Coupled Exterior Acoustics Analysis in MD Nastran

Joe Maronick, Gaowen YeMSC Software

2

Agenda

• Introduction• Theory of Acoustics Analysis • Existing Interior Acoustics Capability• New Exterior Acoustic Capabilities• Some Test Examples• Conclusion Remarks

3

Introduction• Objectives of NVH analysis

• Minimize peak and overall vibration• Minimize noise• Maximize “ride comfort”

• Targets• Component response• Fully assembled vehicle• Powertrain

4

Interior vs. Exterior Acoustics

• Interior Acoustics• Fluid domain is bounded.

• Automotive interiors• Aircraft cabins• Acoustic devices• Etc.

• Exterior Acoustics• Domain is unbounded.

(or “infinite”).• Radiated engine noise• Exhaust pipe system• Acoustic devices

• Loudspeakers• Microphones

Structural

Interior Cavity

5

Agenda

• Introduction• Theory of Acoustics Analysis• Existing Interior Acoustics Capability• New exterior Acoustic Capabilities• Some Test Examples• Conclusion Remarks

6

Theory of Acoustic Analysis

• Basic fluid equations• Euler’s momentum equation• Continuity equation• Isentropic state• Compressibility

( )[ ] pvvv −∇=+∇⋅r&

rrρ

0=⋅∇+∂∂ vt orρρ

:::::

Bcpvρ Density

VelocityPressureSpeed of soundBulk modulus

ρ∂∂= pc2

2cB oρ=

7

Theory of Acoustic analysis (Cont.)

• Acoustic equations in MD Nastran are based on following assumption.

• small motion• negligible convection & net mean flow• Locally isentropic pressure-density

• Speed-of-sound state eq

• With above assumptions obtain:

• Acoustic Wave Equation

uvr&

r=

pu −∇=r&&ρ

( ) ( )ργρ pp =∂∂

oopc ργ /=

011 22

2

=∇−∂∂ p

tp

B oρ

8

Theory of Acoustic analysis (Cont.)

• Structure-Acoustic boundary condition at fluid-structure interface.

• Coupling matrix is generated to satisfy it.

• Equations of motion for both structure and fluid are as follows.

nunP &&ρ−=∂∂ /

∫= S ijij dsNNA

⎥⎦

⎤⎢⎣

⎡=⎟⎟

⎠

⎞⎜⎜⎝

⎛

⎥⎥⎦

⎤

⎢⎢⎣

⎡ −+⎟⎟⎠

⎞⎜⎜⎝

⎛⎥⎦

⎤⎢⎣

⎡QP

pu

KAK

pu

MAM

f

Ts

f

s

&&&

&&

00

9

Theory of Acoustic analysis (Cont.)

• Acoustic Boundary Conditions

• No structure, No BC Rigid Wall•• 1 RB mode • Fully reflecting

• BC p = 0• Free reflecting flow, ie: ocean wave• No RB mode• Fully reflecting

• Infinite element Exterior Acoustics• Non-reflecting

RTp ρ=0≠p

10

Agenda

• Introduction• Theory of Acoustics Analysis • Existing Interior Acoustics Capability• New Exterior Acoustic Capabilities• Some Test Examples • Conclusion Remarks

11

Fully coupled fluid-structure analysisSound pressure from structure vibrationStructure vibration from acoustic sourceInteraction of structure vibration with acoustic pressure

Based on small motion theoryNo fluid flow analysis

Modeling of fluid domain Three dimensional solid element:CHEXA, CPENTA, CTETRAMaterial property: MAT10Solid acoustic absorber and barrier elementsCHACAB/PACABS, CHACBR/PACBAR Shell acoustic absorber CAABSF

Method for Interior Acoustics

Finite Elements : Structure

Finite Elements : Fluid

12

Supported Solutions

• Acoustic eigenvalue analysis (uncoupled) MODES• Boundary of fluid domain is rigid

• Acoustic/Structure eigenvalue analysis (coupled) DCEIG / MCEIG• Coupled structure and fluid analysis

• Frequency response analysis DFREQ / MFREQ• Excitation by force or acceleration to structure• Acoustic excitation to air• Frequency response of acoustic pressure and structural vibration

