Cavity Acoustic Tone Suppression Using Internal Rods
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Solution Overview
Problem
Existing methods for suppressing acoustic tones and resonance in cavities moving relative to ambient fluid, such as those in aircraft, are ineffective as they do not disrupt the formation of large-scale vortices that generate noise, and often require external structures that are not easily removable.
Innovation Solution
A system comprising rods positioned inside the cavity with flow alteration elements that increase the thickness of the shear layer, creating multiple fine-scale turbulences and disrupting the formation of large-scale vortices, thereby reducing acoustic noise. The rods can be configured to be enclosed within the cavity when not in use.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If external suppression structures (pins, spoilers) are placed outside the cavity, then acoustic tone suppression is achieved, but the structures cannot be easily removed when the cavity is closed
Solution Approach 1:
The suppression rods are nested within the cavity itself, extending from the base towards the opening. When the cavity is open, the rods protrude to suppress acoustic tones. When the cavity is closed, the rods are enclosed within the cavity volume, automatically adapting to the closed state without requiring separate removal actions.
2Reliability
If fixed suppression structures are placed outside the cavity, then acoustic resonance is suppressed, but the structures do not disrupt large-scale vortex formation effectively
Solution Approach 1:
Instead of using a single large suppression structure, the invention employs multiple thin rods positioned at different locations within the cavity. Each rod generates fine-scale turbulence independently, and the collective effect of multiple segmented structures more effectively disrupts the organization of large-scale vortices compared to a single fixed structure.
Solution Approach 2:
The rods extend in the vertical dimension from the cavity base, creating flow disruption in a different spatial orientation than traditional external structures. This vertical extension into the shear layer creates fine-scale turbulence that interferes with the horizontal propagation and organization of large-scale vortices.
3Adaptability or versatility
If suppression features are placed inside the cavity, then ease of removal is improved, but the suppression effectiveness may be reduced
Solution Approach 1:
The rods are pre-positioned within the cavity at optimal locations for acoustic suppression. When the cavity is open, they are automatically in the correct position to disrupt vortex formation and suppress tones, eliminating the need for separate positioning actions while maintaining suppression effectiveness.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system effectively suppresses acoustic tones by disrupting the temporal coherence of vortices, reducing noise generation, and allows for easy removal when the cavity is closed, enhancing operational flexibility.
Implementation Method 1
one or more flow alteration elements are provided on one or more of the rods; at least some of the flow alteration elements comprise or provide additional edges at the one or more rods that are in addition to the edge or edges provided by the underlying shape of the rod or rods
Implementation Method 2
A vortex is shed from the cavity leading edge and grows as it travels down the shear layer and impacts on the aft (trailing) wall of the bay resulting in the emission of noise
Implementation Method 3
generating multiple fine scale turbulences i.e. resulting in multiple small vortices, within the shear layer, thereby disrupting the formation of large scale vortices
Implementation Method 4
by providing a thicker shear layer, the vortices closer to the stream flow will propagate downstream faster than those closer to the static air in the bay. The present inventor has realised that therefore at some point downstream the vortices will arrive at different times to each other. The present inventor has realised that this loss of temporal coherence will disrupt the formation of the conventional large scale turbulent structures
Implementation Method 5
The fluctuating pressure of the acoustic wave may either result in vortices being shed from the leading edge cavity lip or an increase in the growth rate of the vortices such that a series of vortices is formed down the shear layer at a preferential rate which is related to the frequency of the upstream acoustic wave
Implementation Method 6
The frequency of the tones may be formulated using Rossiter's equation. It can be seen that there is a feedback loop formed by the passage of the vortices and the upstream propagating acoustic wave
Data Source
AI summary
A cavity system that tends to increase the thickness (28) of the shear layer (22), comprising: a cavity (2) and a plurality of rods (4) extending away from the cavity base (3) to a height extending beyond the leading edge (14); the rods (4) being positioned downstream of, and in the proximity of, the leading edge (14). The rods (4) may extend to different heights and/or be positioned longitudinally offset (e.g. in a zig-zag pattern). The rods may be reversibly movable to a configuration in which they are fully enclosed in the cavity (2) when the cavity (2) is closed. Flow alteration elements (34, 38), for example channels (34) passing through the rods (4) and/or protrusions (38) extending from the rod (4) may be provided on the rods (4).