noise reduction for a high performance military aircraft · fine scale turbulence (g) modelling of...

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Validation of Noise Footprint

Calculation Model for a

High Performance Military Aircraft

Dr. Ernst Grigat, Airbus Defence and Space GmbH

30th May 2017

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Contents

• Why do we (have to) put focus on military aircraft noise?

• Where did we start from?

• What is our overall approach?

• Where do we stand today?

• What are our main results (up to now)?

• How do we plan to proceed?

20.06.2017

Validation of Noise Footprint Calculation Model for a High Performance Military Aircraft

2

The following questions (hopefully) will be answered during the next minutes

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Motivation – Demand for Noise Reduction of Military Aircraft

20.06.2017


3

Noise reduction of military aircraft is of increasing importance

Civil Aircraft

Publications

1970 1980 1990 2000 2010

"Aircraft Noise"

indication of

increasing

relevance

Customer

global (ICAO Annex 16)

european (1592/2002)

national (LuftVZO)

Regulations

ICAO Annex 16 Subsonic airplanes with 4

engines or more

Maximum Noise level of

106 EPNdB for aeroplanes

with maximum certificated

take-off mass of 385 000 kg

and over.

1970 1980 1990 2000 2010

"Aircraft Noise"

"Military Aircraft Noise"

ICAO Annex 16 Supersonic airplanes

SARPs for these aeroplanes

have not been developed.

However, the maximum

noise levels of (…) subsonic

jet aeroplanes may be used

as a guideline.

2010

2000

1990

Swiss RfP 2008

RDAF RBI 2013

Military Aircraft

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General Approach

20.06.2017


4

Generally minor influence on noise emission

(engine/airframe) for military aircraft

Focus on immission

Reduction of aircraft noise ground immission by optimization of the according takeoff

climb (and landing approach) flight paths.

Overall Goal

Due to criticality/classification of military

goods only few information is available

on noise reduction of military aircraft

Starting from scratch

IS – Trajectory Optimization

IS NOT – Calculation of absolute noise values

first step: establish noise calculation model

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Outline of Noise Calculation Model

20.06.2017


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Transmission

Emission

Reflection

Immission S

Generic Modular

Approach

𝑳𝑷 = 𝑳𝑾 +𝑫+ 𝑨 (Metrics)

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Emission – Source Modelling Overview

20.06.2017


6

Modelled Noise Sources

• Engine Jet (Dry & Reheat)

• Engine Fan

• Undercarriage

• Vertical Tail

• Foreplane

• Leading Edge

• Trailing Edge

• Airframe (incl. Wing)

• Stores

Main

Components

Source: Filippone, Antonio: Advanced Aircraft Flight Performance

Assumptions and conditions/constraints

• Separate modeling of source and directivity

• Minor sources (e.g. excrescencies) are neglected

• Breakup of engine noise into jet and fan noise

• Originally based on textbook formulas

• To be validated by flight test

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Example: civil aircraft

Source: Bertsch, Eberhard-Lothar: Noise Prediction within Conceptual Aircraft Design

Emission – Directivity Characteristics (Near Field Behaviour)

20.06.2017


7

Resulting modelling approach (engine components)

• 3-dim directivity correction

• No analytical solution possible/reasonable

• Derivation from noise measurements necessary

• Very expensive and time consuming process

• Cross-read (adapted civil a/c data) as interim solution

• To be validated/refined/corrected by flight test

NO homogenous expansion (at least for engine/fan)

Source: Bräunling, Willy: Flugzeugtriebwerke

Example: general engine noise emission characteristics

All other sources modelled as monopoles Uniform propagation (Directivity correction = 0)

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Flight Test Validation Campaign at Neuburg Airfield – Setup

20.06.2017


8

Fly-Over Beamforming performed by

• 2-day campaign (Nov. 2015)

• 135 microphones ( 300 – 4992 Hz)

• 20 EF2000 Typhoon fly-overs

• 2 distinct configurations

Clean

2 U/W tanks

• 3 engine power settings

Part Dry

Maximum Dry

Maximum Afterburner

• Undercarriage Up & Down

• Fly-over altitude < 200ft

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Environmental Requirements for Beamforming

20.06.2017


9

Weather conditions are critical for fly-over measurements. Recommended requirements are as

follows (10 m above ground resp. 30m above flight path):

