websem aero acoustic simulation dec2013

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Flow-induced noise simulationVirtual.Lab Acoustics Rev12Raphael Hallez – Product Manager Acoustics

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Flow phenomena

What happens in the presence of flow ?

Wave convection:

- Significant at high Mach (M=vflow/c>0.3)

- Flow is NOT the noise source

- Flow influences the wave propagation

- Example: Aeroengine Inlet, muffler,…

Steady

Flow

Noise Source

Wave propagation is modified by flow

Flow-induced vibrations:

- Fluctuations from unsteady turbulent flow

- Flow acts as a loading of the flexible structure

- Example: Aircraft fuselage TBL loading, train

door,…

Turbulent

Flow

Structural vibrations

Structure-borne noise

Flow-induced noise � Aeroacoustics:

- Fluctuations from unsteady turbulent flow

- Flow acts as a noise source

- Acoustic waves propagate in medium at rest or are

convected by the mean flow component

- Example: pantograph, landing gear, cooling fan,…

Turbulent

Flow

Flow fluctuations

Flow-induced noise

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Flow-Induced Noise phenomena

• Flow-induced noise = noise generated by turbulent flow

phenomena

• Vortices, turbulent eddies

• Vortex shedding (von Karman vortex street)

• Turbulent boundary layers and boundary layer separation

• Rotating surfaces in a fluid (propellers, fans)

• Level of turbulence in the flow, characterized by Reynolds number:

• Low Re � Large flow scales

• High Re � Large flow scales + smaller flow scales

• Unsteady vortices on many scales interact with each other and with

steady or moving surfaces �� Noise generation

µ

ρVL=Re

SpeedSound

VelocityFlowMach =

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Flow-induced noise simulation examples

• External Aeroacoustics:

• Turbulent flow interacts with static body and radiates noise outside

• Challenge: Capture the sources and reflection/scattering on large

surfaces

• Train bogie, Landing Gear, Wiper

• Internal Aeroacoustics / Confined flow:

• Propagation of sources (duct noise and blower noise) in ducting

system and radiation through outlet

• Challenge: Capture the acoustic reflections on duct walls (guided

waves)

• HVAC, exhaust, intake, ECS

• Mixed Internal-External Aeroacoustics

• Transmission through flexible panel � Aero-Vibro-Acoustics

• Challenge: Capture Hydrodynamic+Acoustic loading, capture

dynamics of system

• Windnoise (side mirror, A-pillar), fuselage TBL

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Flow-Induced Noise : A Challenge for Numerical Simulation

� Acoustic field = part of the flow field � most straightforward approach: Direct Computational AeroAcoustics (CAA) (=direct numerical simulation of both the unsteady turbulent flow and the noise it generates)

But not practical because :

� High order schemes needed to capture acoustic propagation (numerical instabilities)

� High numerical cost of a direct CAA � prohibitive at low Mach and high Reynolds numbers

� Also, specific issues related to CFD discretisation techniques applied to acoustics

• Dissipation and dispersion errors

• Non-reflecting Boundary conditions

p’ = 4.4934739 Pa

hydrodynamicfield

acousticfield

� at low - moderate Mach numbers: orders of magnitude of difference between

• Length scales: λac = Lturb / M

• Magnitudes: O(M4) of the flow energy radiates into the far field

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AeroAcoustic Analogy : a more suited approach...

� Decoupling flow simulation from the acoustic simulation

� Flow computation : creating source field data

� Acoustic computation : post-processing of source field data

� Fundamental assumption = one-way coupling

� Unsteady flow produces sound and affects its propagation

� BUT: sound waves do not affect flow field significantly

� Principal application of the hybrid approach: flows at low Mach

numbers, no strong coupling like in sunroof buffeting case

sourceregion

observerposition

L

λ

d

� Preferred simulation tools for the flow description

� Reynolds Averaged Navier-Stokes (RANS) solver

� time-averaged data � (some hope with SNGR, RPM)

� Unsteady RANS � unsteady, but only large scale �

� Large Eddy Simulation (LES)

Detached Eddy Simulation (DES)

� unsteady, broadband turbulence (up to grid & scheme cut-off

frequency) ☺

� Important: Low-Mach number limitation

� Incompressible LES / DES data supported to reduce CPU cost

http://www.lmfa.ec-lyon.fr/recherche/turbo

U-RANS

LES

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Aero-Acoustic Sources

Turbulent Flow

Moving Surfaces

Steady Surfaces

No Surfaces

(or smooth surfaces)

Quadrupoles

Dipoles on surfaces (+ Quadrupoles in wake)

Rotating Dipoles (+ Quadrupoles in wake)

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Source Generation in Virtual.Lab

