actran for acoustic radiation analysis · significant noise problems appear
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1 FFT & MSC Software Confidential
Actran for Acoustic Radiation Analysis VPE Workshop: Acoustic Simulation
Ze Zhou
Free Field Technologies, MSC Software Company
2 FFT & MSC Software Confidential
• Overview of Actran Acoustic Applications
• Acoustic radiation & vibro-acoustic coupling
– One way numerical coupling
– Two way strong numerical coupling
• Acoustic radiation into air
– Simulation process
– Techniques: Finite Element, Infinite Elements, (Adaptive) Perfectly Matched
Layers, Ffowcs Williams Hawkings, Discontinuous Galerkin Method
– Examples: powertrain, gearbox, intake manifold, tire
• Acoustic radiation into water
– Added mass effect of heavy fluid
– Example: ship engine room vibration & radiation
Contents
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Overview of Actran Acoustic Applications
Sound from vibration Interior acoustics Material absorption
Duct acoustics
Structure insulation
Acoustic fatigue Aero acoustics Aircraft engine acoustics
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Acoustic Radiation Problems
Car Air Intake Vibration Acoustic radiation
Acoustic radiation into air:
Ship hull vibration from engine room Acoustic radiation into sea water
Acoustic radiation into water:
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One-Way or Two-Way Coupling
Structure
Air
Vibration
induces
noise
Noise
induces
vibration
One-way coupling
(no feedback)
Two-way coupling
(feedback)
Sub-marine under water Engine radiating
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Acoustic Radiation into Air Two-step Weakly Coupled Vibro-Acoustic Approach
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One Way Coupled Problem: Modeling Process
3. Post Processing and Analysis = Actran VI
2. Acoustic computations 1. Structural FEA Analysis
Maps
Mesh &
results
files
FRF Waterfall
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Modeling Process - Inputs
• Structure mesh
• Structure vibration results
– On the structure surface
– Format
• Nastran, Ansys, Abaqus
• Displacement, Velocity or Acceleration
• physical coordinates or modal coordinates (modes shapes + participation factors)
Actran Acoustic Radiation
Structure mesh
& vibration results
Acoustic mesh
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Modeling Process - Acoustic Mesh
• Acoustic mesh is comprised of three parts
– Interior surface: surface wrap mesh of the whole structure, for mapping the
structure vibration
– Exterior convex surface: a convex shape surrounding the interior
– Volume elements between the two surfaces
Exterior Surface
Interior surface
Volume elements
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Infinite Elements (IFE)
• Infinite elements:
– cover an unbounded domain
– have appropriate high order shape
functions in the radial direction
• Infinite elements:
– ensure there are no wave reflections
at the FE/IE interface
– Provide accurate acoustic results beyond the FE
domain
– Provide radiated power across the IFE surface
S
P
1 3
P’ P’’
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Perfectly Matched Layers (PML)
• Alternative / complement to infinite elements
for the radiation in free field
• Extra-layer of finite elements used to
progressively damp the acoustic wave
non-reflecting boundary condition
• PML Leads to symmetric contribution of FEM
matrix
• Far field acoustic pressure by FWH (Ffowcs
Williams Hawkings) computation
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• Automatic creation of the mesh supporting the perfectly matched
layers
• Adaptive thickness and element sizes for each frequency band
• Benefits:
– Reduced meshing effort for modeling sound radiation problems
– Optimized computation time for the each desired frequency
Adaptive Perfectly Matched Layer (APML)
Original acoustic domain
surrounding a gear box
Computation of PML
thickness & elements sizes
based on frequencies
Automatic creation of
PML volume mesh Acoustic computation
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• APML mesh creation on a gear box sound radiation problem
Adaptive Perfectly Matched Layer (APML) – cont’d
APML for 1025Hz ~ 1700Hz APML for 260Hz ~ 510Hz
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• Actran implements a iterative time domain DGM solver, solving
Linearized Euler Equation (LEE)
• Element interpolation order is automatically defined by the software
based on element size, frequency and flow (when applicable)
• Time step is automatically computed on each element, depending on
element size, element order, and flow (when applicable)
Discontinuous Galerkin Method (DGM)
Equivalent 1st order mesh
node of the linear DGM TRI
DGM anchors
FWH surface
Physical domain
Buffer zone
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• Some advantage of DGM: 1) Handle very large problems
into high frequencies, 2) Highly scalable, 3) Low RAM
requirement
• Actran DGM was initially developed for a specific
application: aircraft engine acoustics. With typical
computation involves:
– 100 ~ 200 m3 of air, with shear flow layer
– Large number of CPU’s for parallel computation
– 2 ~ 4 GB of RAM per CPU
• Recently (Actran 14), Actran DGM is extended to perform
acoustic radiation from vibrating structure as well
– Reading structure surface vibration as excitation
– Scattering problem can also be solved (acoustic scattering by a
car or truck)
Discontinuous Galerkin Method (DGM) – cont’d
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Acoustic Radiation Case Studies
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Case Study 1: Truck Powertrain
• A complete truck powertrain with length around 2.5 meters
– The structure vibration is computed using structure FEA software
– The vibration results are used as the excitation of the acoustic radiation problem
solved by Actran
1m20
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• Map the structure results on the acoustic surface
• Mapping based on Integration method
– The geometries might be (slightly) different
– The mesh sizes can be different (no loss of information from FEA)
Actran
inner surface
FEA
outer surface
Case Study 1: Truck Powertrain
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Case Study 1: Truck Powertrain
• Propagation
– Near field: 4 Finite Elements per wavelength ( with special integration rule )
– Far field: the Infinite Elements (free field condition + far field results)
– Note: the infinite elements are surface elements on the boundary
• Tetra volume meshing
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Output specifications
field points
(microphones) field mesh
Case Study 1: Truck Powertrain
• Virtual microphones can be located
anywhere in the finite and/or infinite
element domain
• Multiple control surfaces to compute the
radiated power
• Maps for different frequencies
– on the acoustic mesh or/and
– on a mesh dedicated to the post-processing
(named field mesh in Actran)
– plot acoustic pressure, acoustic intensity, etc.
