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The inlet vortex as a source of tonal rotor noise

University of British Columbia

The thesis describes experimental and mathematical analyses of the noise resulting from the interaction of an axial flow fan with a concentrated inlet vortex. A comparison of results from the two methods reveals the physical phenomena on which the discrete tone noise depends. Vortices were generated with half delta wings mounted in the inlet bell mouth of an axial flow fan. Their properties of vortex strength, core radius, and core axial velocity, deficit were measured in a wind tunnel. The far field spectrum of the fan noise was measured for various combinations of these three parameters, and for different radial positions of vortex entry. An investigation of the origin and effect on the sound levels of naturally occurring inlet vortices was carried out using real time blade pressure measurements and a synchronous display system. The mathematical analysis comprised a linear two dimensional aerodynamic model of a vortex passing through a fan. The characteristics of the blade loading were calculated as a function of the span position and circumferential angle. Levels of overall noise and of the individual discrete tones were calculated. The theoretical dependence of the noise' spectrum on the vortex strength, core radius, and core axial velocity deficit were determined. The variation of sound pressure level with the radial position of vortex entry was also calculated. Comparison of the experimental and theoretical results shows that an artificially generated concentrated inlet vortex increases the overall sound level by as much as eight decibels and some harmonics to a maximum of fourteen decibels, at a typical set of operating conditions. The vortex circulation has the greatest effect on the tone level at the blade pass frequency while the axial velocity deficit exerts more influence on the higher harmonics of the spectrum. Local blade stall caused by large values of core axial velocity deficit is a suspected contributor to the observed pure tone noise at the higher harmonics. A naturally occurring inlet vortex was tracked using the synchronous display system. It was observed to be unsteady in its structure and to move about a preferred position of entry into the fan. Its presence increased the overall far field fan noise by a minimum of three decibels and at the blade pass frequency by 4.5 decibel. No change in sound level occurred at the four significant harmonics of the blade pass frequency.

Text

2010-02-15

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http://hdl.handle.net/2429/20213

Mechanical Engineering

University of British Columbia

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