Aircraft Noise: The major sources, modellÈng capabilities, and reduction possibilities
Bertsch, Lothar; Simons, Dick; Snellen, Mirjam
Publication date 2015
Document Version Final published version
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Bertsch, L., Simons, D., & Snellen, M. (2015). Aircraft Noise: The major sources, modellÈng capabilities, and reduction possibilities. (IB 224-2015 A 110 ed.) DLR.
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Deutsches Zentrum
fur Luft= ynd Raumfahrt e.V.
in der Helmholtz-Gemeinschaft
IB 2 2 4 - 2 0 1 5 A 110
Lothar Bertsch^'^ Dick G. Simons^, and M i r j a m Snellen^
Aircraft Noise: The major s o u r c e s , modellÈng capabilities, and
reduction possibilities
Institut für
Aerodynamik urid Strömungstechnik
verfasser: Lothaf Beftsch^'^ Dick G. Simons^, and M i r j a m Snellen^
Titel: Aircraft Noise: The major s o u r c e s ,
modelling capabilities, and reduction pom Datum: M a r z 2 0 1 5 Auftraggeber: Auftrags-Nr.; Angebot Nr.: Der Bericht u m f a B t : 2 9 Seiten einschl. 7 Tabellen 7 Bildern 23 Literaturstellen
Vervielfaltigung u n d Weitergabe dieser Unterlagen sowie Mitteilung ihres Inhalts an Dritte, auch auszugsweise, nur mit Genehmigung des Auftraggebers
D e u t s c h e s Z e n t r u m
Für L u f t - u n d R a u m f a h r t e. V.
1) DLR, Institut für A e r o d y n a m i k und S t r o m u n g s t e c h n i k , B u n s e n s t r a G e 10, D - 37073 G ö t t i n g e n
2) Section Aircraft Noise a n d Climate E f f e c t s ,
Delft University of T e c h n o l o g y , 2 6 2 9 HS, K l u y v e r w e g 1, Delft, T h e N e t h e r l a n d s
Aircraft Noise: The major sources, modelling capabilities, and
reduction possibilities
Übersicht: . , ,
In October 2 0 1 4 , t h e first "Joint DLR & TU D e l f t A v i a t i o n Noise W o r k s h o p was organized This p u b l i c a t i o n is t h e executive s u m m a r y of this event. Overall, 3 8 invited participants f r o m industry, academia, a n d research institutions have discussed t h e specific t o p i c of this first 3 day w o r k s h o p , i.e " A i r c r a f t Noise Reduction at t h e S o u r c e " .
Four specific tasks w e r e f o r m u l a t e d in o r d e r t o address t h e p r o b l e m , i.e. (1) identification o f main aircraft noise sources o n - b o a r d o f a given reference vehicle, (2) assessment of simulation capabilities f o r noise p r e d i c t i o n , (3) i d e n t i f i c a t i o n a n d assessment of p r o m i s i n g noise r e d u c t i o n concepts f o r t h e reference vehicle, a n d (4) i n t e g r a t i o n o f these measures o n - b o a r d o f t h e reference vehicle. The m a j o r noise sources o n - b o a r d of t h e reference vehicle as identified by t h e participants c o u l d have been r e d u c e d significantly if selected measures are installed o n - b o a r d . These p r o p o s e d measures promise t o reduce t h e system noise by 8 dB a l o n g a t a k e - o f f a n d by 10 dB a l o n g an a p p r o a c h f l i g h t . Yet, t h e almost 6 5 % r e d u c t i o n in perceived noise as specified by ACARE's Flight Path 2 0 5 0 c o u l d n o t be achieved The most e f f e c t i v e measure has b e e n i d e n t i f i e d as structural shielding of engine noise emission.
Deutsches Z e n t r u m f ü r L u f t - u n d R a u m f a h r t e.V
Institut fill- A e r o d y n a m i k u n d Strömungstechnik G ö t t i n g e n
Raffel) (Assoc. Prof. Dr. M. Snellen)
Datum: 17.03.15 Abteilung: Bericht:
I n October 2014, the f i r s t "Joint D L R & T U D e l f t A v i a t i o n Noise Workshop" was or-ganized. This p u b l i c a t i o n is the executive s u m m a r y of this event. O v e r a l l , 38 i n v i t e d participants f r o m industry, academia, and research institutions have discussed the spe-cific topic of this f i r s t 3 day w o r k s h o p , i.e. "Aircraft Noise Reduction at the Source". The concept of the w o r k s h o p was to a v o i d the usual presentation m a r a t h o n b u t enable detailed discussions. The i n v i t e d participants w i t h their various educational, c u l t u r a l , a n d w o r k i n g backgrounds have been assigned into w o r k groups to w o r k on specific a n d p r e d e f i n e d tasks. Four specific tasks were f o r m u l a t e d i n order to address the p r o b -l e m , i.e. (1) i d e n t i f i c a t i o n of m a i n aircraft noise sources on-board of a g i v e n reference vehicle, (2) assessment o f s i m u l a t i o n capabilities f o r noise p r e d i c t i o n , (3) i d e n t i f i c a t i o n and assessment of p r o m i s i n g noise reduction concepts f o r the reference vehicle, a n d (4) integration of these measures on-board of the reference vehicle.
The m a j o r noise sources onboard of the reference vehicle as i d e n t i f i e d b y the p a r t i c i -pants c o u l d have been reduced significantly i f selected measures are installed on-board. These proposed measures promise to reduce the system noise b y 8 dB along a take-off and b y 10 dB along an approach f l i g h t . Yet, the almost 65% r e d u c t i o n i n perceived noise as specified b y A C A R E ' s Flight Path 2050 c o u l d n o t be achieved. The most effective measure has been i d e n t i f i e d as structural shielding of engine noise emission.
O v e r a l l , the w o r k s h o p can be understood as the f i r s t attempt to estabhsh a n e w a n d active n e t w o r k f o r international cooperation i n the f i e l d o f aircraft noise.
