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Research Papers

Effects of Boundary-Layer Ingestion on the Aero-Acoustics of Transonic Fan Rotors

[+] Author and Article Information

Gas Turbine Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139

1Currently MHI Senior Research Fellow and Girton College Lecturer at the Whittle Laboratory, University of Cambridge.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Turbomachinery. Manuscript received July 15, 2012; final manuscript received November 2, 2012; published online June 26, 2013. Editor: David Wisler.

J. Turbomach 135(5), 051013 (Jun 26, 2013) (8 pages) Paper No: TURBO-12-1146; doi: 10.1115/1.4023461 History: Received July 15, 2012; Revised November 02, 2012

The use of boundary-layer-ingesting, embedded propulsion systems can result in inlet flow distortions where the interaction of the boundary-layer vorticity and the inlet lip causes horseshoe vortex formation and the ingestion of streamwise vortices into the inlet. A previously-developed body-force-based fan modeling approach was used to assess the change in fan rotor shock noise generation and propagation in a boundary-layer-ingesting, serpentine inlet. This approach is employed here in a parametric study to assess the effects of inlet geometry parameters (offset-to-diameter ratio and downstream-to-upstream area ratio) on flow distortion and rotor shock noise. Mechanisms related to the vortical inlet structures were found to govern changes in the rotor shock noise generation and propagation. The vortex whose circulation is in the opposite direction to the fan rotation (counter-swirling vortex) increases incidence angles on the fan blades near the tip, enhancing noise generation. The vortex with circulation in the direction of fan rotation (co-swirling vortex) creates a region of subsonic relative flow near the blade tip radius that decreases the sound power propagated to the far-field. The parametric study revealed that the overall sound power level at the fan leading edge is set by the ingested streamwise circulation, and that for inlet designs in which the streamwise vortices are displaced away from the duct wall, the sound power at the upstream inlet plane increased by as much as 9 dB. By comparing the far-field noise results obtained to those for a conventional inlet, it is deduced that the changes in rotor shock noise are predominantly due to the ingestion of streamwise vorticity.

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References

Figures

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Fig. 1

Serpentine inlets used in the parametric study

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Fig. 2

Schematic illustration of computational domain and key data plane locations

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Fig. 3

Generation and ingestion of streamwise vorticity due to boundary-layer ingestion

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Fig. 4

Localized flow accelerations in axial Mach number at the inlet plane. Magenta crosses indicate locations of vortex cores.

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Fig. 5

Axial Mach number at the fan leading edge, showing vortex lift-off for the duct with AR = 1.01 and OR = 0.75

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Fig. 6

Pressure coefficient on duct symmetry plane for AR = 1.01, OR = 0.75 case, schematically depicting the effects of vortex lift-off

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Fig. 7

Relative Mach number versus circumferential angle at outer radius (casing) as influenced by co- and counter-swirl. Top: effect of co- and counter-swirl on shock strength and location.

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Fig. 8

Relative Mach number at 92% span between fan leading edge and AIP. Top: AR = 1.05 and OR = 0.75 case without vortex lift-off. Bottom: AR = 1.01 and OR = 0.75 case with vortex lift-off.

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Fig. 9

Wave attenuation by co-swirling streamwise vortex in the outer span

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Fig. 10

Reduction in sound power decay rate due to counterswirling, lifted-off streamwise vortex

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Fig. 11

In-duct overall sound power level (up to and including the blade-passing frequency) evolution, showing the enhancement in sound power resulting from vortex lift-off (blue line)

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Fig. 12

Overall sound intensity fields (up to and including the blade-passing frequency) at AIP (left) and duct inlet plane (right). Propagation of sound power is increased due to the lifted-off counter-swirling streamwise vortex.

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Fig. 13

Sound power spectra at the AIP and inlet plane, showing the decay of the BPF (f /fshaft = 22) tone. Missing data points indicate that no upstream-propagating sound power is present at that frequency.

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Fig. 14

Far-field spectra at various emission angles Θem; BPF is equal to f /fshaft = 22

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