Aircraft noise reduction by technical innovations Ulf Michel CFD Software GmbH, Berlin formerly at DLR, Propulsion Technology, Berlin AIAA/CEAS Aeroacoustics Conference 2013 Berlin 27-29 May 2013
Constraints for aircraft noise design Noise is only one of the many parameters that have to be considered in the design of aircraft. The aircraft and engine design is the result of a trade-off between Direct operating costs, e.g. Fuel burn Maintenance costs Indirect operating costs, e.g. Purchase price Noise Emissions Range Safety etc 2 2
Risks of aircraft design Development costs of new aircraft and engines are huge. The manufacturers must avoid risks. Risks have to be weighted against their possible financial impact. History shows that a manufacturer can easily go bankrupt if the product does not meet expectations. As a consequence, aircraft design changes only slowly. 3 3
Quiet aircraft Quiet aircraft items discussed in this presentation Low engine noise Low airframe noise Aerodynamic design for low noise Future of quiet aircraft with propeller or counter-rotating open rotor propulsion New concepts for low aircraft noise 4 4
Noise reduction achieved in the past Lateral certification noise levels scaled to equal thrust For comparison of aircraft with different thrusts 23 db noise reduction in 50 years. Significant noise reductions with A380, 787-8, and 747-8.. 7 db noise reduction 747-8 against 747-400 thrustnormalized lateral noise level 23 db The Vision for European Aeronautics 2020 calls for reduction in perceived noise to one half of 2000 levels. 5 5
Main parameters for take-off noise Thrust-specific noise (shown on previous chart) Thrust requirement of aircraft, highly dependent on wing loading Higher wing loading requires more thrust for higher take-off speeds larger accelerations on runway Increase of wing area would reduce required thrust and reduce noise, but would increase weight of wing. A380 747-8 increase A320 A321 Increase Wing loading 6604 Pa 8364 Pa 27% 6039 Pa 7479 Pa 24% Thrust-toweight 0.235 0.268 14% 5 db 0.277 0.304 10% 3.5 db 6 6
Design parameters to be changed to reach Vision 2020 noise goals 7 7
Fan diameter normalized for thrust 100 kn for better comparison between different engines Increase of normalized fan diameter Noise reduction was accompanied with increase of normalized fan diameter Fan diameter increased by roughly 10% between 1990 (1.45 m) and 2010 (Trent 900, GP7270, Trent 1000, GEnx 1B with 1.6 m) Step increase by a further 10% through geared turbofan imminent 8 Fan diameter for 100 kn (m) 2.2 2.0 1.8 1.6 1.4 1.2 geared turbofan A380 787 747-8 A350 777-X 1.0 1960 1970 1980 1990 2000 2010 2020 Year of engine certification
Relation between fan diameter and noise Thrust-normalized EPNL lateral (db) 94 92 90 88 86 84 82 80 78 76 74 72 Vision 2020 70 1.2 1.4 1.6 1.8 2.0 2.2 Normalized fan diameter (m) trend line not valid for engines with high buzzsaw noise Trend line: L p -L p,ref = -120 lg(d/d ref ) 10% increase of fan diameter yields noise reduction of 5 EPNdB Goal of 10 EPNdB reduction requires increase of fan diameter by 21% relative to year 2000 aircraft from 1.58 m to 1.9 m 9 9
Relation between fan diameter and jet speed Larger normalized fan diameter means larger mass flow through the engine Smaller jet speed required for same thrust lower jet noise smaller fan-pressure ratios less fan noise 10 10
Small jet speeds Will smaller jet speeds really reduce noise? Can be investigated with the noise emission in the flyover certification point Engines are run with cutback thrust (about 60 70% of full thrust) Jet speeds as small as for future engines with much larger fan diameters. 11 11
Influence of jet speed on certification noise levels 92 ERJ-190 Normalized EPNL flyover (db) 90 88 86 84 82 80 78 engines with high buzz-saw noise possible combustion noise influence when used at full power 737-700 737-900ER A320(CFM) A321 (CFM) 787-8(RR) 787-8(GE) 777-200(GE) 777-3000ER correlation corr +2dB correl -2 db 76 150 175 200 225 250 Effective cutback jet speed (m/s) Flyover certification measurements with cut-back thrust demonstrate that noise levels continue to decrease when jet speed is reduced. Combustion noise may spoil this behaviour on take-off. 12 12
Effective jet speed Effective jet speed is defined in the figure on the previous chart with jet speed V j and flight speed V f by (derivation in AIAA-2013-2014) a rel =0.62 13 13
