Isotropic and Anisotropic Turbulence Interaction with Flat Plate Serrated Leading Edges

This paper presents the far-field noise radiated by a flat plate with sinusoidal leading edge (LE) serrations immersed in isotropic and anisotropic turbulence. Nine serration patterns have been tested, four with varying wavelength, \lambda (normalised by the span, b) in the range \lambda/b = 0.10 - 0.29 of constant amplitude h, (normalised by chord, c) h/c = 0.06 and five with varying amplitude in the range h/c = 0.03 - 0.17 of constant wavelength \lambda/b = 0.24. The serrated LEs were tested in grid-generated turbulence and the wake of a cylinder positioned at various distances upstream of the LE at chord-based Reynolds numbers from Rec = 1.97 x 10^5 to 6.91 x 10^5. The serrations displayed substantially lower noise levels than the unserrated, straight-edge case at 1 < Stc < 20 in isotropic turbulence and at 3 < Stc < 50 in anisotropic turbulence, where Stc is the chord-based Strouhal number. In both inflows, the greatest noise reduction was achieved by the smallest wavelength serration of \lambda/b = 0.10 (among those with varying wavelength), and by the largest amplitude serration of h/c = 0.17 (among those with varying amplitude). For both optimal cases, the average noise reduction achieved in isotropic turbulence was between 4-6 dB over the chord-based Strouhal number range 1 < Stc < 20 when the Reynolds number was varied from 1.97 x 10^5 to 6.91 x 10^5. In anisotropic turbulence, a gradual noise reduction as a function of Stc occurred over 1 < Stc < 10 and a constant reduction of 8-9 dB was measured at Stc > 10. At Stc > 20, a slight decrease in noise reduction was observed for 2.96 x 10^5 < Rec < 6.91 x 10^5, suggesting that self-noise dominates in this region, reducing the effectiveness of the serrations. Varying the distance between the cylinder and the LE affects noise reduction in a similar way to changing the Reynolds number, with the major differences occurring at the vortex shedding frequency and at high frequencies of Stc > 10. This is attributed to small changes in the flow anisotropy, with only a 16% and 18% difference in the root-mean-square (rms) ratio of urms/vrms and urms/wrms respectively when the cylinder is closest to the LE and when it is further away.