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Case Study 1: Truck Powertrain
• Various maps can be produced
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Case Study 1: Truck Powertrain
Experimental Validation
• For the complete set of frequency, regimes and microphones, a
maximum of 2dB difference has been detected (marks: 5dB)
Sound with
increasing RPM
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Case Study 1: Truck Powertrain
Selective Power Evaluation
• Multiple surfaces can be created in order to measure the power
radiated by each part of the power train.
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• “Waterfall” are diagrams where the
response is plot versus both
frequencies and the engine orders
(RPM)
• Such diagram can be obtained after
a single Actran computation thanks
to the multi-load case capability
• Some phenomena can be identified
as system dependant (vertical lines
on the waterfall), e.g. structure
modes, …
• Some phenomena can be identified
as excitation dependant (diagonal
lines on the waterfall)
Results types: Waterfall Diagrams
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• Panel contribution
Results types: Panel Contribution & Element
Contribution
• Element contribution
Surface 2 Surface 3 Surface 4
Surface 1
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Case Study 2: Floor Effect on Gearbox Sound Radiation
• Floor effect on the pressure directivity
With Floor Without Floor
Real part of the
pressure
Amplitude of the
pressure
Infinite element
surfaces Rigid surfaces
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Case Study 3: Tire sound radiaiton
• Smooth HQ6784 Tire of dimensions :
– radius 0.314 m
– width including sidewalls = 0.355 m
• Tire deformation produced by Chalmers University :
– loaded Tire (3000 N)
– rolling on a rigid or absorbing ground at a speed of 80 km/h
– 256 frequencies (from 0 to 2800 Hz with a step of 11 Hz)
7.5 m
1.2 m
7.5 m
1.2 m
Sound Pressure Level
(SPL) at the standard
pass-by noise test
position
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Effect of Absorbing Ground
• The road is either considered as rigid (perfectly reflecting) or absorbing.
In the latter case the absorption is given, in third-octave band :
Road absorption defined by :
• admittance on surface of mesh ground
• infinite admittance in infinite ground
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Effect of Absorbing Ground – Cont’
Pass-by noise test position : rigid and absorbing road
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• A 2-layers cover is placed near to the
gearbox
– A thin plastic layer of 4 mm thickness
modeled by 2D shell
– A foam layer of 10mm thickness modeled by
volume porous elements
Case Study 4: Adding Cover to Gearbox
Name: Rockwool
Density: 1776kg/m³
Biot factor: 1
Tortuosity: 1.1
Porosity: 0.95
Poisson ratio: 0.3
Young modulus : 3.33 e+08 Pa
Damping : 10%
Name: Plastic
Density 900kg/m³
Poisson
ratio 0.4
Young
modulus 3.33 e+08 Pa
Damping 49%
Thickness 4 mm
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Case Study 4: Adding Cover to Gearbox
• The acoustic mesh is shown below:
Cover: Porous material (10mm) + plastic layer
(4mm)
Infinite elements interface
Gearbox
(skin only)
Acoustic Finite
Elements
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• Directivity plot shows the influence region of the cover (1110 Hz)
Case Study 4: Adding Cover to Gearbox
Effect of the cover on the directivity
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Case Study 5: Manifold
• Mazda has developed a new engine in order to reduce
the fuel consumption as well as the weight
• To achieve this, Mazda decided to use a thin resin intake
manifold
• Consequence: many structure modes occur because of
the low rigidity of the intake manifold and therefore some
significant noise problems appear
• Mazda had to consider many structural modifications in
order to fix this problem
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Case Study 5: Manifold
CAE
Test
Mic2 Mic1
dB(A) scale = 5dB
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Case Study 5: Manifold
• Design improvement was done according to simulation results, which
helped to reduce weight & noise
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315
400
500
630
800
1000
1250
1600
2000
2500
1/3Oct. Band (Hz)
S.P
.L. (
dBA
)
Point1 SPL 2000rpm
BASE
MODIFY 5dB
Element Contribution to sound radiation
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Acoustic Radiation into Water Strongly Coupled Vibro-Acoustic Modeling
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• Calculating radiated acoustic power from engine room
Sound Radiation from Ship Engine Room
Nastran structure model:
Actran strongly coupled vibro-acoustic model
Infinite
elements Symmetric surface
Pressure release condition at
water/air interface: p=0
Actran structure FE model obtained
using “Nastran to Actran translator”
Water FE
Focus on engine room
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Results - Added Mass Effect of Water
• The influence of surrounding air
is negligible compared to the
added inertia of the water
• At higher frequencies, we clearly
see:
– the frequency shift
– the decrease of vibration amplitude
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Results – Structure coupling with Multiple Fluids
Both fluids and shell share the same node coupling handled by Actran
– Duplication of pressure DOF for two fluids
– Based on component identification
– Pressure discontinuity is insured over the shell
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Going Further For other types of acoustic problems
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The Actran software suite
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Thank You !
Ze Zhou Free Field Technologies, MSC Software Company