Dipole Sources

• Flow-induced noise in presence of static surfaces with compact regions

• Requires pressure data on the walls (compressible or incompressible)

Quadrupole Sources :

• Flow-induced noise without presence of surfaces (turbulent jets) or non-

compact regions

• Requires velocity vector data in flow volume

Fan Sources = Rotating Dipoles Sources

• Flow-induced noise caused by rotating surfaces (fan)

• Requires pressure data on one or all blades surface for multiple

revolutions

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Simulation Process based on Aeroacoustic analogy

Fluid Dynamics

AcousticsTransient CFD

Simulation

Aeroacoustic sources

Virtual.Lab Acoustics Computation

Data Mapping + Fourier Transform

Acoustics Results

Turbulent Flow Field

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CFD-Acoustics Coupling in Virtual.Lab

VIRTUAL.LAB supports CFD data stored in CGNS

format files (CFD General Notation System)

Following commercial CFD codes already support

CGNS export for aero-acoustical data :

• CFX (Ansys)

• FLUENT (Ansys)

• STAR-CD 4 / STAR-CCM+ (CD Adapco)

• PowerFlow (EXA)

• CFD++ (Metacomp)

• SCRYU/Tetra (Cradle Software)

• Fine Turbo (Numeca)

• OpenFoam

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What can Virtual.Lab do more than CFD alone?

Effect of acoustics on flow (strong feedback)

far-field scattering

Absorbing materials

Flow-induced vibration

CFD Direct CAA � � � �CFD postprocessing(FWH) � � � �Hybrid Approach

(CFD + Virtual.Lab) � � � �

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Dipole sources from Compressible CFD �� Curle

• Curle’s solution (1955) to Lighthill’s equation in presence of solid rigid boundaries (neglecting viscous effects)

• Quadrupole incident field negligible for low Mach numbers (Power ratio = M2)

• Mathematically exact solution But: Pf must satisfy acoustic boundary conditions

• OK if the flow description is compressible

• if the flow description is incompressible and surface is not acoustically compact, the solution is inaccurate (missing acoustic reflection and scattering effects)

Quadrupoles (negligible

at low Mach numbers)Dipole Surface pressure

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Curle’s analogy: what the compactness assumption means

� Curle assumes the CFD captures all acoustic effects on the source surfaces (hydrodynamic pressure+acoustic scattered pressure)

� Not true for incompressible CFD or non-compact surfaces

� Surface with sources will be seen as acoustically transparent!

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Dipole sources from Incompressible CFD ��Neumann Dipoles

Import CFD pressure Pf and define surface

dipoles

Transform dipoles into

acoustic (Neumann) BCs

Compute acoustic solution

• flow wall pressure can be used to define appropriate boundary conditions of an equivalent acoustic boundary value problem

• If flow is compressible � equivalent to Curle

• If flow is incompressible, G is the Green’s function of Laplace problem (infinite sound speed)

� More flexible: can be applied to Indirect BEM and FEM

� More accurate than standard Curle (recomputes acoustic scattering and reflection effects)

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Example: Trailing edge noise prediction based on incompressible-flow pressure

Reference solution

Curle

New formulation

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Virtual.Lab AeroAcoustics - Slide 16

Flow:

Acoustic:

Example: Rod-Airfoil Noise prediction based on incompressible-flow pressure

FEM mesh

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17 copyright LMS International - 2010

Example: Rod-Airfoil Noise prediction based on incompressible-flow pressure

•Comparison with measurements

Neumann dipoles

1 freq 495 freq 4 cores

FEM Neumann Incompressible 2min+10s 11min+1h20 3min+20 min

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Virtual.Lab Aero-Acoustics

Dipole Source Generation – Step by step

Import CFD surface pressure data

(centroids/nodes)

Map CFD pressure on acoustic mesh + Fourier transform

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Dipole Source Generation – Step by step

Define Surface Dipole Boundary Condition (distributed dipoles are defined on

acoustic mesh � boundary condition)

Solve the acoustic response case (FEM or BEM) + post-process…

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Special feature : Conservative mapping

�� Specific conservative mapping algorithm to preserve information

over a large range of flow scales:

Fine CFD mesh Coarse Acoustic mesh

Map turbulent flow field

� Flow scales very small

� CFD mesh has extremely

small cells (Millions of DOFs)

� Acoustic wavelength large

compared to Flow scales

� Acoustic mesh coarser than

CFD mesh (Lelement=~λ/6)

∫∫ =

MeshAcousticMeshCFD

PdSpds

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Quadrupole sources for more complex problems

(for high Reynolds number, isentropic flow and low Mach

number)

• Lighthill’s equation : incident field from quadrupoles + scattering on surfaces

• more generic :

• non compact source regions

• Aero-vibro-acoustics

• Higher flow speed

• But:

• Difficult to deal with volume data set

• Singularity for sources close to walls

• Mapping from CFD to coarse acoustic mesh

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Quadrupole sourcesNew formulation in Virtual.Lab R13

New implementation in Virtual.Lab R13:

• Best performance: supported by FEMAO solver (Adaptive order FEM)

• Improved Usability: CFD data directly read by solver (linux support)

• Improved Accuracy: No mapping required on intermediate mesh, no singularity issue.

