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Numerical Analysis of the Acoustic Field of a Ducted Axial Fan at Varying Tip Clearances

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The acoustic field generated by a shrouded axial fan is numerically analyzed by a hybrid fluid-dynamics-acoustics method formulated in a rotating frame of reference. In a first step, large-eddy simulations are performed to investigate the dynamics of tip clearance flow for various tip-gap sizes and to determine the acoustic sources. The simulations are performed for a single blade out of five blades with periodic boundary conditions in the circumferential direction on a multi-block structured mesh with 140 × 106 grid points. The results for a Reynolds number of 9.36 × 105 at undisturbed inflow condition are compared with experimental data. The findings show that increasing the tip-gap size results in several vortices in the tip-gap region, i.e., tip leakage, separation, and induced vortices, which enlarge the diameter and the strength of the main tip vortex and decrease the efficiency of the fan. In a second step, the acoustic field is determined by solving the acoustic perturbation equations (APE) on a mesh for a single blade consisting of approx. 1060 × 106 grid points. The acoustic source terms are determined by the flow field. It is shown that the strongest acoustic sources occur in the regions with highest turbulent kinetic energy, i.e., in the tip vortex, blade wake, and hub region. The noise sources generated by the large tip-gap size s/D 0 = 0.01 exhibit higher amplitudes than the smaller tip-gap size s/D 0 = 0.001. The overall agreement of the pressure spectrum and its directivity with measurements confirm the correct identification of the sound sources and accurate prediction of the acoustic propagation. The results prove that increasing the tip-gap size results in an acoustic field with higher frequencies and broadband noise level.
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Document Type: Research Article

Publication date: January 1, 2019

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  • Acta Acustica united with Acustica, published together with the European Acoustics Association (EAA), is an international, peer-reviewed journal on acoustics. It publishes original articles on all subjects in the field of acoustics, such as general linear acoustics, nonlinear acoustics, macrosonics, flow acoustics, atmospheric sound, underwater sound, ultrasonics, physical acoustics, structural acoustics, noise control, active control, environmental noise, building acoustics, room acoustics, acoustic materials, acoustic signal processing, computational and numerical acoustics, hearing, audiology and psychoacoustics, speech, musical acoustics, electroacoustics, auditory quality of systems. It reports on original scientific research in acoustics and on engineering applications. The journal considers scientific papers, technical and applied papers, book reviews, short communications, doctoral thesis abstracts, etc. In irregular intervals also special issues and review articles are published.
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