C o n t e n t s
1 I n t r o d u c t i o n ^ 2 I d e n t i f i c a t i o n o f m a i n aircraft noise sources (Task 1) 8
2.1 Detailed description of task 8
2.2 S u m m a r y of results 8 3 Assessment o f s i m u l a t i o n capabilities (Task 2) 13
3.1 Detailed description of task 13 3.1.1 S u m m a r y of results 13 4 I d e n t i f i c a t i o n a n d assessment o f p r o m i s i n g noise r e d u c t i o n concepts (Task 3) 17
4.1 Detailed description of task 17
4.2 S u m m a r y of results 17 5 I n t e g r a t i o n o f r e d u c t i o n concepts i n t o n e w l o w - n o i s e v e h i c l e (Task 4) 19
6 S u m m a r y & C o n c l u s i o n 22
N omened latare
A N o P P Overall aircraft noise s i m u l a t i o n tool, N A S A A N O T E C A N O T E C consulting, aircraft noise technology A S R I A i r c r a f t Strength Research Institute, China
A R I Aerodynamics Research Institute, China
A z B Overall aircraft noise s i m u l a t i o n t o o l , D L R C A A C o m p u t a t i o n a l Aeroacoustics
C F D C o m p u t a t i o n a l F l u i d Dynamics D N S Direct N u m e r i c a l Simulation
DES Deutsche Flugsicherung G m b H , G e r m a n air navigation service p r o v i d e r
D L R German Aerospace Center
E P N L Effective Perceived Noise L e v e l [EPNdB]
EMPA Swiss Federal Laboratories f o r Materials Science and Technology
F L U L A O v e r a l l aircraft noise s i m u l a t i o n tool, E M P A H E I D I Engine noise s i m u l a t i o n t o o l , D L R
lESTA O v e r a l l aircraft noise s i m u l a t i o n t o o l , O N E R A I N M Integrated Noise M o d u l e , s i m u l a t i o n t o o l , F A A J A X A Japan Aerospace Exploration A g e n c y
LES Large E d d y S i m u l a t i o n
N A S A N a t i o n a l Aeronautics a n d Space A d m i n i s t r a t i o n , U S A N L R N a t i o n a l Aerospace Laboratory, Netherlands
M T U M T U A e r o Engines, company, G e r m a n y O N E R A French Aerospace Research A g e n c y O A S P L (Overall) s o u i i d pressure level, [dB] UPACS-LES C F D / C A A code, J A X A
U - R A N S Unsteady Reynolds Averaged N a v i e r Stokes P A N A M O v e r a l l aircraft noise s i m u l a t i o n t o o l , D L R P I A N O C o m p u t a t i o n a l Aeroacoustics t o o l , D L R Profan A i r f r a m e noise s i m u l a t i o n t o o l , D L R Propnoise Propeller noise p r e d i c t i o n t o o l , D L R R W T H R W T H Aachen U n i v e r s i t y
s o n A I R O v e r a l l aircraft noise s i m u l a t i o n tool, E M P A S O P R A N O O v e r a l l aircraft noise s i m u l a t i o n t o o l , A N O T E C
STAPES A i r p o r t noise exposure s i m u l a t i o n tool, E U R O C O N T R O L SPL (non or A - w e i g h t e d ) Sotmd Pressure Level, [dB] or [ d B A ]
1 Imitjrodiiictioiiii
I n October 2014 the w o r k s h o p " A i r c r a f t Noise Reduction at the Source" was h e l d i n M e i s d o r f , Germany. The event was organized j o i n t l y b y D L R and the D e l f t U n i v e r s i t y of Technology.
The m o t i v a t i o n f o r this w o r k -shop was to investigate the potentials i n low-noise air-craft design b y b r i n g i n g together experts f r o m v a r i -ous fields i n aircraft noise. Selected participants have been i n v i t e d f r o m industry, academia, a n d research i n -stitutions a r o u n d the w o r l d .
T U Braunschweig & DLR
CAL POLY / NASA A crude d i s t i n c t i o n can be
m a d e between engine noise and airframe noise, w i t h m a n y subthemes w i t h i n these t w o (fan noise, jet noise, l a n d i n g gear, flaps, slats). A l s o , a d i s t i n c t i o n between model-based a n d experimen-t a l l y focused research can be made. Further, i n d u s t r y and
research institutes have their o w n , sometimes distinct, interests.
and many more ideas/concepts out there ..
Figure 1.1: Exemplary low-noise aircraft concepts (please note picture copyrights).
Both existing a n d n e w aircraft concepts were discussed, see Fig. 1.1 f o r some exam-ples, a l t h o u g h i n the w o r k s h o p o n l y t u b e - a n d - w i n g configurations were considered. Today n e w aircraft concepts are designed w i t h noise assessment incorporated i n the design process, i n c l u d i n g installation effects. However, even such a state-of-the-art ap-proach w i l l n o t guarantee that the o p t i m u m or best design is i d e n t i f i e d . I n general, con-cepts and n e w ideas are d r i v e n b y i n d i v i d u a l experts or dedicated groups w i t h l i m i t e d
experience i n other fields than their m a i n expertise. This can result i n o n l y component wise o p t i m i z a t i o n and o n l y little or no i m p r o v e m e n t at a system level w i l l be achieved. I n a d d i t i o n , the various s i m u l a t i o n tools that are a p p l i e d have different f i d e l i t y , l i m i t a -tions, a n d accuracy.
Therefore, the relevant questions a n d problems f o r the w o r k s h o p participants were i d e n t i f i e d as the f o l l o w i n g .
o A r e i n d i v i d u a l technologies still "low-noise" i f installed on-board of the aircraft? (e.g. are leading edge devices as tested i n a w i n d - t u n n e l really low-noise on-board?)
o H o w g o o d are our predictions?