• Transient Analysis DTRAN / MTRAN• Excitation by force or acceleration to structure• Acoustic excitation to air• Transient response of acoustic pressure and structural vibration

13

Interior Acoustics Computation Enhancers• Superelements

• External component superelements• Encompassing fluid-structure superelements for interior acoustics• Automatic external superelement optimization (AESO)

• Design optimization• Design targets for acoustic pressures & structural responses achieved• Test-analysis correlation & response matching

• ACMS / SMP / DMP• Eigenvalue analysis for large scale model can be efficiently & effectively

solved with parallel & ACMS within allowable engineering time.

• Auto-FastFR• Frequency response speed-up of modal solution with varying structural

dampings

• Krylov solver• Krylov subspace method for direct frequency response analysis• Matrix decomposition at frequency subset, with algorithm for approximating

remaining frequencies. Fewer dynamic matrix decompositions. • Very efficient for solid models with less modal density & many small forcing

frequencies.

14

Application Example - Interior Acoustics

• Reduction of sound pressure at passenger’s ear location of automotive compartment

• Integration with Design optimization

FR E/ NTG z ex c i t a t i on - SPL a t Pas s en ger ' s ea r

35

40

45

50

55

60

65

20 25 30 35 40 45 50 55 60Fr equency( Hz)

SPL/

Exci

te(d

B)

Bef or e Af t er

Presented at MSC’s VPD Conf by Nissan Motor Co.

15

Agenda

• Introduction• Theory of Acoustics Analysis • Existing Interior Acoustics Capability• New Exterior Acoustic Capabilities• Some Test Examples• Conclusion Remarks

16

Used to model acoustic radiation, reflection & scattering

Study sound pressure in the Far Field and radiated acoustic power through surfaces in the Far Field

Both modal solution and direct solution are supported

Combined interior and exterior acoustic problem can be solved

Can be combined with various high performance computing methods of MD Nastran

Capabilities of Exterior Acoustics

17

Infinite elements are required for exterior acoustics to express“unbounded domain”.Integration of proven and tested infinite elements technology from MSC Actran

No longer necessary to transfer data between different programs.

• Infinite elements are attached to the boundary of the acoustic finite element mesh (vicinity of radiating structure is modeled by solid elements referencing MAT10)

• Provides correct non-reflecting B.C.

Method for Exterior Acoustics

Finite Elements : Structure

Finite Elements : Fluid

Infinite elements: Fluid

18

Infinite Element Terminology

• Geometry is describedby Base and Pole.

• Base is attached to thefinite fluid domain.

• Acoustic pressure isexpanded into a powerseries of ( 1/r ).

• Number of terms in series is called radial interpolation order.

• r = 5 is recommended

r = distance from pole

19

Fluid Mesh, Infinite Elements, and Quarterof Field Point Mesh

20

Results• Standard acoustic results are available in

the finite domain.• Pressure• Velocity

• Wetted structure normal results• Intensity• Power (thru panels)

• Additional results are available at field points inside the infinite domain.

• Pressure• Velocity• Power

• If a field point mesh is defined, the following results are also available

• Intensity component normal to field point mesh• Acoustic power through the field point mesh

21

Exterior Acoustics Computation• Infinite elements available in direct & modal frequency

response solutions.

• Unsymmetric dynamic matrices• UMFPack – high-speed unsymmetric decomposition in MD Nastran• Increased memory requirements

• Mixed modal structure, direct fluid solution is possible• Most accurate with little performance penalty• External structure superelements away from wetted surface• ACMS (structure) compatible with exterior direct solution

• SMP / DMP• Nastran parallel capabilities supported in exterior acoustics

• Design optimization• Exterior acoustic response optimization forthcoming in MD Nastran• Test-analysis correlation & response matching

22

Agenda

• Introduction• Theory of Acoustics Analysis • Existing Interior Acoustics Capability• New Exterior Acoustic Capabilities• Some Test Examples• Conclusion Remarks

