Triple Decomposition of Unsteady Flow in a Low-Speed Mixed-Flow Fan

Abstract

Numerous studies (including theoretical modeling, numerical simulation, and experimental measurement) have confirmed that wake interaction is a primary factor affecting tonal and broadband noise emitted by aeroengine fans. The knowledge and techniques derived from these studies have often been applied to analyze and control the aerodynamic noise of low-speed axial/mixed-flow fans used in ventilation and cooling applications. Despite the structural similarity between aeroengine fans and axial-flow fans for ventilation and cooling, the differences in their rotor–stator spacing and operating parameters raise the question of whether or not they share completely identical or approximately similar unsteady flow and noise generation mechanisms. In this paper, we employ a large-eddy simulation to investigate the low-speed unsteady flow inside a mixed-flow fan. The triple decomposition method is used to analyze the spatiotemporal characteristics of the time-averaged, ensemble-averaged, and stochastic fluctuation velocities and pressures in both stationary and rotating coordinate systems. The results show that the unsteady flow and noise generation mechanisms inside the fan investigated in this paper are significantly different from those of aeroengine fans, mainly due to two aspects. First, the wake effect on the velocity field of the fan studied in this paper is almost negligible, and the potential interaction is the primary source of the nonuniform spatial distribution of the time-averaged velocity field and the periodic velocity disturbance. Second, the turbulent energy spectrum of the stochastic fluctuation velocity is greatly distinct from the classic turbulence spectra in the low-frequency bands.