(e.g. is neglecting mean flow f o r s h i e l d i n g problems allowed?)
• H a v e w e considered a l l relevant noise sources a n d m a j o r interactions? (e.g. is flap side edge noise important?)
• W h a t about the influence of "realistic" flight operation?
(e.g. w h a t is the effect of engine thrust correction a n d / o r speed increase?) • W h a t about counteracting effects?
(e.g. w h a t is the effect of a d d i t i o n a l d r a g a n d w e i g h t of a n e w low-noise h i g h - l i f t system?)
o W h a t about the overaU vehicle noise at a system level? (e.g. is flying at higher altitudes always better?)
I n order to be able to answer these questions, a b r o a d ("holistic") assessment m e t h o d o l -o g y a n d active exchange w i t h vari-ous experts bec-ome essential. I n v -o l v e m e n t -of experts f r o m d i f f e r e n t disciplines w i t h various b a c k g r o i m d s (e.g. academia vs. i n d u s t r y , c u l -t u r a l a n d educa-tional differences) is m a n d a -t o r y
I n order to answer the above m e n t i o n e d questions, the w o r k s h o p attendants were as-signed to w o r k o n the f o u r tasks as l i s t e d i n Tab. 1.1.
Task Description
1 Identification of mam aircraft noise sources on-board of reference vehicle 2 Assessment of simulation capabilities
3 Identification and assessment of promising noise reduction concepts 4 Integration into a new low-noise vehicle concept
1. Introduction 6
The f o l l o w i n g scenario a n d limitations were predefined: the reference aircraft is an ex-isting vehicle, i.e. a conventional, single-aisle, tube-and-wing, medium-range transport aircraft as depicted i n Fig. 1.2 (predicted m a r k e t share of 70% b y 2030, see Refs. [2,3]). Also, the developed n e w low-noise technology s h o u l d be available m 2030 at Technical Readiness Level of 5-6. The overall goal f o r this 2030 scenario is a reduction i n perceived noise level ( w i t h respect to the reference aircraft) of 65% per flight operation as proposed b y the A d v i s o r y C o u n c i l f o r A v i a t i o n Research a n d hnnovation i n Europe (ACARE) i n their ' T l i g h t p a t h 2050"^. This corresponds to a p p r o x i m a t e l y 12 dB reduction i n overall s o u n d pressure level (OASPL) or a level 35 E P N d B cumulative b e l o w Chapter 4^. I n f h e subsequent chapters of this paper, the f o u r tasks are described i n more detail, i n c l u d i n g the m a j o r results of the w o r k s h o p per task.
The w o r k s h o p was n o t a t r a d i t i o n a l conference, i.e. f u l l y f i l l e d w i t h presentations. Ba-sically, such a presentation marathon was a v o i d e d b y dedicating most of the t i m e to ac-tive p a r t i c i p a t i o n i n groups w o r k i n g o n the f o u r tasks above. Five groups were f o r m e d , based o n b a c k g r o u n d a n d research interest (e.g. focus more o n airframe noise or engine noise) a n d m i x e d members f r o m academia, research institutions, a n d industry, where w e t r i e d to separate direct colleagues. The f i v e groups w o r k e d i n parallel o n the f o u r tasks. I n p l e n a r y sessions the results of f h e f i v e groups were discussed per task. I n the plenary sessions, i n d i v i d u a l ideas a n d concepts of each g r o u p were discussed w i t h the a i m to f i n d c o m m o n g r o t m d , a n d to i d e n t i f y the best ideas a n d most p r o m i s i n g con-cepts. To ensure m a x i m u m u n i f o r m i t y i n the outcomes of fhe i n d i v i d u a l groups, the participants were p r o v i d e d w i t h templates f o r d o c u m e n t i n g their discussion results. I n total, there were 38 participants o u t of 10 countries (China, France, Germany, Italy, Japan, Netherlands, Spain, Switzerland, U K a n d US). I n Tab. 1.2 the p a r t i c i p a t i n g i n s t i -tutions are listed.
Industry University Research institutions
ANOTEC Consulting Airbus
DPS MTU Rolls-Royce
Georgia Institute of Technology Peking University
Roma Tre University RWTH Southampton University of Tokyo TU Braunschweig TU Delft TU Muenchen TU Stuttgart ASRI ARI Bauhaus Luftfahrt EMPA DLR JAXA NASA NLR ONERA
Table 1.2: The workshop participants' institutions
^For more information, visit http://www.acare4europe.com/sria/flightpath-2050-goals ^According to ICAO Annex 16.
x[m]
2 Mentificatiomi of mam aircraft imoi^
mutces
(Task
X
2 J Detailed description
Task 1 comprises the iderrtificatiorr of the m a i n noise sources on-board existing aircraft, i.e. the reference vehicle as depicted i n Fig. 1.2, was used as an example case. Partici-pants were asked to i d e n t i f y f h e m a i n sources (airframe or engine noise) along t y p i c a l f l i g h t segments (approach / departure / cruise), t a k i n g hrto account whether sources are classical, parasitic, or due t o installation effects. A l s o the spechal (tonal or b r o a d b a n d contribution, l o w or h i g h frequency) and directional characteristics h a d to be i n d i -cated. For each source, the relevant parameters, b o t h operational ( f l i g h t condition) a n d geometrical, h a d to be specified. I f possible, the importance of each parameter h a d to be ranked.
2o2 Summary of results
The w o r k s h o p participants i d e n t i f i e d the f o l l o w i n g classical aircraft noise sources, see Tabs. 2.1 a n d 2.2. A l s o the noise generating mechanism ( i n c l u d m g the relevant parameters i n descending order of importance) a n d the departure a n d approach conditions u n -der w h i c h these noise sources are i m p o r t a n t are also indicated, see Fig. 2.1. Finally, the level of theoretical t m d e r s t a n d i n g was estimated. A distinction is made between noise sources due to the a i r f r a m e , see Fig. 2.2(a), and engine noise sources, see Fig. 2.2(b).