• temperature between -10° and +35° C 14.3 – 16.6 °C

• no rain, (dew), fog, drizzle or snow

• no ground-based temperature inversions

• humidity between 20% and 95% 61 – 71 %

• wind conditions:

– maximum average wind during test points: 12 kts

– maximum average cross wind component during test points: 7 kts

– maximum wind: 15 kts 6- 15 kts

– maximum cross wind component: 10 kts

Furthermore the following terrain conditions apply:

· terrain: flat, no mounds/furrows

· grass: 7.5m radius circles of mowed grass (< 8cm height) around microphones

· ploughed fields: within a 7.5m radius surface shall be reasonably tamped down

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Flight Test Validation Campaign – Fly-overs

20.06.2017


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Flight Test Validation Campaign - Beamforming

20.06.2017


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Meteo Data

Pass # Wind

Direction

[Deg]

Wind

Speed

[kts]

Temp.

[°C]

Humidity

[%]

1 280 10 16,6 71

2 290 7 16,6 71

3 300 7 16,6 71

4 290 6 16,6 71

9 270 8 16,7 70

SPL

FPR Pod

Flight Path

definition of

source areas

Emission Data

Sound Power

Validation of Noise Sources

Data processing/analysis and

Beamforming performed by

Immission Data

Sound Pressure

Sound Pressure Levels (measurements)

Validation of Overall System

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Flight Test Validation Campaign – Results Immission

20.06.2017


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Exemplary graph for one of a total of 9 branches

Original sound pressure sampling rate 16384Hz

Picture below shows 4Hz resolution (averaging)

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Flight Test Validation Campaign – Influence of Thrust Level on Noise

20.06.2017


13

Distinction of non-engine noise sources

in most cases only feasible for test trials

with PartDry power setting (masking!)

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Flight Test Validation Campaign – Results Emission

20.06.2017


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Sound Intensity Maps Overlay with Sound Power Areas

Deconvolution

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Noise Model Validation – General Approach

20.06.2017


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Flight trials

at Neuburg

Textbook

approaches

Flight trials

by WTD91

Manufacturer

material

Flight trials

by BAeS

Flight trials

armasuisse

Overall system V

a

l

i

d

a

t

i

o

n

E

v

i

d

e

n

c

e

V

a

l

i

d

a

t

i

o

n

E

v

i

d

e

n

c

e

Preliminary Model

Matching

Matching

Y

Adaption of

Model Parameters

N

Validated

Model

Engine Components

Source and Directivity modelled

Non-Engine Components

Modelling as monopole

• Uniform propagation (D=0)

• Only sources modelled

Y

N

Adaption of Model

Preliminary Model

Matching

Matching

Adaption of

Model Parameters

N

Y

Validated

Model

Y

Adaption of Model

Def. of Directivities

N

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Emission – Modelling of Engine Combustion Noise

20.06.2017


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Exit NozzleAfterburner

Low Pressure Turbine

High Pressure TurbineCombustion Chamber

CompressorCompressor

Bypass

Source: http://eurofighter.airpower.at

Combustion Sound Power

4

0

2

0

,

2

,

,,210

T

T

p

p

T

TTmaP TurbinComb

inComb

inComboutCombC

Combe

with exponent Ce: engine specific constant, a: speed of sound, 𝑚 : mass flow,

Tcomb,in/out: combustion chamber in/outlet temperature, pComb,in: combustion

chamber inlet pressure, Tturb: temperature increase in turbine

Combustion Sound Power Level

0

, log10P

PL Comb

CombW

Flight Test Corrections

Analysis of flight test data (see figure below) showed

Combustion noise hardly measurably separately

Combustion contribution to jet noise negligible

Combustion covered by overall jet noise model

Unified re-modelling of jet noise necessary

see jet noise modelling slides

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Emission – Details of Re-Modelling of Engine Jet Noise

20.06.2017


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Noise from large scale turbulence Noise from fine scale turbulence

Similarity spectra for the two components of turbulent mixing noise

Large scale turbulence (F)

Fine scale turbulence (G)

Modelling of engine jet noise

as turbulent mixing noise

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Emission – Directivity for Engine Jet Noise (FlightIdle - MaxDry)

20.06.2017


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Realistically to be determined empirically by respective flight tests

Directivity characteristics for Combustion included

Current approach based on reference model

Measurement of free stream characteristics

Adapted to actual case (military aircraft engine)

Taken from textbook – only rough approximation


Emission

Angle [°]

