Acoustics Lnl 05

1

Aeroacoustics ModelingBy:

Sandeep Sovani, Ph.D.Senior Consulting EngineerFluent Inc., Ann Arbor, MI

March 18th, 2005

Presented at Fluent Lunch and Learn seminar seriesSt. John’s Conference Center, Plymouth, MI

2

Welcome!

Fluent Inc.’s Lunch’N’Learn Seminar SeriesTopical seminars on leading edge CFD applicationsHeld frequently

Aeroacoustics Modeling – March 18, 2005FloWizard – April 22, 2005Unsteady Flow Modeling – April 29, 2005Multiphase Modeling – May 20, 2005

PurposeInform the FLUENT community about the subject

Discuss basics, physics, theory, modeling techniques,Tools available in FLUENT to model the subjectExamples

3

OutlineAeroacoustics BasicsSimulation Methods

Computational Aeroacoustics (CAA)Segregated Source-Propagation Methods (SSPM)

FundamentalsVariational MethodsBoundary Element MethodsIntegral Methods

Stochastic Noise Generation and Radiation (SNGR)SummaryBibliography

Simulation Guide

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Basics: AcousticsDefinitions

Acoustics = The scientific study of sound[1]

Sound = Pressure waves radiating in any material medium

HistorySound was recognized to be a wave phenomenon over 2000 years ago![2]

Chrysippus (Greek philosoper, 240 BC)Vetruvius (Roman architect and engineer, 25 BC)

Sound has essential characteristics of wavesIt has a “source”

Oscillatory disturbanceIt “propagates” in a “medium”

Transports energy without transporting matter

5

Basics: AcousticsSource, Medium, Propagation, Receiver

SourceMediumWave Propagation

SourceMediumWave PropagationReceiver

6

Basics: AcousticsAcoustics is sub-classified based on[2]

SourceAeroacousticsVibroacousticsEtc.

MediumHydroacousticsSeismologyEtc.

Etc…

7

Basics: AeroacousticsAeroacoustics

Sub-area of acoustics where the source of sound is fluid flow

CharacteristicsNo moving boundaries

such as electric speakers, vibrating strings, or vocal chordsUnsteady fluid flow always produces pressure oscillations

therefore is inherently a source of sound

ExamplesWhistlesHVAC vent noiseAutomotive wind noise

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Basics: Aeroacoustics

SoundFlow

Acoustic Medium Receiver

Source

Source ≡ Transient pressure variation caused by the flow

Sound ≡ Pressure waves propagating in the acoustic medium

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Basics: Characteristics of SoundSound waves have several key attributes

Compressible phenomenonWave amplitude is very small

Sound waves carry only a tiny fraction of the energy contained in the mean flow

E.g. Acoustic energy generated by Boeing 747 during take-off is not enough to boil an egg!

0

20

40

60

80

100

120

140

1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02Pressure (Pascal)

SPL

(dB

)

=

ref

rms

pp'20logSPL 10

25 /102 mNpref−×=

1atm = 1E+5 Pa

10





Simulation Guide

11

Outputs typically desired from a aeroacoustics study Source Strengths

Source RankingFrequency Spectrum

At observerDirectivity

Simulation: Objectives

12

Simulation: AspectsTo obtain the desired outputs two aspects need to be simulated

Sound sourceProvides source characteristics and rankings

Sound propagationPropagation of sound from the source to the receiver

Requires input of source characteristicsProvides

» Sound spectrum and receiver» Sound directivity

Aeroacoustics simulation essentially involves computing these two aspects

13

Simulation: ApproachesThere are 3 primary simulation approaches

Computational Aeroacoustics (CAA)Sometimes referred to as Direct Noise Computation (DNC)Sound sources and propagation solved in a single comprehensive model

Segregated Source-Propagation Methods (SSPM)Sound source and propagation solved separately via two separate computations

Integral MethodsBoundary Element MethodsVariational Methods

Stochastic Noise Generation and Radiation (SNGR)

We will discuss each of these methods in detailTheoryApplicabilityAdvantages/DisadvantagesExamples

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Simulation Guide

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CAA: Theory CAA = Computational AeroAcoustics

CAA :: AeroacousticsDNS :: Turbulence

Direct simulation; no models involvedPremise

Fluid flow at sound source and sound propagation, both are fluid phenomena

Therefore both are governed by Navier-Stokes equationsSolve transient N-S equations to calculate both

Sound generationPropagation

Domain spans from sources to receiversMost straightforward in terms of both implementation and usage

A comprehensive CFD code such as FLUENT solves the Navier-Stokes equationsSimply conduct a transient CFD solution and measure static pressure at mike as function of time