(a) Standard departure flights. 2 0 0 0 l l 5 0 0 0) 1 1 0 0 0 B 5 0 0 .0 150
I
" 100 altitude flap I slat ) gear r TAS • thrust H ^ — -s 50 1^ 5 lOOO -30000 distance [m](b) Standard approach flights.
Figure 2.1: Typical and representative operating conditions along departure and approach flights; flight data was recorded during a 2006 fly-over noise campaign by DLR [4].
2, Identification of main aircraft noise sources (Task 1)
Noise source Noise generatmg mechanism
Relevant parame-ters
Conditions under which important
Comments Level of theo-retical under-standing
Landing gear Broadband noise due to turbulent flow on various elements of landing gear and tonal noise due to cavities - Length of strut Diameter of wheels - Number of gears - Gear doors - Nrunber of axles - Number of wheels - Inflow speed
Low engine setting (final approach)
- Heavy aircraft deploy landing gear 15 km before touchdown
- The noise of the main landing gear is directly influenced by circulation around the wing
Medium
Flaps Broadband noise due to turbulence around side edges and gaps
- Flap deflection an-gle
- Local inflow veloc-ity
- Chord length - Airgle of attack - Slat deflection an-gle
- Sweep angle
Low or idle engine setting (approach)
- Flap tracks are of impor-tance and produce excess noise
- Flap side edge noise is dominant compared to flap noise itself
A / T o r i 1' 11 rvi
Slats Broadband noise due to turbulence in gaps
- Local inflow ve-locity
- Chord length - Sweep angle - Geometry between slat and wing, e.g. gap height and overlap
Low or idle engine setting (approach)
- Laminar flow does not al-low slats (therefore future aircraft might have no s'lats) - Slat tracks are of impor-tance and produce excess noise
iVieCllLUlL
A 111
Lift and control surfaces (e.g. wing)
Broadband noise due to turbulence at the trailing edge
- Turbulent inten-sity at the trailing edge
- Sweep angle of the wing
- geometry/shape of the trailing edge, e.g. bluntness of trailing edge
Low engine setting, clean configuration (far approach)
- Limited acoustical data available (difficult to mea-sure because of low noise in-tensity)
- Might not be relevant for current vehicles but for f u -ture designs (e.g. without slats)
iVieCilLUll
Low Spoilers and
speed brakes
Detached flow - Spoiler geometry - Flight velocity
Low engine set-ting (complete approach)
Spoiler noise can be shielded if the gap behind the spoiler and between wing and high-lift system is closed, e.g. with a splitter
blade Low
Krueger (lead-ing edge de-vice)
Not understood - Geometry - Inflow velocity - Sweep angle
Heavy use of spoil-ers during standard approaches, domi-nant during low or idle engine setting
Track system might domi-nate Krueger itself
Table 2.1: Overview of airframe noise sources.
Noise source Noise generating meclra-nism
Relevant parame-ters
Conditions un-der which im-portant
Conrments Level of theoretical under-standing Fan -Thickness and loading
noise
- Interaction rotor-stator - Stator vane
- struts
- Fan-intake interaction, e.g. engine inlet or pylons - Tonal noise due to shock cells on blades (harmonic) - Shock cell interaction with nacelle (not a har-monic sequence)
- Inlet geometry - Nmnber of blades - Number of vanes - Fan pressure ratio - Relative Tip Mach number
- Inlet flow distor-tion, e.g. due to an angle of attack or due to a pylon in front of the engine inlet
Always - For current engines both tones and broadband noise important. The broadband contribution becomes more important for f u -ture designs
- Buzzsaw (tonal) is relevant - Fan noise increases due to in-creased inflow distortion by en-gine installation
- Fan noise is reduced due to lin-ing
- Fan noise can be subject to sig-nificant noise shielding due to stmctural elements - Medium for tones - Low for broadband contribu-tion
Jet - Turbulent mixing - Shock noise (only in cruise condition)
- Velocity differ-ences between the streams, i.e. free, core, and bypass stream
- Temperatiue - Nozzle diameter - Nozzle type
Take-off Jet noise is a distributed source behind engine Good (under subsonic conditions) - Medium (under sonic condi-tions) Combustion - Mainly broadband noise
- Direct contributioir due to the expansion of the gas mixture in the combustion chamber
- Indirect noise contribu-tion due to the convec-tion of non-uniformities through pressure gradi-ents in the turbine
- Temperature - Pressure ratio - Combustor type (lean, rich) - Approach - Departure af-ter thrust cut-back
- Side-line
Becomes more important since all other sources are being re-duced
Low
Turbine Tonal and broadband noise (due to same mecha-nism as fan noise genera-tion)
- Number of blades - Number of vanes - Mach number - Shaft speed - axial stage spacing - Number of stages - Exit area - Shaft power Mainly ap-proach and then departure after , thrust cutback
- Becomes more complex due to multi-stage design
- Haystacking might be of im-portance, i.e. a characteristic spectral broadening effect of tur-bine tones due to the jet shear layer
Low-Medium
Compressor Tonal and broadband noise similar to fan
Same as fan Departure after thrust cutback and approach
Medium
Table 2.2: Overview of engine noise sources.
I n Tab. 2.3 w e list possible interaction and installation effects, i n c l u d i n g the relevant d r i v i n g parameters. I n general, the theoretical u n d e r s t a n d i n g of the corresponding noise generating a n d / o r the noise shieldmg effects is l o w .
2. Identification of main aircraft noise sources (Task 1) 12
(a) Airframe cor\tributiori.
(b) Engine contribution.
Figure 2.2: The various noise generating components on-board of the aircraft, i.e. the "classical" noise sources [1].