� FEMAO = FEM Adaptive Order solver:

� Start from coarse mesh (less than 1 element /

wavelength!)

� Solver automatically increases the element order

at high frequency

� Most efficient FEM solver for broad frequency

computation

� Most accurate scattering modeling

� Up to 20 times Less memory and faster

computation time than standard FEM!

FEMAO Acoustic mesh

36 000 nodes

Max freq FEM: 200 Hz

Max freq FEMAO: 4000 Hz

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Fan noise components

• Aeronautical

• Energy

• Automotive

Fan Noise

Tonal (Discrete Frequency)

Broadband

Unsteady pressure fluctuations on the blade surface

Incoming turbulence (Leading edge)

Self-noise (Trailing edge)

Tip vortex shedding

Component Source

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Rotor-stator configuration – noise generation mechanisms

1 Rotor leading-edge noise 2 Rotor trailing-edge noise 3 Wake interaction noise on stator

� Interaction of inflow turbulence with leading edge

� Depends on inflow turbulence

� Modeled with fan source

� Interaction of boundary layer with trailing edge

� Important for high rotation speed

� Modeled with fan source

Flow

RROOTTOORR SSTTAATTOORR

� Interaction of rotor wake with stator

� Modeled with surface dipoles

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FWH formulation for fan noise

Constructive interference:

sound of the total fan =

B x (sound of a single blade)

Sound

emitted at

BPFHs

Sum over

BLHs

Bessel function: modulation of

the Doppler frequency shift

during blade revolution

Radius where

force is applied Thrust

harmonic

Drag

harmonic

� Tonal fan noise formulation:

• Fan blade represented by a rotating point dipole (force obtained by

integration of CFD surface pressure)

• Rotation effects (Doppler shift) accounted for analytically (no rotating

mesh)

• If blade is large, automatically split into compact segments

• Captures both Tonal and Broadband components

• Needs unsteady pressure for multiple blade revolutions

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Industrial Case Study

Radial Fan Noise : internal pressure distribution

Internal SPL distribution in the blower

ducts (around 3 kHz)� Observe intake and outlet

noise maxima in external SPL distribution

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7 8 9

Microphones

Pre

ssu

re L

evel

dB

(A)

Measurement

Computed

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Fan noise applicationContra-rotating open rotors

� Interesting fuel efficiency

� Very loud tonal noise: “interaction

tones” from each rotor:

� Freq1=1BPF1+1BPF2



� …

� Each tone has a specific directivity

(interaction tones tend to radiate

radially)

� Incident field captured with

Aeroacoustic Fan source

� Installation effects captured with

FEMAO solver

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Aero-acousticsExample HVAC Duct

Source regions

(vorticity)

Flow:

Acoustic:

Dipoles on the flap Far-field acoustic radiation

101

102

103

30

40

50

60

70

80

90

100

110

frequency (Hz)

SP

L (

dB

)

CFDexperiments (Jaeger et al. 2008)

Instantaneous velocity

From consortium of German car manufacturers - Audi, BMW, Daimler, Porsche and

Volkswagen where LMS participated

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10

110

210

330

40

50

60

70

80

90

100

110

frequency (Hz)

SP

L (

dB

)


CFD results: pressure on the wall

C

101

102

103

30

40

50

60

70

80

90

100

110

frequency (Hz)

SP

L (

dB

)


101

102

103

30

40

50

60

70

80

90

100

110

frequency (Hz)

SP

L (

dB

)CFDexperiments (Jaeger et al. 2008)

B

A

B

C

- Very good agreement downstream the flap

- Overprediction in the elbow separation region

A

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Acoustic modeling

Source region (dipoles)

exterior field points

Inlet (absorbent panel Z = ρc)

� Aeroacoustic sources: distributed dipoles defined from CFD pressure

� Implementation in FEM: transformation of CFD pressure into equivalent

Neumann BCs (more details in AIAA2012-2070)

� 267 030 TETRA4 elements

� computation time: 10 s/freq

PML

FEM model

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HVAC Flap Application case - accuracy

Acoustic radiation – Averaged over all measurement points:

New Neumann-based source modeling

Experimental

Numerical (FEM)

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Noise Radiated from a Train Bogie

High speed train - CFD done with EXA Powerflow (Lattice-Boltzman) – Courtesy of Bombardier

Surface Velocity magnitude Vorticity

Pressure coefficientVelocity magnitude

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Noise Radiated from a Train Bogie

Comparison with measurements

(Pressure at 6 m on the side):

Source distribution on the train bogeySound radiated in far-field

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Flow-induced Vibrations

� Flow acts as a structural pressure loading:

� pump vibration

� Windnoise (turbulence around A-pilar, mirror)

� turbulent Boundary layer loading on fuselage or hull

� How to get pressure loading?

