Actran for Acoustic Radiation Analysis

Actran for Acoustic Radiation Analysis 

VPE Workshop: Acoustic Simulation 

Ze Zhou 

Free Field Technologies, MSC Software Company 

FFT & MSC Software Confidential 

1

Contents 

• 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 


2

Overview of Actran Acoustic Applications 

Sound from vibration Interior acoustics 

Structure insulation Material absorption 

Duct acoustics 

Acoustic fatigue 

Aero acoustics 

Aircraft engine acoustics 


3

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: 


4

One-Way or Two-Way Coupling 

Structure 

Noise 

induces 

vibration 

One-way Two-way coupling 

(no (feedback) feedback) 

Vibration 

induces 

noise 

Air 

Engine radiating 

Sub-marine under water 


5

Acoustic Radiation into Air 

Two-step Weakly Coupled Vibro-Acoustic Approach 


6

One Way Coupled Problem: Modeling Process 

1. Structural FEA Analysis 

Mesh & 

results 

files 

2. Acoustic computations 

3. Post Processing and Analysis = Actran VI 

Maps 

FRF 

Waterfall 


7

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 


8

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 

Interior surface 

Exterior Surface 

Volume elements 


9

Infinite Elements (IFE) 

• Infinite elements: 

– cover an unbounded domain 

– have appropriate high order shape 

functions in the radial direction 

P’’ 

P’ 

1 3 

• 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 


10

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 


11

Adaptive Perfectly Matched Layer (APML) 

• 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 

Original acoustic domain 

surrounding a gear box 

Computation of PML 

thickness & elements sizes 

based on frequencies 

Automatic creation of 

PML volume mesh 

Acoustic computation 


12

Adaptive Perfectly Matched Layer (APML) – cont’d 

• APML mesh creation on a gear box sound radiation problem 

APML for 260Hz ~ 510Hz 

APML for 1025Hz ~ 1700Hz 


13

Discontinuous Galerkin Method (DGM) 

• 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) 

FWH surface 

node of the linear DGM TRI 

DGM anchors 

Equivalent 1 st order mesh 

Buffer zone 

Physical domain 


14

Discontinuous Galerkin Method (DGM) – cont’d 

• 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 m 3 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) 


15

Acoustic Radiation Case Studies 


16

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 


17


• 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 


18


• 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 


19


• Virtual microphones can be located 

anywhere in the finite and/or infinite 

element domain 

Output specifications 

• 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. 

field points 

(microphones) 

field mesh 


20


• Various maps can be produced 


21


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 


22


Selective Power Evaluation 

• Multiple surfaces can be created in order to measure the power 

radiated by each part of the power train. 


23

Results types: Waterfall Diagrams 

• “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) 


24

Results types: Panel Contribution & Element 

Contribution 

• Panel contribution 

• Element contribution 

Surface 1 

Surface 4 

Surface 3 

Surface 2 


25

Case Study 2: Floor Effect on Gearbox Sound Radiation 

• Floor effect on the pressure directivity 

With Floor 

Without Floor 

Real part of the 

pressure 

Rigid surfaces 

Infinite element 

surfaces 

Amplitude of the 

pressure 


27

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 

Sound Pressure Level 

(SPL) at the standard 

pass-by noise test 

position 


28

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 


29

Effect of Absorbing Ground – Cont’ 

Pass-by noise test position : rigid and absorbing road 


30

Case Study 4: Adding Cover to Gearbox 

• 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 

Name: 

Density 

Poisson 

ratio 

Young 

modulus 

Plastic 

900kg/m³ 

0.4 

3.33 e+08 Pa 

Damping 49% 

Thickness 

4 mm 

Name: 

Density: 

Rockwool 

1776kg/m³ 

Biot factor: 1 

Tortuosity: 1.1 

Porosity: 0.95 

Poisson ratio: 0.3 

Young modulus : 

3.33 e+08 Pa 


31 

Damping : 10%


• The acoustic mesh is shown below: 

Cover: Porous material (10mm) + plastic layer 

(4mm) 

Infinite elements interface 

Gearbox 

(skin only) 

Acoustic Finite 

Elements 


32


Effect of the cover on the directivity 

• Directivity plot shows the influence region of the cover (1110 Hz) 


33

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 


34


Mic1 

Mic2 

Test 

dB(A) scale = 5dB 

CAE 


35

S.P.L. (dBA) 

315 

400 

500 

630 

800 

1000 

1250 

1600 

2000 

2500 


• Design improvement was done according to simulation results, which 

helped to reduce weight & noise 

Point1 SPL 2000rpm 

Element Contribution to sound radiation 

90 

85 

80 

75 

70 

65 

60 

55 

50 

5dB 

BASE 

MODIFY 

1/3Oct. Band (Hz) 


36

Acoustic Radiation into Water 

Strongly Coupled Vibro-Acoustic Modeling 


37

Sound Radiation from Ship Engine Room 

• Calculating radiated acoustic power from engine room 

Nastran structure model: 

Actran strongly coupled vibro-acoustic model 

Actran structure FE model obtained 

using “Nastran to Actran translator” 

Focus on engine room 

Pressure release condition at 

water/air interface: p=0 


Symmetric surface 

38 

Water FE 

Infinite 

elements

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 


39

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 


40

Going Further 

For other types of acoustic problems 


41

The Actran software suite 


42

Thank You ! 

Ze Zhou 

Free Field Technologies, MSC Software Company 

ze.zhou@fft.be 


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Actran for Acoustic Radiation Analysis

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