"Noise source" Relevant parameters
Jet with flap - Flap-jet vertical distance
- Mach numbers (of jet and flight speed) - Pylon design and position
Engine pylon with wing - pylon design
- location of engine installation
Spoiler on flap and slat - flow conditions around flap and slat due to
spoiler deflection
Landing gear with flap - influence on flow conditions around the flap
due to the extracted main landing gear Shielding effect of engine noise - location of engine installation
Concerning the stateoftheart m o d e l l i n g capabilities of aircraft noise, i n task 2 the f o l -l o w i n g questions were addressed:
o W h a t are the m o d e l l i n g techniques f o r f h e various noise sources obtained f r o m task 1?
c W h a t are the available s i m u l a t i o n capabilities?
e W h a t tools have been developed and a p p l i e d already? • W h a t are the m a i n applications of these tools?
I n a d d i t i o n , task 2 s h o u l d have also addressed the most urgent gaps m s i m u l a t i o n ca¬ p abilities:
• Can i n d u s t r y p r o v i d e a w i s h - l i s t f o r h i t u r e s i m u l a t i o n developments? © Wl-iat accuracy is required?
H o w e v e r , this second topic was h a r d l y covered d u r i n g the w o r k s h o p . For this specific task, the discussion groups were f o r m e d based o n the participants' expertise, i.e. m o d e l developers a n d software users.
3.1.1 Summary of results
I t was proposed to d i s t i n g u i s h f o u r d i f f e r e n t approaches w i t h i n the current f u l l range of m o d e l l i n g capabilities. A w e l l - l m o w n d i s t m c t i o n is that of Farassat [5 , b y w h i c h the followml 4 d i f f e r e n t approaches are distiiaguished (specifically d e r i v e d f o r a i r f r a m e noise b u t m p r m c i p a l applicable to engine noise as weU):
14 3. Assessment of simulation capabilities (Task 2)
. Fully numerical, where the source and propagation are s i m u l a t e d simultaneously S one ü m e - d e p e n d e n t C o m p u t a t i o n a l F l u i d Dynamics (CFD) and ComputaUona Aeroacoustics ( C A A ) r u n . ^ e s e type of simulations require the c o m p — ^ d o m a i n to be large e n o u g h for b o t h c a p h i r i n g the sound source regions and the propagation of the sound to the receiver.
o A CFD step combined loith application of the acoustic analogy, i.e. the source arid p r o p -agation are simulated i n t w o different steps. The aerodynamic J c^^^^^^^^ fi?st for the region where the origins of the sound are expected to be located. Based on post procefsing the aerodynamic
field
results, the s o u n d sources are calculated e g using LighthlU's acoustic analogy [6,7]. The t e r m analogyrefers
here to the n f e t h o dof
c a p t u r i n g processes i n theflow
that are capable to generate s o u n d b y aTound
source term that can then be usedfor
calculating the acoustic propagation. TWs second type is based o n the assumption that there is n ofeedback from
the acousticfield
on the turbulence.e Fully analytical This g r o u p comprises a h approaches w h e r e b o t h the flow a^^^^ acoustic
field
are d e r i v e d analytically The source m o d e l ^ ^ ^ ^ ^ ^ ^ ^ ^ °^ monopoles, dipoles a n d quadrupoles, based on the flow f - f f ^ ; - ^ ^ ^ ^ ject geometry The s o u n d at the receiver location is t y p i c a l l y calculatedfrom
the Green's f u n c t i o n .. Sem-empirical. M e t h o d s i n this class are based on databases ^ ° - t a i n i n g m^^^^^^^^ acoustic data, either
from
component w i n d - t u n n e l tests orfrom
fuU-scale a i r c i a f t and for v a r y i n g operational conditions.This classflication was discussed d u r i n g the w o r k s h o p . The outcome was t^^ r e t d n classes 1 a n d 2
conform
Farassat [5], b u t toredefine
class 3 as semt-analyttcal as thei T w n
models that are based o n analytical approaches areoften -^^^^^^^^
other approach. Class 4 was split i n t w o , i.e. 4a, w h i c h was denoted as the class of
X ^ i c ^
^nethods, and 4b, containing thefast
(semi-empirical) s c ^ ^ ^ ; ^ ^Class 4a is solely based o n measurements, whereas for class 4b a c o m b m a t i o n is made b ween acoustfc data for those elements i n the calculation for w h i c h - analytica or numerical tools are avaUable, a n d analytical or n u m e r i c a l methods for the r e m a m m g
steps, i.e. a physics-based approach^.
The various exiting methodologies and tools as developed or a p p l i e d b y the w o r k s h o p
Jartictpa^^^^^^
aresuLiarised
i n Fig. 3.1. The tools hsted i n Fig. 3.1 are explamed m m o r e d e t a f l i n T a b . 3 . 1 .Noise source Tool A p p l i c a t i o n Landing gear 1, 2, 4fa Flaps 1, 2, 'ih Slats 1, 2, 4b
Lift and Control surface SpoilersS Speed brakes
'lb Krueger 1,2 • Fan 2, 3, flb Jet 2, 3, 4b • Combustor 4b Turbine 2,4b Compressor 2,4 b Interaction/Shielding 2,3
1 Simulating the source
(source and propagation in one simulation)
DNS, LES, U-RANS (unsteady CFD)
2 IVlodeling the source (acoustic analogY)
Piano, UPACKS-LES
3 Semi-analytical componential method (new: combined with CFD or empirical data)
Propnoise, ANOPP, PAI^iAM
4a Fully empirical method (data based, best practice)
INIVI,FLULA-2, AzB, STAPES/IMPACT
4b Fast (semi-) empirical scientific (parametric)
ANOPP, SOPRANO,
lESTA, sonAIR, PANAM
U n d e r s t a n d i n g source physics N e w aircraft p h a s e 3 ^ ( = detailed aircraft or engine design) N e w aircraft p h a s e 2 ( = preliminary aircraft or engine design) N e w aircraft p h a s e 1 ( = c o n c e p t u a l aircraft or engine design) Existing aircraft c o m p o n e n t s ^ Existing aircraft
•significant modifications are required
Figure 3.1: Tire existing methodologies and tools (middle column). For the direct numerical simulation (DNS), Large Eddy Simulation (LES) and unsteady RANS approaches (U-RANS) various tools are used which are not further specified. I n the left column one finds the noise sources identified i n task 1 (uirder each noise source the current available modelling method-ologies from the middle column are indicated). The right column indicates the applications that are possible w i t h each tool.
