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Experimental Investigation of the Flow Structure over a Delta Wing Via Flow Visualization Methods

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Published on: April 23, 2018

Slow sound laser in lined flow ducts.

Antonin Coutant¹, Yves Aurégan¹, Vincent Pagneux¹

¹Laboratoire d'Acoustique de l'Université du Maine, Unite Mixte de Recherche 6613, Centre National de la Recherche Scientifique, Avenue O. Messiaen, F-72085 LE MANS Cedex 9, France.

The Journal of the Acoustical Society of America

|November 2, 2019

Summary

This study reveals a laser-like acoustic instability in waveguides with specific impedance walls. This subwavelength instability, analogous to black hole physics, is triggered by a subsonic resonant cavity and persists despite shear flow effects.

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Area of Science:

Acoustics
Fluid Dynamics
Wave Propagation

Background:

Acoustic wave propagation in waveguides is influenced by wall impedance, especially at low frequencies.
Wall impedance can lead to effective supersonic flow conditions in ducts at low Mach numbers.
Varying impedance can create unique flow regimes, including supersonic-subsonic-supersonic transitions.

Purpose of the Study:

To investigate acoustic wave propagation and instability in a waveguide with spatially varying impedance.
To analyze the characteristics of a laser-like instability generated by a subsonic resonant cavity within the waveguide.
To examine the influence of shear flow layers on this instability and its properties.

Main Methods:

Theoretical analysis of sound propagation in a waveguide with an impedance wall.
Modeling of a specific impedance profile creating supersonic-subsonic-supersonic flow regions.
Investigation of instability onset, wave characteristics (subwavelength), and static instability conditions.
Analysis of the effect of a shear flow layer on the instability.

Main Results:

A subsonic region within a waveguide with varying impedance acts as a resonant cavity, triggering a laser-like instability.
The instability is found to be highly subwavelength.
If the subsonic region is sufficiently small, the instability can become static.
A shear flow layer near the wall significantly alters the instability but does not eliminate its fundamental properties.

Conclusions:

A novel acoustic instability, analogous to the 'black hole laser' phenomenon in fluid dynamics, is identified and characterized.
The instability's subwavelength nature and dependence on the resonant cavity size are key findings.
The robustness of the instability's main properties in the presence of shear flow is demonstrated.