Directivity

Corr. [dB]

0

60

90

100

110

120

130

140

150

160

170

180

Offset model-FT between ~90°-130° emission angle

Re-modelling (additional dependency on thrust lever)

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Emission – Modelling of Undercarriage Noise

20.06.2017


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Assumption: monopole model uniform propagation Directivity correction D = 0


Landing Gear Sound Pressure

)(2 fFr

AMaqp

ref

LG

withq: dynamic pressure, Ma: free stream Mach number,

A: equivalent landing gear surface area, rref: reference length

: empirical reference amplitude for frequency spectrum,

and the frequency (f) correction function using Strouhal number St

Landing Gear Sound Pressure Level

0

, log20p

pL LG

LGp

q

qq

StStq

StStqfF

0

0

1/

V

lffStSt

ref

with lref: reference length, V: free stream velocity

,,q: empirical parameters

Parameter adaptation: , , St0

rod system

tyre

attachment

parts

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Emission – Modelling of Foreplane Noise

20.06.2017


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Foreplane Sound Pressure (Trailing Edge Noise)


5.2

02225.01548.0

V

uC

r

AMaqp L

ref

FP


A: total foreplane surface area, rref: reference length

CL: lift coefficient, u0: local velocity variation du to turbulence,

Foreplane Sound Pressure Level

0

, log20p

pL FP

FPp

Complemented by vortex noise model

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Emission – Modelling of Trailing Edge Noise

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21



Trailing Edge Sound Pressure

5.2

02225.01548.0

V

uC

r

AMaqp L

ref

TE


A: total trailing edge surface area, rref: reference length


Trailing Edge Sound Pressure Level

0

, log20p

pL TE

TEp

No isolated noise signal for trailing edge extractable

Actual sound power model seems resonable (see below)

Read-across of spectral form from foreplane feasible

including vortex noise model complement

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Emission – Modelling of Leading Edge Noise

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22



Normalized LE Sound Pressure Level from WTT TELEnormp StL 225.0)log(35.93,

withcLE: average LE chord, LE: slat deflection, TE: flap deflection

geopVpnormpLEp LLLL ,,,,

refle

le

ref

VpV

VL

,

,cos

coslog30log50

with sLE: span slat, LE: leading edge sweep

• LE de facto a dipole showing distinctive directivity

• Currently no WT / FT data for military aircraft available

• Simplification of directivity feasible for T/O, not for landing

• To be refined in a later stage of model development

• Semiempirical approach

• Based on wind tunnel tests for commercial aircraft

• Adapted to high performance military aircraft

Leading Edge Sound Pressure Level

)log(181.115, StL normp

sStSt

sStSt

21

225.06.21

10TELE

sSt

V

cfSt LE

2,cos

log100.5refLE

LELEgeop

r

scL

Velocity (Lp,V) and geometric (Lp,geo) correction

Flight Test data prove unrealistic LE modelling

LE noise contribution is modelled much too high

Not enough FT data for realistic re-modelling

LE retracted: noise masked by other sources

LE extended: no measurements ( neglected)

Incorporation of LE noise into surface noise feasible

Increment on roughness height in surface noise

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Emission – Modelling of Vertical Tail Noise

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23



Vertical Tail Sound Pressure (Trailing Edge Noise)

Currently no correctio/refinement/validation

No according flight test data available

Effect negligible due to minor relevance

No significant contribution to overall noise

5.2

02225.01548.0

V

uC

r

AMaqp L

ref

Fin


A: total vertical tail surface area, rref: reference length


Vertical Tail Sound Pressure Level

0

, log20p

pL Fin

Finp

Surface noise component / contribution neglected!

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Emission – Modelling of Airframe Noise

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24


Airframe Sound Pressure (Surface Noise)

)(25.01

3.0 2 fRCr

AMaqp relD

ref

Airframe


A: airframe lower surface area, rref: reference length

CD: drag coefficient, : roughness density, Rrel: relative roughness height,

and the frequency (f) correction function using Strouhal number St

Airframe Sound Pressure Level

0

, log20p

pL

Airframe

Airframep

9.57.1

8.0

00044.01.41

28.82

StSt

Stf

VffStSt

2

with : boundary layer thickness, V: free stream velocity


Model adaptation: correction function

5.45.3

9.1

4.0001.0

5.52

StSt

Stf

spectral shape

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Emission – Modelling of Stores Noise