Assessment of simulation capabilities (Task 2) 16
Tool Type Description Origin Reference
INM 4a Integrated Noise Model Federal
Avia-tion Adminis-tration
Olmstead et al. [8]
FLULA 4a Fluglaerm, acoustic
investi-gation of complex scenarios such as yearly air traffic
Swiss Federal Laboratories for Materials Testing and Research Pieti'zko and Buetikofer [9]
ANoPP 3,4b Aircraft Noise Prediction
Program
NASA Gillian [10]
ANoPP 2 3,4b Aircraft Noise Prediction
Program, new version
NASA Burley [11]
SOPRANO 4b Silencer Common Platform
for Aircraft Noise calcula-dons
ANOTEC con-sulting
Van Oosten [12]
lESTA 4b Infrastructure for Evaluating Air Transport Systems
ONERA Rozenberg
and Bulté [13]; Brunetet al. [14]
SonAIR 4b Model for predicting single
flight events to investigate and optimize noise abate-ment procedures by using ei-ther generic data, e.g. from a full flight simulator, or cock-pit data from real flights
Empa, Swiss Federal Labo-ratories for Ma-terials Science and Technol-ogy, and Swiss Laboratory for Acous-tics/Noise Control Zellmann, Wunderli and Schaeffer [15]
PANAM 3,4b Aircraft system noise model-ing
Airframe noise model: Pro-fan
Engine noise model: HEIDI
DLR Bertsch [1]
and Bertsch & Isermann [16] (PANAM); Rossignol, Lummer, and Delfs [18] (Pro-fan); Bassetti and Guérin [17] (HEIDI)
AzB 4a German calculation standard
(e.g. implemented in com-mercial codes Soundplan, Cadna, and IMMI)
DLR Isermann and
Vogelsang [19]; Bertsch and Isermann [16]
STAPES 4a SysTem for AirPort noise
Exposure Studies (in I M -PACT: An Integrated Aircraft Noise and Emissions Mod-elling Platform)
EUROCONTROt ECACDoc. 2 9 / ICAO Doc. 9911
Propnoise 3 Propulsion Noise DLR Moreau and
Guérin [20]
Picino 2 Computational
Aeroacous-tics code
DLR Caro [21]
UPACS-LES 2 Computational Fluid
Dy-namics / Aeroacoustics code
JAXA Imamura [22]
•promising noise jreduiiction concepts
(Taslc 3)
401 Detailed description of task
This task concerned the i d e n t i f i c a t i o n and assessment of p r o m i s i n g noise r e d u c t i o n con-cepts. The f o l l o w i n g issues were addressed:
© W h i c h n e w technologies or systems are k n o w n to result i n noise r e d u c t i o n (the noise sources obtained f r o m task 1 are considered)?
o W h a t are f h e implications w h e n installed on-board of f h e aircraft?
• W l i a t is the operational impact, e.g. is i t effective o n l y i n s l o w f l i g h t w h e n the engines are idle?
402 Summary of results
Tab. 4.1 gives f h e o v e r v i e w of a l l discussed noise reducing measures and the impHcation f o r f h e aircraft.
. Identification and assessn:ient of promising noise reduction concepts (Task 3) 18
Noise reduction measure Estimated reduction
Implications for the aircraft Landing gear mesh fairings
(add-on device)
3-bdB Landing gear design, weight,
maintenance Flap-side-edge noise: Porous
device at the edge Maintenance
Slats: Setting optimization (overlap, gap)
3-bdB Additional complexity/weight
with respect to kinematics and tracks
Fan: Optimized fan speed, im-proved liner design for wide-band noise reduction, design for by-pass-ratio (bpr) 15, pressure ratio 1.2 (reference is 1.6)
5 dB (mainly attributed to fan rpm); higher reduction possible with increasing bpr
Engine weight, nacelle design, drag increase
Jet: Increase bpr, add chevrons 1-2 dB (chevrons); higher
reduc-tion possible with increasing bpr Bigger nacelle, weight Engine noise shielding
(espe-cially fan noise)
10 dB and more Aerodynamic disadvantages
due to location of engine installation
into new low-noise vehicle (Task 4)
The objective of f h i s task was f o i d e n t i f y f h e most p r o m i s i n g low-noise technologies a n d concepts and h o w to integrate these on-board of the reference aircraft.
The noise source contributions f o r the reference vehicle are depicted m Fig. 5.1 f o r ap-proach and i n Fig. 5.2 departure. The noise source contributions o n the g r o u n d are evaluated f o r t w o t y p i c a l a n d representative observer locations. Depicted are P A N A M s i m u l a t i o n results [1]. The vehicle is simulated u n d e r t y p i c a l operating conditions along approach a n d departure, respectively. A l o n g the s i m u l a t e d f l i g h t p a t h , observer loca-tions that are t y p i c a l l y subject to increased c o m m t m i t y noise annoyance have been se-lected. The approach observer is approx. 7 k m p r i o r t o u c h - d o w n whereas the departure observer is located approx. 3 k m after take-off.
