The flow-fields around airfoils in a uniform flow under the generation of noise were numerically studied and compared with experimental data. The numerical simulation was carried out by a large-eddy simulation that employs a deductive dynamic model as a subgrid-scale model. The result for a symmetrical airfoil at small angle of attack α = 3°–6° indicates that the discrete or narrow-banded frequency noise is generated when the separated laminar flow reattaches near the trailing edge of the pressure side and a strong instability thereafter affects positive vortices shed near the trailing edge. This type of forced transition or late transition instabilities near the trailing edge of the pressure side, interacting with convected vortices in an attached T.B.L. on the suction side, can be found in many practical airfoils of impellers rotating at moderate speeds under design conditions. The sound spectra derived from the aero-acoustic computations of airfoils indicate a dipole nature of sound having a narrow-banded or discrete peak by laminar instability and turbulent vortex shedding from their trailing edges of finite thickness at a Strouhal frequency, a quadrupole sound by turbulent broadband boundary-layer noise, or a mixed mode depending on flow conditions near the T.E.

This content is only available via PDF.
You do not currently have access to this content.