Reduced-order analysis of the acoustic near field of a ducted axial fan

Abstract

Spatio-temporal and modal analyses of the tip-vortex system of a ducted axial fan are performed based on highly resolved large-eddy simulations by proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). The Reynolds number based on the tip speed of the blade and the diameter of the fan Do is Re=9.36×105, the Mach number is M=0.136, and the rotational speed Ω=3000 rpm. The tip clearances s/Do=0.001,0.005, and 0.01 are investigated for the design and off-design operating conditions. Only at the off-design condition, a blade-tip-vortex interaction is observed for s/Do=0.01 and s/Do=0.005 while further reduction of the tip clearance to s/Do=0.001 prevents the cyclic interaction of the neighboring blades tip vortex with the current blade and a fully transitional region develops on the suction side of the blade. The POD results capture the narrowband cyclic interaction of the blade wake at the off-design condition. The first two energetic mode pairs can be linked to the interaction of the blades with the incoming wakes of the preceding blades. Compared to the POD results, the DMD modes corroborate the findings of blade-wake interactions. Furthermore, they highlight the connection to boundary layer transition on the suction side of the blades. The spatio-temporal DMD analyses at the characteristic frequencies of the blade-wake interaction reveal the generation of pressure waves which evolve in the azimuthal direction like isolated bubbles and propagate upstream of the blade. It causes two peaks in the spectrum of the acoustic pressure in the near field acoustics in agreement with the experimental sound spectra for s/Do=0.01 and s/Do=0.005 at the off-design condition.