On the Generation of Discrete Frequency Tones by the Flow around an Aerofoil

✍ Scribed by A. McAlpine; E.C. Nash; M.V. Lowson

• Publisher: Elsevier Science
• Year: 1999
• Tongue: English
• Weight: 327 KB
• Volume: 222
• Category: Article
• ISSN: 0022-460X
• DOI: 10.1006/jsvi.1998.2085

✦ Synopsis

Tonal noise, the self-induced discrete frequency noise generated by aerofoils, is investigated. It is heard from an aerofoil placed in streams at low Mach number ¯ows when inclined at a small angle to the stream. It is a piercing whistle, commonly up to 30 dB above the background noise level. Previous authors have attributed tonal noise to a feedback loop consisting of a coupling between laminar boundary-layer instability waves and sound waves propagating in the free stream. Boundary-layer measurements have shown the presence of tonal noise is closely related to the existence of a region of separated ¯ow close to the trailing edge of the aerofoil. An analysis of the linear stability of the boundary-layer ¯ow over the aerofoil is presented. The ampli®cation of the instability waves is shown to be controlled almost entirely by the region of separated ¯ow close to the trailing edge. In light of these new experimental and theoretical results the suitability of the aero-acoustic feedback model is discussed.