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CAA: Theory

Sound Source

Receiver

p’(t)Propagation

Computational Domain

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Practical problems in using CAA1] Frequency range (20 Hz ~ 20,000 Hz)

Acoustic timescales are often orders of magnitude greater than turbulence timescalesSimulation needs to be run for long real time with a small timesteps, i.e. for large no. of timesteps

2] Radiation to Far FieldDomain needs to extend from source to receiverLarge mesh sizes for far-field sound problems e.g. aircraft noise heard on the ground

CAA: Applicability

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3] Acoustic Pressure MagnitudeMagnitude of the acoustic pressure is much less than the hydrodynamic pressureNecessitates use of very high order discretizationschemes to propagate sound over long distances

Still then, can only propagate sound over limited distance

CAA: Applicability

0

20

40

60

80

100

120

140

1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02Pressure (Pascal)

SPL

(dB

)

patm ~ 1E+5 Pa

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CAA: Applicability

CAA is practically applicable only to cases where these 3 obstacles are relatively minor

Frequency rangeLower the better

Distance between source(s) and receiver(s)Smaller the better

Sound pressureLarger the better

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CAA: ApplicabilityRegion of practical applicability

Quantitative expressions for the bounding lines of the “region of practical applicability” are still a open matter for research

Region of practical applicability

Distance Between Source and Receiver

Freq

uenc

y Increasing Sound Pressure

21

CAA: Advantages/Disadvantages

AdvantagesSimple to implement

Single simulation solves sound generation as well as propagation

Can account for flow-sound “coupling”Cases where sound has backward effect on flow

DisadvantagesLimited applicability

As discussed on previous slidesComputationally expensive

Large meshesLong transient computations

Mesh needs to be carefully prepared to capture sources properly

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Gaussian pulse initialized, σ = 1m; Mesh size ∆x = 0.2m, 2σ/∆x = 10

Coupled explicit solver, CFL=0.75; Inviscid, 2nd order upwind

Pressure outlet BC with NRBC

Pulse leaves domain with no reflection

Standard pressure outlet BC

Pulse reflects as expansion wave at open boundary

CAA: Example1: 1D-Pressure Pulse

Courtesy: Dr. Thomas Scheidegger

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Circular piston in infinite wallr =0.1m, 2D axisymmetric

Domain size 2m, ∆x=0.005m, 160,000 grid pointsVibration frequency f =3000Hz, amplitude 0.5mmλ=0.11m, λ/∆x=22, Coupled explicit solver, CFL=0.75Inviscid, 2nd-order upwind discretization

– Analytical solution for first minimum in directivity: sin Θ1= 3.83/kr kr >> 1

CAA: Example2: Circular Piston

MDM mesh motion


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CAA: Example2: Circular Piston


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λ/∆x=44kr = 2.72Θ1 ≈ 80.7o

160,000 grid points

Marginally resolved propagation

λ/∆x=11

160,000grid points

– Acoustic beaming for higher frequencies correctly predicted

– Analytical solution for first minimum in directivity: sin Θ1= 3.83/kr kr >> 1

f =1500Hz

λ/∆x=22kr = 5.43Θ1 ≈ 44.8o

160,000 grid points

f =3000Hz

λ/∆x=22kr = 10.86Θ1 ≈ 20.7o

640,000 grid points

f =6000Hz


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CAA: Example3: Side Window BuffetingBuffeting

Loud throbbing sound/pulsation felt inside a car cabin when the window(s) or sunroof are openAlso known as “wind-throb”

ExampleAimed at predicting sound pressure spectrum at driver’s and passenger’s ears when side window is openParametric studies

Effect of position inside the cabinDifferent window openingsDifferent mirror designs

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CAA: Example3: Side Window BuffetingReferences:

Passenger Car:Hendriana, D., Sovani, S.D., and Scheimann, M. On simulating passenger car side window buffeting, SAE International Paper 2003-01-1316 (2003)[3]

SUV:An, C.-F., Alaie, S.M., Sovani, S.D., Scislowicz M., Singh, K., Side window buffeting characteristics of a SUV, Vehicle Aerodynamics, Vol. SP1874, pp. 43 - 53, SAE International Paper 2004-01-0230 (2004)[4]

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CAA: Example3: Side Window Buffeting

The figure that originally appeared on this page has been removed.

See Figures 1 and 2 from SAE paper 2003-01-1316

Courtesy: DaimlerChrysler Corp.

