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This paper presents the acoustic performance of a novel approach for the passive adaptive control of tonal noise radiated from subsonic fans. Tonal noise originates from non-uniform flow that causes circumferentially varying blade forces and gives rise to a considerably larger radiated dipolar sound at the blade passage frequency (BPF) and its harmonics compared to the tonal noise generated by a uniform flow. The approach presented in this paper uses obstructions in the flow to destructively interfere with the primary tonal noise arising from various flow conditions. The acoustic radiation of the obstructions is first demonstrated experimentally. Indirect on-axis acoustic measurements are used to validate the analytical prediction of the circumferential spectrum of the blade unsteady lift and related indicators generated by the trapezoidal and sinusoidal obstructions presented in Ref. [A. Gérard, A. Berry, P. Masson, Y. Gervais, Modelling of tonal noise control from subsonic axial fans using flow control obstructions, Journal of Sound and Vibration (2008), this issue, doi:10.1016/j.jsv.2008.09.027.] and also by cylindrical obstructions used in the literature. The directivity and sound power attenuation are then given in free field for the control of the BPF tone generated by rotor/outlet guide vane (OGV) interaction and the control of an amplified BPF tone generated by the rotor/OGV interaction with an added triangular obstruction between two outlet guide vanes to enhance the primary non-uniform flow. Global control was demonstrated in free field, attenuation up to 8.4dB of the acoustic power at BPF has been measured. Finally, the aerodynamic performances of the automotive fan used in this study are almost not affected by the presence of the control obstruction. [Copyright &y& Elsevier]

This paper investigates the analytical calculation of blade unsteady lift spectrum when interacting with a neighboring obstruction, designed to control tonal noise. The approach used in this paper is to add a secondary unsteady lift mode, of equal intensity but opposite in phase with the primary unsteady lift mode which radiates most of tonal noise, so that the resultant of both primary and secondary modes is null. To control one unsteady lift mode (consequently an acoustic tone) without affecting the harmonics of the controlled mode (consequently the harmonics of the acoustic tone to be controlled), it is important for the secondary unsteady lift to be harmonically selective. We have therefore evaluated the harmonic content of the blade unsteady lift generated by the proposed control obstructions. To this purpose, an original equation is derived using the infinitesimal radial strips theory coupled with the one-dimensional Sears gust analysis. The spectrum of the blade unsteady lift is then analyzed for three types of obstructions: a series of -trapezoidal obstructions, a -periodic sinusoidal obstruction and a series of -rectangular obstructions (where is the number of blades). The use of salient obstructions leads to a large unsteady lift harmonic content. An optimized wake width of the trapezoidal obstruction leads to a low harmonic content rate of . A Gaussian approximation of the measured inflow velocity profile generated by a sinusoidal obstruction leads to a relatively low harmonic content rate of , which indicates that most of the energy is contained in the fundamental mode of the blade unsteady lift. Finally, a rotor/rectangular interaction shows that the use of small-width rectangular obstructions leads to a higher harmonic content rate of . [Copyright &y& Elsevier]