that spatially evolving turbulent flow should be taken into account when studying acoustic–flow interaction and when measuring impedance. They also suggest that the different flow conditions experienced by upstream- and downstream-propagating waves are important for interpreting liner behavior.
What the researchers tested
The researchers used lattice-Boltzmann very-large-eddy simulations to study the interaction between acoustic waves and turbulent grazing flow over a single-degree-of-freedom acoustic liner with 11 streamwise-aligned cavities. The simulations reproduced reference experiments from the Federal University of Santa Catarina and examined the effects of grazing flow, acoustic wave amplitude, frequency, and propagation direction relative to the mean flow.
What worked and what didn't
The direct in situ impedance method showed strong spatial variation across the sample, but averaged values showed minimal differences between upstream- and downstream-propagating waves. The model-fitting mode-matching method gave different results from the direct method. The flow analyses also showed that the orifices displaced the flow away from the face sheet, that acoustic waves amplified this effect, and that the boundary layer displacement thickness increased downstream with local humps after each orifice.
What to keep in mind
The abstract does not report detailed numerical values beyond the centreline Mach number of 0.32. It also does not provide a full account of experimental uncertainty or broader applicability beyond the simulated liner configuration and the reference conditions.
Key points
- Turbulent grazing flow changed acoustic liner impedance in a spatially varying way.
- Flow asymmetry depended on whether the acoustic wave traveled upstream or downstream.
- Direct in situ impedance and model-fitting mode-matching gave different results.
- The orifices displaced flow away from the face sheet, and acoustic waves amplified this effect.
- Boundary layer displacement thickness increased downstream and showed local humps after each orifice.
Disclosure
- Research title:
- Turbulent grazing flow changes liner impedance measurements
- Image credit:
- Photo by Zelch Csaba on Pexels
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