Fan diameter to achieve Vision 2020 goals 10 % diameter increase required for A380: from 2.95 m to 3.24 m (cabin width 6.54 m) 787: from 2.85 m to 3.13 m (cabin width 5.77 m) Diameter increase for aircraft currently in development: A320neo (PW1100G): from 1.74 m to 2.11 m 21% 10 EPNdB A320neo (Leap-1A): from 1.74 m to 2.00 m 15% 7 EPNdB 737max (Leap 1B): from 1.55 m to 1.75 m 12% 6 EPNdB Both aircraft will miss Vision 2020 goals by roughly 5 EPNdB 14 14
Necessary diameter increase for A350 to achieve Vision 2020 goal Required fan diameter 3.45 m Current fan diameter 3 m Current fan diameter Fan diameter required (only 20 cm more than GE90) 3.00 m 3.45 m Current normalized fan diameters (depend on thrust) 1.48 m 1.65 m (for comparison 787-8: 1.62 m 1.75 m) 15% fan diameter increase required for A350 (7 db) 1.7 m 1.9 m to achieve Vision 2020 goal. 15 15
Installation effects Jet installation below a wing has a negative effect on noise emission This problem will likely get even bigger when the jet diameter is increased. Currently investigated in the European research project JERONIMO 16 16
How can increase of fan diameter be achieved? 17 17
Consequences of diameter increase Benefits Propulsive efficiency increases Noise decreases Drawbacks Weight of engine increases Weight of nacelle increases (D 3 ) Nacelle friction drag increases (<D 2 ) Wave drag in cruise increases Engine-wing interference drag increases Jet-Flap interaction noise may increase 18 18
Possible solutions Geared turbofan lowest core weight Contra-rotating geared turbofan smallest fan diameter for given mass flow, but heavier Three spool direct-drive turbofan Stage count increase on low-pressure turbine (LPT) Two spool direct-drive turbofan Stage count increase on LPT low-pressure turbine and compressor LPC 19 19
Possible solutions Above a yet unknown thrust-normalized diameter (strongly related to bypass ratio) the geared version will be the only choice. Nacelle length increase may have to be limited. With consequences to available liner surface. Lightweight materials for nacelle required Laminar flow on nacelle to reduce its drag 20 20
Variable area fan nozzle (VAN) A variable area fan nozzle will be a must when normalized fan diameter is increased substantially over current values. Benefits: Fan working point can be moved away from surge line. Fan works at best efficiency in any flight regime. Smaller core possible due to better efficiency at top-of-climb. Weight of fan blades can be reduced (certification issue, fail safe mode for VAN required). 21 21
Benefits of variable area fan nozzle Big fuel burn advantage on ground and initial climb due to better fan efficiency and better propulsive efficiency Top-of-climb with best fan efficiency Cruise always with best fan efficiency Higher propulsive efficiency at lower than optimal altitude Fuel burn advantage on long-range aircraft at least 2% (up to 1.5 tonnes on 787), larger for medium range aircraft, more than compensates weight penalty of nozzle Reduced jet speed on take-off yields 2 db noise benefit Reduced fan broadband noise by at least 2 db Substantial reduction of flow separations in highly loaded fan. Reduced cabin noise in cruise. 22 22
Disadvantages of variable area fan nozzle VAN has also draw-backs Weight Maintenance costs Possible reduction of dispatch reliability Reduced surface areas for acoustic duct lining Possibly higher fan tone noise, because rotor wakes are more uniform between different blades Careful noise design of VAN required Avoid noise increase on take-off due to a ring slit Capability to adjust nozzle area in the percent range in cruise required 23 23
Detailed discussion of turbofan noise sources 24 24
Noise sources of a turbofan Jet noise is external noise source. Reduction primarily through reduction of jet speed. achieved by increasing thrust-normalized fan diameter. Noise sources inside engine have to be reduced similarly. Fan noise (tones, broadband) Core noise Combustion noise (low frequency, broadband, sometimes tones) Turbine noise (high-frequency tones, high-frequency broadband) Compressor noise (high-frequency tones, broadband) Bleed valve noise Goal of reducing internal noise in parallel to jet noise was achieved in the past by technical innovations. 25 25
Technical innovations for fan noise in the past 1. Relocation of fan guide vanes downstream of rotor. 2. Forced mixers for long cowl nozzles. 3. Acoustic treatment of flow ducts 4. Increase of stator vane count (cut-off design). 5. Increase of distance between rotor and stator. 6. Reduction of tip Mach number of fan blades 7. Swept fan leading edges 8. Serrated nozzles (Chevrons) 9. Splice-less liners 26 26
