Vortex Shedding Noise Reduction of a Mixed Flow Fan: Experimental and Numerical Investigation

C2 - Numerical Methods

The support for this paper is a mixed flow fan unit designed for use in the Dyson Air MultiplierTM desk fan. Early versions of the unit had a hissing noise perceptible to the user. As sound quality is a key feature of a desk fan, it was important to reduce this noise, and if possible, eliminate it all together. This paper explores the mechanisms and geometrical parameters that affect the frequency and level of the hissing noise. A combination of computational fluid dynamics and experimental in-duct measurements are used to confirm the noise production as vortex shedding off the trailing edge of the impeller. Using compromised fidelity CFD and acoustic measurements of various prototypes, a design was proposed to reduce the coherence of the vortical structures to eliminate the hissing noise. Although it was shown that adjusting simple geometric parameters, such as the trailing edge thickness, could change the frequency content from the vortex shedding, this alone was not enough to completely remove the hissing noise. The different iterations of design attempting to understand and improve the vortex shedding hissing noise have been prototyped. They have been tested across a full operational map, varying flow rates and running speeds, collecting performance and acoustic data which are reported in this paper. The final design provided an elegant solution which kept the original thickness distribution. Throughout this investigation it was imperative that the aerodynamic performance of the compressor unit was not decreased in any way. Furthermore, psychoacoustic metrics were used to evaluate the sound quality of the different designs explored. Resulting performance, acoustics and sound quality were assessed for the mixed flow fan in isolation. The methodology described in this paper can be used as a basis for identifying trailing edge vortex shedding noise and ways of decreasing its acoustic impact.