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 finite thickness is applied to investigate the noise produced by turbulent flow 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 absent. 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 spectrum for both fully attached flow (that remains attached right up to the trailing edge) and for cases where separation occurs on the rounded profile. When premature separation occurs the amplitude of the edge-generated sound decreases exponentially with increasing frequency, and predicted edge-noise levels are significantly smaller than estimates obtained when the airfoil is modelled by a rigid half-plane. For attached flow turbulence on the suction side of the airfoil always passes close to the edge and interacts strongly with it, but contributions from the interaction of the same turbulence with the pressure side of the airfoil are reduced because of the finite airfoil thickness. In this case sound levels fall short of those for a rigid half-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 airfoil thickness.