� Directly from CFD (compressible)

� From Aeroacoustic source propagation (side mirror

noise)

� From analytical models (Corcos, Chase)

� How to compute vibro-acoustic response

� Apply loading on structural model (modal or direct)

� Compute vibration in a weakly or strongly-coupled

model

� Apply vibration as Boundary Condition for acoustic

model with FEMAO

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Windnoise application case Description and modeling process

A-pillar Turbulence

Mic.

Glass Interior Walls

Outer Walls (Rigid)

External CFD

Model – Transient

Flow

Vibrating Surfaces

(Side Glass, Windshield)

Acoustics Model

(Car Interior)

Coupled Vibration & Acoustics Model

LMS Virtual.Lab Acoustics

Inflow

CFD Model

ANSYS Fluent

• Noise prediction at interior of the HSM (Hyundai Simplified Model released by

HKMC) caused by transient external aerodynamic sources around A-pillar

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CFD Model

• CFD Domain consists of 45 million cells

• 2nd order implicit transient formulation (time step: 2.0e-

5s)

• Total physical time = 1 s (5120 time steps)

• 4 different cases: 110 and 130 km/h, 0 and 10 deg.

Yaw

• Solver : ANSYS Fluent (Pressure Based, Double

Precision, Transient , Gradient – Least Square Cell

Based)

• Turbulence Model Transient : DDES – SST K-Omega

• Total computation time: 15 days for the transient run

and 12 Hours for steady run with Intel 2*6 core Xeon

5680, 4 m/c connected via infiniband (total 48 cores)

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Transient Flow Field of 110kph, 10deg Yaw

Velocity Contours at Z = 0.5 m Pressure Contours at Z = 0.5 m

Iso-surface of Q-Criterion colored by velocity magnitude

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Flow

Vibration

Acoustics

Aero-Vibro-acoustic modeling strategy

� compressible flow simulation captures Unsteady Turbulent flow

� contains both hydrodynamic and acoustic components �

Directly applied as structural pressure loading

� Assume vibration has no influence on the flow � uncoupled flow-

vibration approach

� Structural FE model captures dynamics of structure

� Modal approach is used (with uniform modal damping)

� Windows vibration defined as Boundary condition for Acoustic

model.

� Assume Vibration is independent from fluid loading � weakly

coupled vibro-acoustic approach (OK for target frequencies)

� Acoustic radiation computed with FEM-AO solver (adaptive

order)

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Structural model CFD loading

� CFD Surface Pressure is mapped onto Structural mesh in LMS Virtual.Lab

� 5120 time steps, dt = 2e-5s, T = 0.1s

� Pressure is transformed from time to frequency (df=10Hz) and applied as distributed

pressure loading

� No time averaging is performed here (could be done if time history is long enough to

improve convergence of predictions)

� Structural modes computed with LMS Virtual.Lab structural solver

CFD Pressure 500 Hz Structural mode shape Window FRF

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SPL Results

• Excellent match with measurements both for SPL and effect of flow speed

• Computation time: 6 hours for 400 frequencies with 4 cores Win64

• Thanks to Ashok Khondge and Myunghoon Lee from Ansys Inc. for running CFD.

• More details: See proceedings of KSNVE Conference 2013

0 deg. Yaw – 130 km/hEffect of flow speed

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Conclusions

� Virtual.Lab Acoustics powerful tool for aero-acoustics simulation

� Various aeroacoustic sources for accurate modeling:

� Dipole sources for compressible and incompressible flow description

� Quadrupole sources in FEMAO solver

� Fan sources for tonal and broadband noise

� Virtual.Lab for Flow-induced vibrations:

� Integrated vibro-acoustic solver

� Poro-elastic and visco-elastic material modeling

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Pour toutes informations complémentaires

Pour toutes informations complémentaires, vous pouvez contacter :

� Yohann MESMIN :

� [email protected]

�T : +33 (0) 1 34 52 17 55

�M : +33 (0) 6 18 55 17 60

websem aero acoustic simulation dec2013

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