Noise from fluid flow over rough surfaces is an important consideration in the design and performance of certain vehicles with high surface-area-to-perimeter ratios. A new method of noise control based on the anatomy of owls is developed and consists of fabric or fibrous canopies suspended above the surface. The method is tested experimentally and is found to reduce the total far-field noise emitted by the surface. The treatment also is found to reduce the magnitude of pressure fluctuations felt by the underlying surface by up to three orders of magnitude. Experimental investigations into the effects of geometric parameters of the canopies lead to an optimized design which maximizes noise reduction.
The results obtained during the canopy experiment inspired a separate new device for the reduction of trailing edge noise. This type of noise is generated by flow past the wing of an aircraft or the blades of a wind turbine, and is a source of annoyance for those in surrounding communities. The newly developed treatment consists of small fins, or "finlets," placed near the trailing edge of an airfoil. The treatment is tested experimentally at near-full-scale conditions and is found to reduce the magnitude of far-field noise by up to 10 dB. Geometric parameters of the finlets are tested to determine the optimal size and spacing of the finlets to maximize noise reduction. Follow-up computational and experimental studies reveal the fluid mechanics behind the noise reduction by showing that the finlets produce a velocity deficit in the flow near the trailing edge and limit the magnitude and spanwise correlation lengthscale of turbulence near the trailing edge, factors which determine the magnitude of far-field noise.
In a final experiment, the finlets are applied to a marine propeller and are found to reduce not only trailing edge noise, but also noise caused by the bluntness of the trailing edge. The results of this experiment show the potential usefulness of finlets to reduce noise from rotating systems, such as fans or propellers, as well as from structures which feature blunt trailing edges. / Ph. D. / As vehicles and other engineering structures, such as wind turbines, pass through the atmosphere or ocean, noise is produced when fluid is disturbed by their passage. The dominant source of this noise may be a certain geometrical or structural feature depending on the type of vehicle or structure in question. The noise from marine vehicles can be dominated by interaction between the fluid flow and any roughness present on the surface of the vehicle, and this is termed roughness noise. This noise can be detrimental to the performance and efficient operation of marine vehicles, and few options exist to suppress this noise apart from removing the roughness itself. As this is not always feasible if the structure’s design depends on the presence of roughness (for example, rivet heads which fasten structural components of the vehicle), other methods of noise control would be valuable.
The noise from large, rotating wind turbines is dominated by interaction between the fluid flow and the sharp trailing edges of the turbine blades, termed trailing edge noise. This noise can travel significant distances from wind turbines and can be a source of annoyance for those living in nearby communities. New methods of noise control would significantly improve the quality of life in these communities and increase the viability and popularity of wind energy.
This work takes inspiration from the anatomical features of silently-flying owls to develop new methods to control both roughness noise and trailing edge noise. Experiments and simulations were carried out to prove the effectiveness of these methods and to gain scientific understanding of the fluid mechanics responsible for noise reduction. The developments described in the present work give engineers new tools for designing future vehicles and wind turbines which operate more quietly and more efficiently.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/77531 |
| Date | 27 April 2017 |
| Creators | Clark, Ian Andrew |
| Contributors | Aerospace and Ocean Engineering, Devenport, William J., Alexander, William Nathan, Glegg, Stewart, Lowe, K. Todd |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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