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See Figure 4 from SAE paper 2003-01-1316


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See Figure 3 and Tables 2,3 from SAE paper 2003-01-1316


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Pressure Variation60 mph, 5 degree yaw





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Velocity Magnitude Variation60 mph, 5 degree yaw





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SPL spectrum at driver’s ear60 mph, 5 degree yaw





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Effect of Position in the Cabin60 mph, 5 degree yaw





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Effect of Exhauster and Slightly Opening the Rear Window60 mph, 5 degree yaw





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Difference between two mirror housing designs




See Figures 7 and 15 from SAE paper 2003-01-1316

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Difference between two mirror housing designs60 mph, 5 degree yaw




See Figures 18 from SAE paper 2003-01-1316

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CAA: Example4: Side View MirrorSound radiated from a generic automotive side-view mirror[14]

SVMs are strong contributors to wind noiseBluff bodies prominently protruding from vehicle surface

Produce highly turbulent, transient wakes that are sources of sound (unsteady pressure variation)

39

CAA: Example4: Side View MirrorGeneric Side View Mirror

Half cylinder (0.2m dia. and height), topped with aquarter sphereMounted on a flat plateExperimental flow/sound measurements reported in literature

Hold et al. (AIAA-99-1896)[5] and Seigert et al. (AIAA-99-1895)[6]

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Boundary ConditionsInlet velocity = 200 km/hrRe = 7 × 105

CAA: Example4: Side View Mirror

Velocity Inlet

Pressure Far-Field

Symmetry

Walls

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Inlet Mirror

Mesh All hex mesh, 1.4 million cells

42

Solution Settings:CFD code: Fluent 6.1

Finite Volume Method based Navier-Stokes Solver

Solver: Segregated ImplicitTurbulence Model: LES

Smagorinsky-Lilly sub-grid scale modelDiscretization schemes:

Time: 2nd order implicitMomentum: 2nd order upwindPressure-Velocity Coupling: SIMPLE

Transient Solution:Timestep size: 60 microsecondTotal timesteps: 2100Run time: 4.75 daysHardware: 2 processors, Intel P4, 2.2 GHz, RedHat Linux


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CAA: Example4: Side View MirrorFlow Velocity and Pressure

Velocity magnitude on horizontal planeStatic pressure on vertical plane

44

Microphone Locations


Side View

Top View

Pt 101

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CAA: Example4: Side View MirrorSound Pressure Spectrum[14]

Reference for Experimental Data: Hold et al. (AIAA-99-1896)[5] and Seigertet al. (AIAA-99-1895)[6]10

30

50

70

90

110

0 500 1000 1500 2000Frequency (Hz)

SP

L (d

B)

ExperimentalCFD - CAACFD - AA

Point 101

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Inlet Throttle Body

Throttle Plate

SidebranchCavity

IntakeManifold

Outlet

Inclined Face

Automotive air intake manifolds can produce loud whistles (tonal noise)In this example we study such whistle production with CAA[7]

CAA: Example5: Air Intake Whistle

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CAA: Example5: Air Intake WhistleTwo cases were studied

Baseline (Produced Strong Whistle)Modified (Attenuated Whistle)

Simulation were carried in two stages

2D (Along centerline cut of the geometry)3D (Work in progress)

Airflow

θ=43°Baseline

θ=43°

Airflow

Modified

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43°

418000 2D Quad Cells, 4k-ε RNG Turbulent Model4Unsteady Simulation4Coupled Solver 42nd Order Upwind4Time Step = 2.5e-05s4Run Time = 0.04s4 Ideal Gas Law4Double Precision

Solver

High Mesh Density

Inlet (101325 Pa) Throttle

Plate Sump

Zip TubeOutlet92325PaδP = 9kPa

Monitor Point A


49

CAA: Example5: Air Intake WhistleVelocity Magnitude in Modified Geometry

Ref: SAE Paper 2005-01-2364 Kannan et al. “Computational Aeroacoustics Simulation of Whistle Noise in An Automotive Air-Intake System”

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CAA: Example5: Air Intake WhistleBaseline Design

Sound spectrum measured at sump bottom

Experimental - 3D

159dB @ 2125Hz

80

90

100

110

120

130

140

150

160

170

180

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Frequency (Hz)SP

L (d

B)

Computational - 2D

153dB @ 1710 Hz


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CAA: Example5: Air Intake WhistleModified Design

Sound spectrum measured at sump bottom

Experimental - 3D Computational - 2D

137dB @ 2125Hz

80

90

100

110

120

130

140

150

160

170

180

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Frequency (Hz)

SPL

(dB

)

150 dB @ 1710 Hz


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Difference between CFD and experimentally measured spectra

Primarily due to strong 3D effects in experimentsIn reality flow passes around the sides of the throttle platesSide flow affects the shear layer on the sump

Present CFD simulations are only 2DAll air flow has to pass above or below the throttle plateImpingement length is different, therefore Strouhal number (St = fL/U) is differentExcitation happens at fixed St, so greater L in 2D causes peak to occur at lower f

