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SUMMARY:A global stability analysis of tonal noise in the flow around an a
 erofoil: instability and receptivity mechanisms - Miguel Fosas de Pando\, 
 LadHyX\, Ecole polytechnique
DTSTART:20140117T160000Z
DTEND:20140117T170000Z
UID:TALK50137@talks.cam.ac.uk
CONTACT:Dr C. P.  Caulfield
DESCRIPTION:Even in low-turbulence environments\, aerofoils can radiate su
 bstantial levels of noise from the sole interaction between the boundary l
 ayers and the wake: experiments show that for moderate-to-high Reynolds nu
 mber and small angles of incidence\, the sound spectrum is characterised b
 y discrete tones that are commonly perceived as a whistle.  Previous inves
 tigations have suggested that the acoustic emission is correlated to the r
 inging of coherent structures near the trailing edge\, suggesting a nontri
 vial interplay between acoustics and hydrodynamics. Although this problem 
 has received considerable attention since the mid-seventies\, a satisfacto
 ry explanation for the physical origins of the tones is still under debate
 .\n\nIn this presentation\, we address the instability and receptivity mec
 hanisms involved in the tonal noise problem by means of global stability t
 heory. It is found that the flow response to incoming disturbances exhibit
 s important transient-growth effects that culminate into the onset of aero
 acoustic feedback loops\, involving instability processes on the suction- 
 and pressure-surface boundary-layers together with their cross interaction
  by acoustic radiation at the trailing edge. The features of the aeroacous
 tic feedback loops and the appearance of discrete tones are then related t
 o the features of the least stable modes in the global spectrum: on the on
 e hand\, the spatial structure of the direct modes display the growth of h
 ydrodynamic instabilities on the suction surface and the near wake\; on th
 e other hand\, the associated adjoint modes display increased receptivity 
 of the flow on the pressure surface. Finally\, the analysis of the wavemak
 er region highlights\, in agreement with previous experimental investigati
 ons\, the sensitivity of the flow to the pressure-surface boundary layer.
LOCATION:MR2\, Centre for Mathematical Sciences\, Wilberforce Road\, Cambr
 idge
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