Trailing edge noise at low Mach numbers part 2: Attached and separated edge flows

The diffraction theory of Part 1 [1] for estimating the sound generated by low Mach number flow past the trailing edge of an airfoil of compact, but f inite thickness is applied to investigate the noise produced by turbulent f low over an edge whose upper surface profile (the suction side) is rounded. The sound is expressed in terms of the "upwash" Velocity fluctuations that the same boundary layer turbulence would generate if the airfoil were abse nt. An approximate method is proposed for expressing these fluctuations in terms of local properties of the blocked pressure generated on the surface exposed to the turbulent flow. Predictions are made of the edge-noise spect rum for both fully attached flow (that remains attached right up to the tra iling edge) and for cases where separation occurs on the rounded profile. W hen premature separation occurs the amplitude of the edge-generated sound d ecreases exponentially with increasing frequency, and predicted edge-noise levels are significantly smaller than estimates obtained when the airfoil i s modelled by a rigid half-plane. For attached flow turbulence on the sucti on side of the airfoil always passes close to the edge and interacts strong ly with it, but contributions from the interaction of the same turbulence w ith the pressure side of the airfoil are reduced because of the finite airf oil thickness. In this case sound levels fall short of those for a rigid ha lf-plane typically by about 5-10 dB, the precise values being dependent on frequency, and on the ratio of the boundary layer thickness to the mean air foil thickness. (C) 2000 Academic Press.