Fan Broadband Noise Generation and Suppression€¦ · Introduction to Fan Noise Generation Mechanisms Suppression Techniques Summary 4 . Noise data are presented here using a variety
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Fan Broadband Noise Generation and Suppression
Ed Envia June 24, 2015
https://ntrs.nasa.gov/search.jsp?R=20150022910 2020-05-01T23:00:20+00:00Z
Outline
Introduction to Fan Noise Generation Mechanisms Suppression Techniques Summary
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Outline Introduction to Fan Noise Generation Mechanisms Suppression Techniques Summary
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Much of the data presented here is from NASA wind tunnel tests and FAA databases. Engine and fan noise data are company proprietary and not publicly available.
Outline Introduction to Fan Noise
Generation Mechanisms
Suppression Techniques
Summary
4
Noise data are presented here using a variety of metrics including sound pressure level (SPL) spectra, sound power level (PWL) spectra, and Effective Perceived Noise Level (EPNL).
Motivation
5
Aircraft noise has an adverse effect on the environment and as a result it is regulated.
A Growing Problem
Source: FAA Report
6
Projected growth of passenger traffic in the U.S.
Community Noise Metric Approach Reference
Lateral Reference
450 m (1,476 ft)
2,000 m (6,562 ft)
6,500 m (21,325 ft)
Flyover Measured Noise Level
Lateral Measured Noise Level
Approach Measured Noise Level
Flyover (with Cutback) Reference
Flyover Certification Noise Level
Lateral Certification Noise Level
Approach Certification Noise Level
Flyover Noise
Margin
Lateral Noise
Margin
Approach Noise
Margin
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Aircraft Design
Aircraft MTOW
Noise Margin
Cumulative noise (CUM) margin is the sum of the individual margins.
(Airbus 380-842 CUM Margin = 16.4 EPNdB)
Engine Noise Sources
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Fan is one of the several engine noise sources.
Engine Source Levels
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Estimated Source Level Breakdown for a High Bypass Ratio Turbofan
Fan is a significant contributor to the overall engine noise emissions.
Fa
n In
let
Fa
n Ex
haus
t
Com
bust
or
Turb
ine
Jet
Directivity of Fan Noise
Compressor
Jet
Combustor (dashed line)
Turbine
Fan Inlet
Fan Exhaust
Lateral Condition
Fan Inlet
Turbine
Fan Exhaust
Jet
Combustor (dashed line)
Approach Condition
Characteristics of Fan Noise
Blade Passing Tones (Fan Tones)
Source: NASA Data 11
Fan noise has rich content and characteristics.
Spectral Content of Fan Noise
Fan Tones
Source: NASA Data 12
Fan broadband noise is the non-tonal component of the spectrum (i.e., part not coherent to the fan shaft rate).
Fan Broadband Noise Fan noise is principally produced as a result of
unsteady flow perturbations interacting with the fan blades and the outlet guide vanes.
Fan broadband noise is generated by the
interaction of flow turbulence with the blades and vanes.
Important sources of fan broadband noise
include …
13
Rotor Sources
Inlet Boundary Layer (BL)
~V6
~V5
Blade BL
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Blade BL turbulence is scattered into sound at the trailing edge.
Inlet BL turbulence is scattered into sound by the rotor blade tips.
Inlet turbulence impinging on the blades is another noise source.
Rotor / Stator Interaction Source
Rotor Stator
~V6
Rotor Wake Turbulence
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The principal source of fan broadband noise is the interaction of rotor wake turbulence with the fan exit guide vanes.
Source Hierarchy
Extraneous External Noise
Stage Configuration
Rotor-Only Configuration Approach Tip Speed
Source: NASA Data 16
Generally rotor/stator interaction noise is more important than rotor self-noise though the latter should not be ignored.
Effect of Rotor Transmission
Inlet/exhaust power noise split is partly governed by the rotor acoustic transmission which is controlled by the rotor geometry and flow swirl downstream of the rotor …
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Effect of Rotor Transmission
Inlet/exhaust power noise split is partly governed by the rotor acoustic transmission which is controlled by the rotor geometry and flow swirl downstream of the rotor …
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Low Blade Count Fan Moderate Blade Count Fan
As the rotor blade count decreases, swirl becomes the primary barrier against the rotor acoustic transmission.
Effect of Tip Clearance
Source: NASA Data 19
Tip gap does not have a significant influence on noise.
Noise Mitigation
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Cycle Change
The advent of high bypass ratio engines has been a major factor in reducing both fan and jet noise.
Levels corrected for engine count and thrust level.
Source: FAA Data 21
Stage 2 Stage 3 Stage 4
Aver
age
Noi
se L
evel
R
el. t
o Tu
rboj
ets,
EPN
dB
Liners
Acoustic liner is a common noise reduction technology used in aircraft engines today.
NASA ADP Fan Rig
Inter-Stage Liner Exhaust Liner Inlet Liner
Single Degree of Freedom (SDOF)
Double Degree of Freedom (DDOF)
NASA Liner Testbed (ADP Fan) 22
Liner Impact
Overall, no significant difference was seen in the performance of SDOF, DDOF, and Bulk liners over a wide range of frequencies.
Source: NASA Data 23
Inlet Noise Only
Inlet + Exhaust Noise
Liner Impact (Cont’d)
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Substantial noise reduction can be achieved using liners over a wide range of frequencies and tip speed conditions.
Inlet Noise Only
Inlet + Exhaust Noise
Vane Count
Cut-Off OGV
Cut-On OGV NASA SDT Fan Rig
Cut-On OGV (Swept)
Reduce Broadband Noise
Reduce Tone Penalty 25
Vane Count Impact
Source: NASA Data 26
Vane count reduction can reduce R/S interaction broadband noise.
Operating the fan close to its highest efficiency point at each tip speed should reduce fan noise by improving flow incidence on the fan blades.
Variable Area Nozzle (VAN)
Nominal Nozzle
Open Area Nozzle*
* Open area exaggerated for illustration purposes. 27
VAN Impact
Source: NASA Data 28
Broadband noise level reductions were measured for all fan operating conditions over a wide range of frequencies.
Soft Vane
Soft Vane
Soft Vane
Vane Internal Cavities
Positive levels denote noise reduction.
Broadband Noise Reduction
Source: NASA Data 29
Over-The-Rotor (OTR) Treatment
FJ44 Engine
Standard Fan Case
Stainless Steel Foam Metal
OTR
OTR-Treated Fan Case
Frequency Range 0.5BPF – 1.5BPF
Frequency Range 1.5BPF – 2.5BPF
Source: NASA Data 30
Broadband Noise Reduction
Concluding Remarks
Fan is an important source of aircraft engine noise whose importance is likely to grow with increasing engine bypass ratio.
A better understanding of its source mechanisms
and scaling laws should provide deeper insight for devising methods for mitigating it.
Noise reduction benefits drawn from cycle
change will likely reach a plateau requiring more reliance on noise reduction technology.
More innovative noise reduction techniques may have to be brought to bear to make substantial breakthroughs in reducing fan broadband noise.
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Questions?
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