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25


Flight Test Corrections Noise Characteristics for a 1000l tank

Strategy for including other stores:

scaling with Drag Index

1000l tank Sound Pressure (Surface Noise)

)(25.01

3.0 2 fRCr

AMaqp relD

ref

Tank


A: tank surface area, rref: reference length, CD: drag coefficient,

: roughness density, Rrel: relative roughness height,

and the frequency (f) correction function with Strouhal number St

1000l tank Sound Pressure Level

0

, log20p

pL Tank

Tankp

9.57.1

8.0

00044.01.41

28.82

StSt

Stf

VffStSt

2

with : boundary layer thickness, V: free stream velocity

During flight trial data evaluation

only trailing edge type noise

could be identified. Surface

noise is completely masked.

Re-definition of noise model

Read-across from foreplane trailing edge validation

But: additionally tonal components due to periodical

flow separations

The general approach for covering other stores

by scaling with Drag Index cannot be directly taken

Tonal Components

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Immission – Modelling and Implementation Aspects

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26

Reflection (mountains) is executed by Transmission algorithm

(but not yet implemented)

Noise immission is calculated by summing up the contributions

from the different/distinct noise sources impacting on ground.

Evaluation of immissions based on different metrics, e.g.

- Sound Pressure Level (SPL)

- A-weighed SPL

- Effective Perceived Noise Level

- Psychoacoustic metrics

Phenomena not considered/implemented

due to reduction of complexity

Diffraction Diffusion (molecular

level)

Calculation results for each parameter

(e.g. SPL) are stored in datacubes

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Immission – Graphical Representation of Noise Footprints

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27

Based on Matlab using Google Maps Api Functions

WGS (World Geodetic System) 84 Coordinates

Terrain Representation potentially based on NASA SRTM

currently checked (resolution ~30m)

interface yet provided

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Transmission – Modelling of Noise Propagation

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28

Reference Point

d0 distance

Time t0 Original approach: timmission = temission

• Noise emission @t0 immission @t0

• Noise emission @t0+125ms immission @t0+125ms

Improved approach:

Simplified (linearised) Ray Tracing

Modelled noise “loss” by absorption (ISO 9613-1)

• Geometric (depending on distance from source)

• Atmospheric (molecular absorption)

• Soil damping (for small immission angles)

Doppler Effect included

Noise reflection not yet modelled, but planned for

Wind & temperature effects (refraction) neglected

d1

But obviously more realistically (a = speed of sound)

• Noise emission @t0 immission @t0+d0/a

• Noise emission @t0+125ms immission @t0+125ms+d1/a

Inconsistencies due to discretisation

t0+125ms+d1/a

t0+250ms+d2/a

t0+375ms+d3/a

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Transmission / Propagation Algorithm Validation

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Validated transmission algorithm

Comparison with

Refinements / Adjustments

• Geometric absorption

• Atmospheric absorption

• Doppler effect

• …

Noise Footprint Program

- Original Version -

Program SOPRANO by

Noise Footprint Program

- Validated Update -

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Transmission / Propagation Algorithm Validation - Results

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Example: noise footprint of a turning flight 15 secs after takeoff

Original implementation Refined algorithm

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Summary

Resuming the introductory questions

• Why do we (have to) put focus on military aircraft noise?

Increasing public pressure / flight regulations / customer requirements

• Where did we start from?

Starting from scratch

• What is our overall approach?

Noise reduction by flight path optimisation

Strictly modular analytical modelling approach to be validated by flight test

• Where do we stand today?

Simplified (flying) aircraft noise model almost completely validated

• What are our main results (up to now)?

First calculations look promising

... and one more question …

20.06.2017


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Outlook

How do we plan to proceed?

• Final refinements of emission model including directivity characteristics

• Complete validation of the noise model based on flight test noise measurements

• Revision / additional investigation on feasibility of monopole assumption for landing gear

• Integration of wind and temperature effects into transmission algorithm

• Parallelization

• Integration of a terrain data base into the model

• Integration of a terrain data base into the graphical noise footprint representation

• Comfortable graphics based user interface for noise calculation model

• Embedding of the noise calculation model into an flight path optimisation algorithm

… and many more

20.06.2017


32

Current work in progress …

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Thank you for your attention!

Any questions?

20.06.2017 33


noise reduction for a high performance military aircraft · fine scale turbulence (g) modelling of...

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