Example 1: Circumferential tip Mach number Fan tip Mach numbers reduced substantially in recent years past M=1.45 Airbus A340-500/600 (Trent 500) recent M=1.28 Airbus A380 (Trent 900, GP7200) present M=1.15 Boeing 787 (Trent 1000) future M=1.05? Bombardier CSeries (PW1500G) Rotor-alone noise emission reduced or even eliminated Smaller M requires larger swirl in flow between rotor and stator. Swirl reduces rotor-stator interaction tones. Cut-off design requirement may no longer be required, noise reduction potential for broadband noise. 27 27
Example 2: Passive acoustic liners Acoustic liners very important. Reduce sound emission of internal sound sources by up to 18 db. 6 db broadband Progress: Surface of perforated plates replaced by wire meshes: sound absorbing performance less dependent on flow speed above liner. Two layers of liners in some areas: better performance over larger frequency range. Liners in inlet manufactured in one piece without splices. Eliminates mode scattering. Reduces radiation of rotor-alone tones. Sources: Google; Rienstra; Pratt & Wittney; Hennecke 28 28
Acoustic liners for core flow Combustion noise is difficult to reduce. Acoustic liners will be needed to attenuate combustion noise in future aircraft engines since all other noise sources are reduced Turbine noise hot-stream liner often used (single layer) Applied in core jet exit (outer and inner wall) and for exhaust cone (long cowl engines) Hot-stream liner for core noise in development 29 29
Innovations for reduction of airframe noise Airframe noise is dominant on far approach, when engines are flight idle. Airframe noise and engine noise both influence final approach noise, when engines have to maintain glide slope. (Proof : certification noise level depends on engine option for A320, A321, A330, A380, 787-8) 30 30
Elimination of cavity tones Cavity tones are a frequent noise problem and can be the loudest sound sources during the approach of some aircraft. Cavity tones can always be suppressed, if their origin is known Cavity tones are hard to predict during design Measurements necessary. Origin of cavity tones should be localized before noise certification. Measuring technique: phased microphone array Source: DLR 31 31
Cavity tones Flow direction Quiet air intake (NACA inlet) Very loud tone, fuel tank pressure relief valve Source: DLR De-ice air outlets on nacelle generate tone Change of outlet shapes on next engine model eliminated tone Source: DLR 32 32
Elimination of cavity tones Bombardier Regional Jet 100/200 Fuel drain valve in wing, loudest noise source on approach (Michel et al. 1998) Airbus A319/A320 2 fuel overpressure valves on each wing loudest sound source on approach before high-lift devices are extended (found by DLR 2001), solution with vortex generators proposed by DLR) Airbus A319/320 with CFM56-5A engine exhaust holes for de-icing air in engine nacelle (problem solved on CFM56-5B) Boeing 747-400 front landing gear with two loud tones (problem still exists, see AIAA-2013-2115, Dedoussi et al) Boeing 777-300ER Holes for de-icing air in slat (problem solved) Boeing 787-8 Cavity in main landing gear (problem solved) 33 33
Technical innovations for the reduction of airframe noise 34 34
Source of slat noise Interaction of turbulence from the free shear layer in the slat cove with the slat trailing edge. (Verified through space-time cross-correlations within simulated flow field by Knacke AIAA-2013-2162) 35
Slat noise scaling Slat noise is generally studied experimentally by changing tunnel speed and keeping the angle of attack constant. Conclusion: slat noise scales at U f 5 This is not the situation in flight, where the lift remains constant, when the airspeed changes. The lift coefficient is proportional to V f -2. Slat-noise Mach exponent must be much smaller. This may also be concluded from noise certification data. 36 36
Reduction of slat noise Replacement of slats by drooped leading edges (slats without gaps) on part of wing (reduces lift, but increases lift-to-drag ratio) (A380, A350?) Increasing slat chord Increasing wing area (reducing wing load), decreases required lift coefficient, further benefits: shorter field length, smaller engine. Reduction of slat gap size Serrations on slat trailing edge Slat cove filler 37 37