3D CFD simulations are in progressShow great improvement in accuracy

Sample results seen in “FLUENT6 for Acoustics Modeling”[8]

Results to be presented in Fluent CFD Summit 2005 to be held in Dearborn, MI, June7-9 2005


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CAA: Examples Overview

Region of practical applicability

Distance Between Source and Receiver

Freq

uenc

yIncreasing Sound Pressure

Air-Intake Whistle

Generic Side View Mirror

Side Window Buffeting

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Simulation Guide

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SSPM: TheorySegregated Source-Propagation Methods

Sound generation and propagation are independent phenomena in most cases

They happen at vastly different scalesFlow pressure ~ 1 kPa; Acoustics pressure ~ 1 mPaTurbulence length scales ~ 1µm; Acoustic wavelengths ~1mTurbulence time scales ~ 1 µs; Acoustic timescales ~ 1ms

Problem domain can be thought to be composed of two “layers”

Flow fieldGoverns sound generationNavier-Stokes equations

Acoustic fieldGoverns sound propagationWave equation

AdvantagesReduced computational effortExpanded applicability to a wide variety of problems

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Derivation of the Wave EquationLinearized Continuity Equation (For fluctuations)

Linearized Momentum Equation (No convection, no body forces, no viscous stresses)

Eliminate

Use isentropic relation for speed of sound

Wave Equation

0''=

∂∂

+∂

∂

i

i

xu

tρρ

'' 2 ρcp =s

pc

∂∂

=ρ

2

0''2

22

2

2

=∂∂

−∂

∂

ixpc

tp

iou'ρ0''

2

2

2

2

=∂∂

−∂

∂

ixp

tρ

0''0 =

∂∂

+∂

∂

i

i

xp

tuρ

SSPM: Theory

57

Connection between the two segregated parts of the problem: source and propagation

Sir James Lighthill provided the mathematical foundation for connecting the source and propagation parts

Famous “Lighthill’s Acoustic Analogy” [9]

SSPM: Theory

58

Lighthill’s Acoustic Analogy[9]

Continuity Equation

Momentum Equation (Convection included, but no viscous stresses)

In a conservative form

Eliminate

0)( =∂∂

+∂∂

ii

uxt

ρρ

ij

ij

i

xp

xuu

tu

∂∂

−=

∂∂

+∂

∂ρ

ij

jii

xp

xuu

tu

∂∂

−=∂

∂+

∂∂ ρρ

iiji

ji

xxp

xxuu

t ∂∂∂

+∂∂

∂=

∂∂ 22

2

2 )( ρρ

SSPM: Theory

iuρ

59

Lighthill’s Acoustic Analogy (continued)The previous equation can be cast in the form of a wave equation in an undisturbed medium at rest by subtracting from both sides:

This gives “Lighthill’s Equation”

Where

This is referred to as “Lighthill’s tensor”For nearly isentropic flows:

SSPM: Theory

iio xx

c∂∂

∂ '22 ρ

ji

ij

ii xxT

xxc

t ∂∂

∂=

∂∂∂

−∂∂ 22

202

2 '' ρρ

ijojiij cpuuT δρρ )( 2−+=

jioij uuT ρ≈

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Lighthill’s Acoustic Analogy (continued)Lighthill’s equation can be thought of as a wave equation with a source term

Wave Equation

Lighthill’s Equation

Lighthill’s tensor representing the sound source can be calculated by solving Navier-Stokes equations using CFD

0'' 2202

2

=∂∂

∂−

∂∂

ii xxpc

tp

SSPM: Theory

ji

ij

ii xxT

xxc

t ∂∂

∂=

∂∂∂

−∂∂ 22

202

2 '' ρρ

Propagation Source

61

Computing PropagationOnce CFD provides sound source information the problem reduces to solving for sound propagationSeveral methods exist for this with varying level of simplification

Rigorous Lighthill equation solutionFinite Difference Methods Variational Methods (Finite Element)[10]

Boundary Element MethodsIntegral Methods

Kirchoff’s Method[11]

Ffowcs-Williams and Hawkings Method[12]

Discussion on development of most of these methods is beyond the scope of this presentation.

