Actran for Acoustic Radiation Analysis · significant noise problems appear

1 FFT & MSC Software Confidential

Actran for Acoustic Radiation Analysis VPE Workshop: Acoustic Simulation

Ze Zhou

Free Field Technologies, MSC Software Company


• 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


Overview of Actran Acoustic Applications

Sound from vibration Interior acoustics Material absorption

Duct acoustics

Structure insulation

Acoustic fatigue Aero acoustics Aircraft engine acoustics


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:


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


Acoustic Radiation into Air Two-step Weakly Coupled Vibro-Acoustic Approach


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


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


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


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’’


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


• 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


• 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


• 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


• 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


Acoustic Radiation Case Studies


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


• 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




• 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


Output specifications

field points

(microphones) field mesh


• 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.



• Various maps can be produced



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



Selective Power Evaluation

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

radiated by each part of the power train.


• “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


• Panel contribution

Results types: Panel Contribution & Element

Contribution

• Element contribution

Surface 2 Surface 3 Surface 4

Surface 1


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


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


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


Effect of Absorbing Ground – Cont’

Pass-by noise test position : rigid and absorbing road


• 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



• The acoustic mesh is shown below:

Cover: Porous material (10mm) + plastic layer

(4mm)

Infinite elements interface

Gearbox

(skin only)

Acoustic Finite

Elements


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


Effect of the cover on the directivity


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



CAE

Test

Mic2 Mic1

dB(A) scale = 5dB



• Design improvement was done according to simulation results, which

helped to reduce weight & noise

50

55

60

65

70

75

80

85

90

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


Acoustic Radiation into Water Strongly Coupled Vibro-Acoustic Modeling


• 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


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


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


Going Further For other types of acoustic problems


The Actran software suite


Thank You !

Ze Zhou Free Field Technologies, MSC Software Company

[email protected]

Date post:	31-May-2019
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Actran for Acoustic Radiation Analysis · significant noise problems appear

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