Reduction of landing gear noise Landing gear dominant, when used as air brake at high airspeeds with clean wing, while engines are at flight idle. Noise increase of 12 db measured on A319 by DLR at 120 m/s Landing gear noise reduces with local airspeed U 6. L_p landing gear (arbitrary db) 110 105 100 95 90 85 80 20 db reduction typical approach speed 60 70 80 90 100 110 120 Air speed (m/s) L_p MLG L_p FLG Main landing gear (MLG) has larger exponent than front landing gear (FLG) due to influence of wing circulation on local U. 38 38
Relation to slat noise While landing gear noise reduces with airspeed, slat noise remains rather flat. Slat noise becomes relatively more important on final approach (noise certification conditions) Low noise landing gear design important for flight at higher airspeeds with engines flight idle. Componentn L_p 110 105 100 95 90 85 L_p MLG L_p FLG Lp_slat Landing-gear noise has relatively small contribution on final approach: AIAA-2013-2014, Michel AIAA-2013-2115, Dedoussi et al 80 60 70 80 90 100 110 120 Air speed (m/s) 39 39
Flight procedures 40 40
Increasing glide angle on approach Approach noise can be reduced substantially by increasing the glide angle Airspeed unchanged Airframe noise unchanged Engine power decreased Noise reduction through decreased engine power increased flyover altitude Safety must be preserved May require substantial changes on aircraft and engines. 41 41
Propellers and counter-rotating open rotors 42 42
Propeller aircraft Propeller aircraft have a principal noise problem: Noise increases with the square of aircraft mass if tip speed and wing loading are held constant. Noise of turbofan increases linearly with aircraft mass, if jet speed and wing loading are held constant. 90 Quelle: Airbus 80 turbofan Propeller Arbitrary SPL 70 60 50 8 32 128 Maximum take-off mass (t) 43
Propeller aircraft Propeller aircraft save fuel, but are loud for larger aircraft Airbus A400M was just able to meet Chapter 4 noise limits. Noise reduction for propellers possible Quelle: Airbus increasing blade count decreasing tip Mach number Decreasing tip Mach number requires increase of propeller diameter with consequences for aircraft weight and size of vertical stabilizer. Decreasing tip Mach number increases swirl and thus reduces efficiency. Fuel burn benefit erodes, when tip Mach number decreased. Vertical stabilizer size can only be reduced if the two engines are coupled with a shaft like in tilt-rotor aircraft. Weight and reliability problem. 44
Quelle: Airbus Source: ISVR Source: CFMI Counter-rotating open rotors Higher flight Mach numbers require counter-rotating propellers. Second rotor removes swirl of first rotor. Like a propeller the noise increases with the square of engine power for a given tip speed. Noise much higher than of turbofan. However, no weight and drag penalty of a nacelle. Tip speed can be reduced without significant efficiency penalty But size and weight will increase when tip speed is further reduced. It may be doubted that remaining fuel burn benefit is large enough.
New concepts for far future 46 46
New aircraft concepts Source: European Commission EU project NACRE Noise reduction by shielding of noise radiation from engine inlets Shielding reduces cumulative EPNL by 4 EPNdB But: Rear engines increase aircraft weight Servicing of engines more difficult. Source: Silent Aircraft Initiative, Cambridge-MIT Institute Quelle: ISVR Center of gravity of aircraft more difficult to control. Noise alone will not suffice to warrant this concept. 47
New aircraft concepts Hybrid wing body aircraft Source: Airbus Shielding of fan forward noise High angle-of-attack during approach will likely create much slat noise (AIAA-2013-0462, Guo et al) Complete change of airport infrastructure necessary There must be compelling reasons other than noise. Source NASA 48
New aircraft concepts Source: Airbus Buried engines: Disturbed inflow might increase fan noise by up to 20 db Exceeds far the benefit of shielding 49
Conclusions Substantial noise reduction is only possible with larger fan diameters But quieter aircraft can only be realized under the current boundary conditions, if the costs are acceptable to the market. Larger wings is an additional way to reduce noise during take-off and approach. Research and Development is necessary for reducing length and weight of large nacelles decreasing fan noise by the same fraction as achievable for jet noise. developing liners for the hot stream nozzle Airframe noise must be reduced since being an important contribution on approach. Steeper approaches should be developed. Propeller and counter-rotating open rotor propulsion is limited to relatively small aircraft due to the fast rise of their noise emission with aircraft weight. 50 50