See references for more information

SSPM: Theory

62

FLUENT provides features to compute sound propagation using several of these methods

Variational Methods (Finite Element Method)ACTRAN-LA is a third party Lighthill’s equation solver code developed by Free Field Technologies

Fluent has an interface to export Lighthill’s tensor and other variables to ACTRAN

Boundary Element MethodSYSNOISE is a third party BEM code developed by LMS International

Fluent has an interface to export surface pressure fluctuation to SYSNOISE

» Being a Boundary Element code SYSNOISE needs only the pressure fluctuation on the boundaries

Integral MethodsFLUENT has an inbuilt sound propagation module based on the Ffowcs-Williams and Hawkings (FWH) Method[12]

Part of the standard FLUENT package, no add-on components required

SSPM: Implementation

63

SSPM Implementation follows these steps for each methodPerform well resolved transient simulation of flow in and around the sound source regions

Save required data (such as Lighthill’s tensor, or time-varying surface pressure) on source regions

Read saved data into the sound propagation solver (ACTRAN, SYSNOISE, or FLUENT’s inbuilt FWH module)

Perform sound propagation computationPost-process acoustic results

Sound spectra, Directivity

SSPM: Implementation

Source

Receiver

p’(t)

CFD Domain

Sound Propagation

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Simulation Guide

65

FLUENT’s FWH moduleThe most general Acoustic Analogy-based integral formulation today is the Ffowcs-Williams Hawking’s(FW-H) method[13]

AllowsMoving surfacesPermeable surfaces

Lighthill-Curle’s integral is a subset of the FW-H formulation

Essentially an integral methodStore time-varying pressure at all points on the identified source surfaces during a transient CFD simulationAfter transient CFD simulation is completed, an automatic “one-click” routine provides sound pressure signal at predefined receiver locations

SSPM: FWH Method

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SSPM: FWH Method

BenefitsLess computation expense

Small CFD Domain only covering source area

Ideal for far-field applications

DisadvantagesCannot account for backward effect of sound on flowCannot account for reflectionsNeeds straight line of sight from source to receiver

67

Fluent 6.2 implements the FW-H integral method in its most general form

Allowable source surfacesWalls (impermeable)Permeable source surfaces (interiors, inlets, outlets, sliding interface)

Can be used to account for quadrupole sources by wrapping an interior source surface around a quadrupole volume source

Moving/rotating source surfaces Special addition

steady state fan noise (Gutin noise) Multiple source surfaces and multiple receivers allowed

SSPM: FWH Method

68

Fluent6.2 FWH ImplementationTime-domain implementation

Forward-time formulationallows for ‘on the fly’ simultaneous noise calculations

Usable with segregated and coupled implicit solversCompressible or incompressible source data3D and 2D planar implementation

FW-H not available for axisymmetric solver

Targeted applicationsExternal aerodynamic noise

Side view mirrors, Louvers, ….Landing gear, high-lift devices, …

Fan/rotor noiseJet noise

SSPM: FWH Method

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10

30

50

70

90

110

0 500 1000 1500 2000Frequency (Hz)

SP

L (d

B)

ExperimentalCFD - CAACFD - AA

Point 101

Generic Side View Mirror[14]

Source Surfaces

SSPM: FWH Method: Example 1

70

Landing gear scale model M=0.2, ReD=1.23x106

Segregated solver, incompressibleLES, Smagorinsky, Cs=0.12nd-order in time, ∆t=2.5x10-6 sBounded central-differencing in space

Mesh:5.3M cells173,000 surfaces tri-elements, ∆s=0.0135D6 prism layers, h1=1.6x10-3D

Run time:3min 40s per time step, 4 nodes9950 time steps for flow to pass throughdomain (18D), ≈ 25 days run time

FW-H analysis:Source data extraction after one flow passSource data sampled during 0.9 flow passes


71

SSPM: FWH Method: Example 2Cells are clustered in wake and near landing gear

72


Vortical structures, (ω2 - SjiSij)/2 = (u0/D)2

73


p(t)

receivers 1, 3

93.4 dB

98.3 dB92.9 dB

96.8 dB

Receiver OASP

SPL receivers 1, 3

92.3 dB

74

Automotive A-pillar Rain Gutter[15]

Air speed = 22.35 m/s Reynolds number of 40,000

β A

B

C D

E F

G

H

I

J

K

L

α

Flow

Microphone


75

Rain gutter: Flow structure

Static pressure contours

Vorticity Iso-surface colored by velocity mag.

Static pressure contours

Rain gutter


76

Rain gutter: Flow structure

Rain gutter

Contours of Velocity Magnitude on the symmetry plane


77

Rain Gutter: Surface pressure fluctuation

0

20

40

60

80

100

120

0 500 1000 1500 2000

x (m)

SPL

(dB

)

Experiment (Kumarasamy and Karbon)

Fluent (LES Central Differencing)

Kumarasamy and Karbon CFD

Rain GutterLESCentral DifferencingRun 2Model A

Timestep size = 8E-5 sec

POINT -5; 0.05 m upstream of the raingutter's vertical side

CFD spectrum is average of 5 samples. Each sample was measured for 0.1 sec.