A p p l y i n g the selected noise measures as i d e n t i f i e d i n Tab. 4.1, the g r o u n d noise i m p a c t can be significantly reduced. I t is assumed, that a i r f r a m e noise contributions can be re-duced b y the m a x i m u m as i d e n t i f i e d b y f h e experts. This is a 5 d B level r e d u c t i o n f o r each source, i.e. l a n d i n g gear, flap-side edge, a n d l e a d i n g edge noise c o n t r i b u t i o n . Fur-thermore, jet noise can be reduced b y 6 dB^ and modifications to the f a n can y i e l d noise level reductions i n f h e order of 10 dB^. Obviously, the r e d u c t i o n of one i n d i v i d u a l noise source c o n t r i b u t i o n w i l l y i e l d another d o m i n a t i n g noise source so that a l l measures have to be i m p l e m e n t e d simultaneously. Finally, f o r f h e selected o p e r a t i n g conditions a n d at the corresponding representative observer location, an overall level r e d u c t i o n of 8.5 dB along the take-off a n d 6.2 dB along the approach can be achieved. Yet, i t has t o be m e n t i o n e d , that the l a n d i n g gear remains as the d o m i n a t i n g noise source f o r f h e approach case. If f h e gear is n o t d e p l o y e d , a level r e d u c t i o n of almost 10 dB is p r e d i c t e d a l o n g the approach case. Take-off noise is still d o m i n a t e d b y f a n noise c o n t r i b u t i o n even after application of the measures as i d e n t i f i e d i n Tab. 4.1. E x p l o i t a t i o n of noise shield-i n g effects promshield-ises f u r t h e r sshield-ignshield-ifshield-icant noshield-ise r e d u c t shield-i o n to the f a n noshield-ise shield-i m p a c t o n the g r o u n d . So overall, i t can be concluded, that the technology as i d e n t i f i e d b y the w o r k
-^Here it is assumed, that a 2 dB reduction is achieved due to nozzle modification and additional 4 dB reduction due to an increase in BPR.
^It is assumed, that 10 dB reduction are achievable due to increased BPR, a reduced fan rpm, and advanced fan design.
5. Integration of reduction concepts into new low-noise vehicle (Task 4)
(a) Reference vehicle. (b) Ref. with installed measures. Figure 5.1: Typical take-off noise source ranking.
shop participants w o u l d not f u l l y meet the f i r s t w o r k s h o p goal, w h i c h is a 12-13 reduction of the m a x i m u m A - w e i g h t e d s o u n d pressure level f o r each flight operati( i.e. along approach and departure.
The certiflcation noise i n EPNdB is u s u a l l y d o m i n a t e d b y tonal f a n noise contributi( A p p l y i n g the i d e n t i f i e d measures to the f a n noise contribution, i.e. i n c l u d i n g shie i n g , promises significant reduction of f h e t o n a l f a n noise. I t can be concluded, that 1 E P N L at the certification points could be s i g n i f i c a n t l y reduced. The selected level redi tions f o r each measure m i g h t n o t yet reach the order of 35 EPNdB c u n u n u l a t i v e bek Chapter 4^ as specified as another w o r k s h o p goal, b u t i t gets close. I n conclusion, 1 i d e n t i f i e d measures promise to reduce the u n d e r l y i n g noise sources s i g n i f i c a n t l y b u t n o t reach the A C A R E goals.
6 SvrmMiMj iz CcMiclMgioB
A w o r k s h o p was organized b y D L R and T U D e l f t i n order to b r i n g together experts f r o m i n d u s t r y academia, and research institutions. The participants were organized into w o r k i n g groups i n order to a l l o w f o r detailed discussions and a v o i d a presenta-tion m a r a t h o n . W i t h i n the w o r k i n g g r o u p , the experts h a d to w o r k o n p r e d e f i n e d tasks i n order t o (1) i d e n t i f y the existing noise sources onboard of a g i v e n reference v e h i -cle, (2) i d e n t i f y available and still missing s i m u l a t i o n capabilities, (3) i d e n t i f y possible measures to reduce these noise contributions, and f i n a l l y (4) evaluate the i m p a c t of f h e reduction measures i f a p p l i e d to the reference vehicle.
Classical d o m i n a t i n g noise sources have been assessed and parameters i d e n t i f i e d , that dominate their inherent noise generation. For the a i r f r a m e noise sources, i t can be con-cluded, that g o o d to m e d i u m u n d e r s t a n d i n g and data is available f o r most sources. Yet, spoilers a n d speed brakes as w e U as Krueger leading edge devices are n o t y e t f u l l y understood. These sources require more detailed investigation i n f h e near f u t u r e . Espe-cially, because spoilers are h e a v i l y used along so-called "low-noise" or steep approach procedures w h i l e their i m p a c t o n the o v e r a l l g r o u n d noise is still u n k n o w n . Krueger devices o n the other h a n d m i g h t become v e r y i m p o r t a n t i f l a m i n a r - f l o w w i n g s are still of interest f o r f u t u r e aircraft^.
W i t h respect to the engine noise sources, i t s h o u l d be noted, that more emphasis s h o u l d be p u t on the so-caUed core noise sources, i.e. combuster and turbine. Since signiflcant level reductions seem achievable f o r the jet and f a n noise, f h e core noise sources w i l l r e m a i n as d o m i n a t i n g noise sources i n f h e f u t u r e . Therefore, detailed research o n these sources w i l l become essential i n the f u t u r e .
A n o t h e r v e r y interesting noise source has been i d e n t i f i e d b y the participants. The counter-rotating open r o t o r concept (CROR) is v e r y p r o m i s i n g w i t h respect to a reduc-t i o n i n f u e l c o n s u m p reduc-t i o n compared reduc-to a convenreduc-tional 2015 reduc-t u r b o f a n engine^. The noise generation is v e r y complex and n o t yet f u l l y understood. Tlie CROR concept w o u l d easily fill u p a separate a n d dedicated w o r k s h o p , hence was not i n the scope of this ^Krueger flaps are very promising high-lift devices for laminar wings because they keep the wing surface protected from insect and dirt impact, therefore keep them clean,
reduction in fuel consumption in the order of 10% seems possible.
event. Yet, the i n d u s t r y participants indicated confindence that the noise levels of an advanced CROR design w i l l meet the restrictions of Chapter 4^.
