acoustics lnl 05
TRANSCRIPT
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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
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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
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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
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Basics: AcousticsSource, Medium, Propagation, Receiver
SourceMediumWave Propagation
SourceMediumWave PropagationReceiver
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Basics: AcousticsAcoustics is sub-classified based on[2]
SourceAeroacousticsVibroacousticsEtc.
MediumHydroacousticsSeismologyEtc.
Etc…
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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
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120
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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
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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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Outputs typically desired from a aeroacoustics study Source Strengths
Source RankingFrequency Spectrum
At observerDirectivity
Simulation: Objectives
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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
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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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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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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
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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
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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
Courtesy: Dr. Thomas Scheidegger
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CAA: Example2: Circular Piston
Courtesy: Dr. Thomas Scheidegger
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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
Courtesy: Dr. Thomas Scheidegger
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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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CAA: Example3: Side Window Buffeting
The figure that originally appeared on this page has been removed.
See Figure 4 from SAE paper 2003-01-1316
Courtesy: DaimlerChrysler Corp.
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CAA: Example3: Side Window Buffeting
The figure that originally appeared on this page has been removed.
See Figure 3 and Tables 2,3 from SAE paper 2003-01-1316
Courtesy: DaimlerChrysler Corp.
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Pressure Variation60 mph, 5 degree yaw
CAA: Example3: Side Window Buffeting
Courtesy: DaimlerChrysler Corp.
The figure that originally appeared on this page has been removed.
See Figure 6 from SAE paper 2003-01-1316
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Velocity Magnitude Variation60 mph, 5 degree yaw
CAA: Example3: Side Window Buffeting
Courtesy: DaimlerChrysler Corp.
The figure that originally appeared on this page has been removed.
See Figure 7 from SAE paper 2003-01-1316
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SPL spectrum at driver’s ear60 mph, 5 degree yaw
CAA: Example3: Side Window Buffeting
Courtesy: DaimlerChrysler Corp.
The figure that originally appeared on this page has been removed.
See Figure 10 from SAE paper 2003-01-1316
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Effect of Position in the Cabin60 mph, 5 degree yaw
CAA: Example3: Side Window Buffeting
Courtesy: DaimlerChrysler Corp.
The figure that originally appeared on this page has been removed.
See Figure 13 from SAE paper 2003-01-1316
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Effect of Exhauster and Slightly Opening the Rear Window60 mph, 5 degree yaw
CAA: Example3: Side Window Buffeting
Courtesy: DaimlerChrysler Corp.
The figure that originally appeared on this page has been removed.
See Figure 14 from SAE paper 2003-01-1316
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Difference between two mirror housing designs
CAA: Example3: Side Window Buffeting
Courtesy: DaimlerChrysler Corp.
The figure that originally appeared on this page has been removed.
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
CAA: Example3: Side Window Buffeting
Courtesy: DaimlerChrysler Corp.
The figure that originally appeared on this page has been removed.
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)
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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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CAA: Example4: Side View Mirror
Inlet Mirror
Mesh All hex mesh, 1.4 million cells
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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
CAA: Example4: Side View Mirror
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CAA: Example4: Side View MirrorFlow Velocity and Pressure
Velocity magnitude on horizontal planeStatic pressure on vertical plane
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Microphone Locations
CAA: Example4: Side View Mirror
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
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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
CAA: Example5: Air Intake Whistle
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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
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0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Frequency (Hz)SP
L (d
B)
Computational - 2D
153dB @ 1710 Hz
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 WhistleModified Design
Sound spectrum measured at sump bottom
Experimental - 3D Computational - 2D
137dB @ 2125Hz
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0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Frequency (Hz)
SPL
(dB
)
150 dB @ 1710 Hz
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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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
CAA: Example5: Air Intake Whistle
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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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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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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
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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
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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
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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
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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
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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
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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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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
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
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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
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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
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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
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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
SSPM: FWH Method: Example 2
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SSPM: FWH Method: Example 2Cells are clustered in wake and near landing gear
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SSPM: FWH Method: Example 2
Vortical structures, (ω2 - SjiSij)/2 = (u0/D)2
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SSPM: FWH Method: Example 2
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
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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
SSPM: FWH Method: Example 3
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Rain gutter: Flow structure
Static pressure contours
Vorticity Iso-surface colored by velocity mag.
Static pressure contours
Rain gutter
SSPM: FWH Method: Example 3
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Rain gutter: Flow structure
Rain gutter
Contours of Velocity Magnitude on the symmetry plane
SSPM: FWH Method: Example 3
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Rain Gutter: Surface pressure fluctuation
0
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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]
SSPM: FWH Method: Example 3
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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.
SSPM: FWH Method: Example 3
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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
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
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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
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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
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SNGR: ExampleGeneric HVAC duct
Baseline designThree additional design variationAim: determine noise ranking with BB models and compare with experimentally determined noise rankings
The figure that originally appeared on this page has been removed.
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.
See SAE paper 2005-01-2495 “On Predicting the AeroacousticPerformance of Ducts with Broadband Noise Source Models.”
Design 2
Design 4
88
SNGR: ExampleMesh
The figure that originally appeared on this page has been removed.
See SAE paper 2005-01-2495 “On Predicting the AeroacousticPerformance of Ducts with Broadband Noise Source Models.”
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
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SNGR: ExampleFlow Structure: Contours of velocity magnitude
The figure that originally appeared on this page has been removed.
See SAE paper 2005-01-2495 “On Predicting the AeroacousticPerformance of Ducts with Broadband Noise Source Models.”
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
The figure that originally appeared on this page has been removed.
See SAE paper 2005-01-2495 “On Predicting the AeroacousticPerformance of Ducts with Broadband Noise Source Models.”
93
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
94
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.
95
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
96
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)
97
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)
98
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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OutlineAeroacoustics BasicsSimulation Methods
Computational Aeroacoustics (CAA)Segregated Source-Propagation Methods (SSPM)
FundamentalsVariational MethodsBoundary Element MethodsIntegral Methods
Stochastic Noise Generation and Radiation (SNGR)SummaryBibliography
Simulation GuideCAAFWH Method
100
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
5
Source Region
Receiver
p’(t)TransmissionRegion
Simulation Guide: CAA
101
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
102
OutlineAeroacoustics BasicsSimulation Methods
Computational Aeroacoustics (CAA)Segregated Source-Propagation Methods (SSPM)
FundamentalsVariational MethodsBoundary Element MethodsIntegral Methods
Stochastic Noise Generation and Radiation (SNGR)SummaryBibliography
Simulation GuideCAAFWH Method
103
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
5
Simulation Guide: FWH Method
104
MeshInterior source surfaces need to completely enclosedominant sources and important scattering surfaces
wall source surface
Interior source surfaces
interior source surface
Duct
Simulation Guide: FWH Method
105
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
106
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
Simulation Guide: FWH Method
107
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
Simulation Guide: FWH Method
108
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
Simulation Guide: FWH Method
109
Simulation Guide: FWH Method
1(B) SolutionTransient
Time step = 1/(max frequency)/10Run simulation for total real time = (1/(min frequency))*10
LES preferred, but not necessary
110
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
Simulation Guide: FWH Method
111
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
.
Simulation Guide: FWH Method
112
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
Simulation Guide: FWH Method
113
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
Simulation Guide: FWH Method
114
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
Simulation Guide: FWH Method
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Simulation Guide: FWH Method
Original Data Pruned Data