Surface Pressure Spectrum(Upstream Microphone)

Reference:Kumarasamy S. and Karbon K., “Aeroacoustics of an Automobile A-Pillar Rain Gutter: Computational and Experimental Study,” SAE Paper 1999-01-1128, (1999)[16]


78

Rain Gutter: Sound at far-field microphone

Reference:Kumarasamy S. and Karbon K., “Aeroacoustics of an Automobile A-Pillar Rain Gutter: Computational and Experimental Study,” SAE Paper 1999-01-1128, (1999)[16]

0

20

40

60

80

100

0 500 1000 1500 2000

Frequency (Hz)

SPL

(dB

)Experiment (Kumarasamy and Karbon)

Fluent (LES - Central Differencing)

Kumarasamy and Karbon - CFD

Rain GutterLESCentral DifferencingRun 2Model A

Timestep size = 8E-5 sec

Far-field microphone location

CFD spectrum is average of 5 samples. Each sample was measured for 0.058 sec.


79





Simulation Guide

80

RationaleUnsteady simulations (LES and DES) are time-consumingSteady RANS results contain a fair amount of useful information

mean velocity components, mean pressure, turbulent kinetic energy, rate of dissipation, etc.

This information can be used to shed some light on broadband (turbulence) noiseCan’t avoid approximationYet potentially very useful to:

screen “noisier” designsIdentify the primary source of the noise

SNGR

81

FLUENT6.2’s Offering:Broadband Noise Source Models

Source terms in the acoustic equations

Lilley’s equationLinearized Euler equation (LEE)

Proudman’s formula for turbulence noise [17]

Turbulent boundary layer noise model [17]

Jet noise source modelsRibnerGoldstein

All these models require steadyRANS results only.

SNGR

82

Steady RANS results used to “synthesize” turbulent velocity fields with the stochastic noise generation and radiation (SNGR) method.FLUENT reports the r.m.s. values of the source terms.

Self-noise sourcesShear-noise sources

. Iso-surface of Lilley’s acoustic source (total) strength

SNGRBB Model 1: Lilley’s Acoustic Sources

83

Source terms in LEE (Linearized Euler Equation)

Steady RANS (k-ε, k-ω, RSM) results are used to “synthesize” turbulent velocity fields with the SNGR method.FLUENT reports r.m.s. values of the source terms in the individual coordinate directions.

Self-noise sourceShear-noise source

.

4342144 344 21noiseSelfnoiseShear −−

∂∂

−∂∂

−∂∂

−=⋅⋅⋅+∂∂

+∂

∂

j

titj

j

itj

j

tij

j

aij

ai

xuu

xUu

xuU

xuU

tu

BB Model 2: LEE Source Terms

SNGR

84

Originally derived by Proudman (1952) for noise due to isotropic turbulence (quadrupole sources)

Recently re-derived (Lilley, 1993) and confirmed using DNS (Sarkar and Hussaini, 1993)

Simple yet very useful to determine the local contribution to the total acoustic power.

Developed by Fluent Inc. Based on Lighthill-Curle formulation

BB Model 3: Proudman Acoustic Power

BB Model 4: Boundary Layer Noise

SNGR

85

SNGR: ExampleBroadband noise source models are practically useful for

Determine prominent noise generating regions in a flow domainDetermine noise rankings of different variations of a design

Current example demonstrates how the BB models can be used to determine noise rankings amongst several ducts[17]

Acknowledgements: Study conducted in collaboration with Delphi Thermal and Interior Systems

86

SNGR: ExampleGeneric HVAC duct

Baseline designThree additional design variationAim: determine noise ranking with BB models and compare with experimentally determined noise rankings


See SAE paper 2005-01-2495 “On Predicting the AeroacousticPerformance of Ducts with Broadband Noise Source Models.”

87

SNGR: ExampleDesign Variations

Baseline: Design 1

Design 3

The figures that originally appeared on this page have been removed.


Design 2

Design 4

88

SNGR: ExampleMesh



89

SNGR: ExampleSolver Settings and Boundary Conditions

Steady state, Segregated Implicit

Double PrecisionRNG k-e2nd order 2nd order upwindSIMPLECAir (incompressible)

• Solver• Precision• Turbulence Model• Pressure discretization• Momentum

discretization• Pressure-velocity

coupling• Fluid

SettingFunction

7.507 m/s(=300 cfm)0 Pa (gage)

Constant VelocityInteriorNo slip wallConstant

PressureNo slip wall

• Duct Inlet• Duct Outlet• Duct boundaries• Plenum Outlet• Plenum

boundaries

ValueBoundary Condition

Boundary

90

SNGR: ExampleFlow Structure: Contours of velocity magnitude



91

SNGR: ExampleAcoustic power generated inside the duct as estimated from the BB models

(1) Proudman Formula (Representative of quadrupolecontribution)(2) Turbulent Boundary Layer Noise (Representative of dipole contribution)

Design4

Design3

Design2

Design1

Dipole Source PowerSurface Integral of Surface Acoustic Power

Quadrapole Source PowerVolume Integral Volumetric Acoustic Power

Design

The data that originally appeared on this page has been removed.