The importance of advanced s i m u l a t i o n capabilities f o r overall noise p r e d i c t i o n is ac-centuated b y the fact that most organizations and institutions r u n their o w n software developments i n that area. A n i m p o r t a n t step t o f u r t h e r i m p r o v e the overall noise p r e d i c t i o n is the c o m b i n a t i o n of methods w i t h d i f f e r e n t fidelity. Interfaces between overall system noise p r e d i c t i o n tools a n d measured data or h i g h - f i d e l i t y s i m u l a t i o n ap-proaches, e.g. C A A , promises to be an essential step t o w a r d s m o r e reliable s i m u l a t i o n results.
The i d e n t i f i e d measures to reduce k n o w n noise sources are listed i n Tab. 4.1. A p p l i -cation of these measures on-board of the reference vehicle promises a significant noise r e d u c t i o n of 6.2 dB a n d 8.5 dB along approach a n d departure, respectively The re-d u c t i o n along the approach can be f u f h e r i m p r o v e re-d t o 10 re-dB w i t h o u t the gear re- de-p l o y e d . Yet, the i d e n t i f i e d measures to the reference vehicle do n o t reach the order of 12 dB O A S P L r e d u c t i o n w h i c h corresponds to 35 E P N d B c u m m u l a t i v e b e l o w Stage 4 as specified i n f h e A C A R E goals. A d v a n c e d vehicle concepts w i t h engine noise shielding promise even higher level reductions f o r the specific noise source subject to shielding, therefore m i g h t help to f i n a l l y come close to the A C A R E goals, see Ref. [23].
A n o t h e r p r o b l e m that has been i d e n t i f i e d d u r i n g f h e w o r k s h o p is the lack of an ap-p r o ap-p r i a t e noise metric. Available metrics, e.g. E P N L at f h e certification ap-points, w i l l n o t always do f h e job. By s i m p l y considering the certification points, other significant flight segments are n o t accounted for. For example, i t is a k n o w n fact that c o m m u n i t y noise annoyance is d o m i n a t i n g along the c o m m o n approach p a t h t o w a r d s any m a j o r airport. Yet, this s i t u a t i o n is still far a w a y f r o m any certification p o i n t , hence not even consid-ered f o r a "conventional" noise assessment.
The w o r k s h o p participants have f i l l e d o u t an a n o n y m o u s survey about the w o r k s h o p after f h e event. For this survey, special attention was p u t o n f h e concept of the w o r k -shop, i.e. a v o i d presentation m a r a t h o n b u t enable detailed discussions. A l l of the par-ticipants gave the concept 8-10 points w i t h 10 b e i n g the highest grade. Furthermore, the participants indicated that they w o u l d n o t have been able to d r a w such an "holistic" o v e r v i e w o n aircraft noise, i.e. f h e m a j o r sources, m o d e l l i n g capabilities, a n d r e d u c t i o n possibilities, b y themselves. The presented event was the f i r s t "Joint D L R & T U D e l f t A v i a t i o n Noise Workshop". For more i n f o r m a t i o n o n f o l l o w - u p events, the interested reader is referred to directly contact the editors.
6. Summary & Conclusion 24
Acknowledgments
The authors greatly acknowledge the c o n t r i b u t i o n of the i n v i t e d participants. The w o r k -shop attendants i n alphabetical order are: Eckhard A n t o n ( R W T H Aachen U n i v e r s i t y Germany); Jason Blinstrub (DLR, Germany); D o m i n i k Broszat ( M T U , Germany); Casey Burley ( N A S A Langley, USA); Bao Chen (Aerodynamics Research Institute - A R I , China); Jan Delfs ( D L R and T U Braunschweig, Germany); P h i l i p p Ernstberger ( A i r b u s De-fense and Space G m b H , Germany); Roland E w e r t (DLR, Germany); Sebastien G u e r i n (DLR, Germany); A n d r e w H a h n ( N A S A L a n g l e y U S A ) ; Michaela H e r r (DLR, Ger-many); Fredi Holste (Rolls-Royce, GerGer-many); X u n H u a n g (Peking University, China); Umberto l e m m a (Roma Tre University, Italy); Taro I m a m u r a (University of Tokyo, Japan); Hernando Jimenez (Georgia Institute of Technology, U S A ) ; Carsten Liersch ( D L R , Ger-m a n y ) ; Partrice M a l b e q u i ( O N E R A , France); Luis M e l i v e o (Anotec C o n s u l t i n g , Spain); M i t s u h i r o M u r a y a m a (JAXA, Japan); Yan Q u n ( A i r c r a f t Strength Reserach Institute -ASRI, China); Johann Reichenberger ( A i r b u s Defense and Space G m b H , Germany); K a r l - S f é p h a n e Rossignol (DLR, Germany); A b h i s h e k Sahai ( R W T H Aachen University, Germany); Laurent Sanders ( O N E R A , France); Reinhold Schaber ( M T U , Germany); Ste-fan Schwanke (DFS, Germany); A r n e Seitz (Bauhaus L u f t f a h r t , Germany); Christian Stanger ( U n i v e r s i t y of Stuttgart, Germany); Russell Thomas ( N A S A Langley, U S A ) ; Fe-l i x W i Fe-l Fe-l ( T U M u n i c h , Germany); R i k Wijntjes ( N L R , NetherFe-lands); C h r i s t o p h Z e Fe-l Fe-l m a n n (EMPA, Switzerland); X i n Z h a n g (University of Southampton, GB); Thomas Z i l l (DLR, Germany).
Finally, f h e authors w o u l d l i k e to express their g r a t i h i d e towards Andreas D i l l m a n n , head of the D L R Institute of Aerodynamics and F l o w Teclinology i n Goettingen, for s u p p o r t i n g these extracurricular activities and f o r a f i n a n c i a l c o n t r i b u t i o n .
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