See SAE paper 2005-01-2495 “On Predicting the Aeroacoustic Performance of Ducts with

Broadband Noise Source Models.”

92

SNGR: ExampleExperimental Measurements

Sound spectrum measured at a point 1 m directly downstream of the duct’s outlet

Acknowledgements: Study conducted in collaboration with Delphi Thermal and Interior Systems



93





Simulation Guide

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Summary

FLUENT offers a fully integrated, comprehensive aero-acoustics capability.1. Direct CAA using transient flow solvers2. FW-H integral method – most general acoustic analogy

based method3. Suite of broadband noise source models4. Source data export in an universal format to 3rd party

codes (ACTRAN, SYSNOISE)Spectral analysis utility (FFT)All these can be done within FLUENT

No add on modules necessary

We are committed to offering best-in-class aero-acoustics capability fully integrated to CFD.

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Simulation Guide

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Bibliography1. Oxford English Dictionary2. Pierce AD, Acoustics: An introduction to its physical principles and

applications, Acoustical Society of America, Woodbury, NY (1994)3. Hendriana, D., Sovani, S.D., and Scheimann, M., “On simulating

passenger car side window buffeting,” SAE International Paper 2003-01-1316 (2003)

4. An, C.-F., Alaie, S.M., Sovani, S.D., Scislowicz M., Singh, K., “Side window buffeting characteristics of a SUV,” Vehicle Aerodynamics, Vol. SP1874, pp. 43 - 53, SAE International Paper 2004-01-0230 (2004)

5. Hold R., Brenneis A. Eberle A., Schwarz V., and Siegert R., “Numerical simulation of aeroacoustic sound generated by generic bodies placed on a plate: Part I – Prediction of aeroacoustic sources,” AIAA Paper no. 99-1896, 5th AIAA/CEAS Aeroacoustics Conference, Seattle WA, May 10 –12 (1999)

6. Siegert R., Schwarz V., and Reichenberger J., “Numerical simulation of aeroacoustic sound generated by generic bodies placed on a plate: Part II – Prediction of radiated sound pressure,” AIAA Paper no. 99-1895, 5th AIAA/CEAS Aeroacoustics Conference, Seattle WA, May 10 – 12 (1999)

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Bibliography7. Kannan V., Sovani S.D., Greeley, D., and Khondge A., “Computational

Aeroacoustics Simulation of Whistle Noise in an Automotive Air-Intake System,” SAE International Paper No. 2005-01-2364 (2005)

8. Fluent Inc., Brochure “FLUENT6 for Acoustics Modeling”, Lebanon, NH (2005)

9. Lighthill M. J., “On Sound Generated Aerodynamically, I. General Theory,”Proc. Roy. Soc. London, A211, page 564, (1952).

10. Caro S., Ploumhans P. and Gallez X., “Implementation of Lighthill’sacoustic analogy in a finite/infinite elements framework,” 10th AIAA/CEAS Aeroacoustics Conference, AIAA Paper number 2004-2891 (2004)

11. Lyrintzis A.S., “Surface integral methods in computational aeroacoustics –From the (CFD) near-field to the (Acoustic) far-field,” International Journal of Aeroacoustics, vol. 2, pp. 95-128, (2003)

12. Kim S.-E., Dai Y., Koutsavdis E., Sovani S., Kadam N., Ravuri K.M.R., “A versatile implementation of acoustic analogy based noise prediction method in a general-purpose CFD code,” AIAA paper 2003-3202 (2003)

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Bibliography13. Ffowcs Williams, J.E. and Hawkins, D.L., “Sound generation by

turbulence and surfaces in arbitrary motion,” Proceedings of the Royal Society of London A, vol. 264, pp. 321-342 (1969)

14. Lokhande B.S., Sovani S.D., Xu J., “Computational Aeroacoustic Analysis of a Generic Side View Mirror,” SAE Paper 2003-01-1698 (2003)

15. Sovani S.D. and Chen K.-H., “Aeroacoustics of an "Automobile" A-Pillar "Rain Gutter": A Numerical Study with the Ffowcs-Williams and HawkingsMethod,” SAE Paper 2005-01-2492 (2005)

16. Kumarasamy S. and Karbon K., “Aeroacoustics of an Automobile A-Pillar Rain Gutter: Computational and Experimental Study,” SAE Paper 1999-01-1128, (1999)

17. Khondge A.D., Sovani S.D., Kim S.-E., Farag A.A., and Guzy S.C., “On Predicting the Aeroacoustic Performance of Ducts with Broadband Noise Source Models,” SAE Paper 2005-01-2495 (2005)

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Simulation GuideCAAFWH Method

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Mesh:Mesh needs to be carefully prepared In source region

Mesh edge length = length scale of turbulent eddieswhose timescale is 1/(max frequency)

In transmission regionMesh edge length = {shortest sound wavelength of interest}/10

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Source Region

Receiver

p’(t)TransmissionRegion

Simulation Guide: CAA

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Solution:Transient; Compressible

Time step = 1/(max frequency)/10Run simulation for total real time = (1/(min frequency))*10

LES preferred, but not necessaryMonitor static pressure at microphone location

Pressure vs. Time signalTake FFT to get SPL spectrum

Simulation Guide: CAA

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Simulation GuideCAAFWH Method

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Mesh:Required only in source regionNeeds to be carefully prepared

Mesh edge length = length scale of turbulent eddieswhose timescale is 1/(max frequency)

Wall source surfacesInclude all walls that will experience transient pressure fluctuation

Interior source surfacesneed to enclose dominant sources and important scattering surfacesQuadrupole sources inside permeable source surfaces are accounted forAcoustic pressure field needs to be resolved accurately within the region enclosed by interior source surfaces

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Simulation Guide: FWH Method

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MeshInterior source surfaces need to completely enclosedominant sources and important scattering surfaces

wall source surface

Interior source surfaces

interior source surface

Duct


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Simulation Guide: FWH MethodSimulation ProcedureSTEP1: Processing

1(A) Setup Fluent case; Activate FW-H acoustics model

Select ‘Export Acoustic Source Data’ optionSelect source surfaces from available Fluent zones in Acoustic Sources panelSpecify write frequency

1(B) Run transient simulation• Acoustics source data written in

• .index file and• .asd files

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Simulation ProcedureSTEP2: Post-Processing

2(A) Specify receiver locations in AcousticReceivers panel

2(B) Read .index and .asd filesCompute/write acoustic signals

• pressure vs. time at each receiver2(C) Use FFT tool

• transform acoustic signal to frequency spectrum• PSD vs. frequency


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1(A) Setup FWH ModelDefine → Models → Acoustics…

Select source data export or simultaneous FW-H calculation

Set model parametersFar-field densityFar-field speed of sound

Set references values for SPL calculation


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1(A) Setup FWH ModelDefine → Models → Acoustics…→ Sources…

Select all source surfaces‘On the fly’ FW-H calculation requires consistent surface selection at all time stepsIf source data is exported, redundant surfaces may be selected

– Allows identification of contributions from different sources

For permeable (interior) surfaces,Fluent requires the specification of the ‘inner’ cell zoneSelect write frequency for source data export

Source data does not need to be exported every time step

fwh3


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1(B) SolutionTransient

Time step = 1/(max frequency)/10Run simulation for total real time = (1/(min frequency))*10

LES preferred, but not necessary

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2(A) Set receiversDefine → Models → Acoustics…→ Receivers…

Specify receiver locationsEach receiver generates a signal (.ard) fileReceiver location can be inside or outside the CFD domain


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2(B) Compute sound from saved source dataSolve → Acoustic Signals…

Load .index file– automatically updates the available source data list

Select the source data (.asd files) to be processed– can use subset (in time) of source data

– Choose the source zones (i.e source surfaces) to be used

– ‘Compute/Write’ calculates acoustic signal for the selected receivers and writes out the receiver (.ard) files

.


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0cyxt ii −+=τiyx −

Raw receiver signal Auto-pruned signal

Receiver signal is calculated ‘forward in time’All sources radiate at emission time τSignals from different sources arrive at the receiver at different times , depending on the source-receiver distanceTails of assembled receiver signal are automatically trimmed where signal is incomplete (pruning)

Auto-pruning control in TUI /define/models/acoustics/auto-prune


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2(C) Convert sound signal to spectrumPlot → FFT…General FFT utility

Process receiver data directly or read any Fluent xy-fileY-axis functions: Power Spectral Density (PSD), Sound Pressure Level (SPL), A-, B-, C-WeightedX-axis functions: Frequency, Strouhal Number, Fourier Mode,Octave,1/3-Octave Band


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Plot → FFT… → Plot/Modify Input Signal…Allows modification of signal beforesubsequent FFT analysis

Subtract mean valueClip to rangeWindowing options

HammingHanningBarlettBlackman

Reports signal statisticsmin/max, mean, variance

Plot/Write acoustic signal


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Original Data Pruned Data

Acoustics Lnl 05

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