23

EX1: A Simple Acoustic Box Model

81928192air_box163844096plate3968027392Total

16641664infinite1344013440 air

Plate128Plate64

• Interior/Exterior-Structural Coupling• Acoustic Source within the lower box

• Source Strength:0.01• MD Nastran

• SOL111, Iterative• 246 modes

• MSC Actran • Krylov solver

• Number of Elements

24

Results: FRF of the Center Plate Grid

Nastran vs Actran : Plate 64

1.00E- 05

1.00E- 04

1.00E- 03

1.00E- 02

1.00E- 01

1.00E+000.0 50.0 100.0 150.0 200.0

Hz

Mob

ility

air32_plate64_nast_Amair32 plate64 actran Am

Nastran vs Actran : Plate 128

1.00E- 05

1.00E- 04

1.00E- 03

1.00E- 02

1.00E- 01

1.00E+000.00E+00 5.00E+01 1.00E+02 1.50E+02 2.00E+02

Hz

Mob

ility

air32_plate128_nast_Amair32 plate128 actran Am

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Results: Sound PressureComparison of Acoust ic Pressure

1.0E- 5

1.0E- 4

1.0E- 3

1.0E- 2

1.0E- 1

1.0E+0

0 20 40 60 80 100 120 140 160 180 200Frequency (Hz)

dB

P64 NastranP64_ActranP128 NastranP128 Actran

MD Nastran results are in good agreement with those from MSC Actran

MD Nastran vs. MSC Actran

26

Performance Comparison(SGI Altix, ProPack 3.04)

CPU time

020000400006000080000

100000120000140000

Plate64 Plate128

CPU

[sec

]actranSol111 iter

MD Nastran is very powerful for coupled simulations which have many structure elements.

Other codes have difficulty in solving large acoustic problems

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171.41.8334

10.8036.06

SparseR1A

R2R2R2Version

171.91.5336

3.40 11.37

Sparse

B

59.51.4250

0.882.99

UMFPACK

D

5.83Elapsed time (hr)Jacobi (Iter)Method

122.61.061

Mem (MB)Disk (GB)I/O (GB)

1.74FRRD1 Time per Freq (min)

CRun Case

36.1

11.45.8 3.0

05

10152025303540

R1 R2 R2 R2

Sparse Sparse Iterative UMF

Elap

sed

time

(hr)

SOL 111, Plate 128, No-ACMS

Performance Comparison by Diff. Methods

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EX2: Powertrain (Volvo)

• Model Descriptions• Total nodes 473,000

External acoustic nodes 36,200• Total elements 480,000

External acoustic elements 194,000Infinite elements 3,800

• Analysis Conditions• Modal frequency (SOL 111), iterative• 185 structural modes (0 ～3000Hz)• Forcing frequencies : 51 Freq.• IBM Power5 1.85GHz CPU• MEM=12GB, SMEM=5GB

• Performance Results • Serial run & 1MDACMS• Elapsed time = 4.6 hr / 3.9 hr• CPU time = 2.8 hr / 3.3 hr

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Performance Comparison by Diff. Methods

2.232577

2.916.22

UMFPACKC

2.232499

5.258.18

Jacobi(Iter)B

2.332636

16.017.33

SparseA

Elapsed (hr)

Method

Mem (GB)Disk (GB)I/O (GB)

FRRD1 per Freq. (min)

Test case

17.33

8.186.22

0

5

10

15

20

Sparse Iterative UMFPACK

Elap

sed

(hr)

• IBM Power4 1.45GHz 4CPUs; 32GB RAM• MEM=4GB, SMEM=2GB, MIO=2GB• Structural Modes by ACMS

30

EX3: Full Vehicle Acoustics • Model Descriptions

• Total nodes 421,000External acoustic 58,800Internal acoustic 3,300

• Total elements 708,000External acoustic 329,000Internal acoustic 2,500Infinite elements 7,700

• Analysis Conditions• Modal Frequency (SOL 111)• 2646 structural modes (0 ～375Hz)• Forcing frequencies : 100 Freq.• HP DL585 : AMD Opteron 2600MHz• MEM=6GB, SMEM=1GB

• Performance Results• DMP=4 (ACMS)• Elapsed time = 12.6hr• CPU time = 12.5 hr

Interior/exterior-Structural Coupling

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Agenda

• Introduction• Theory of Acoustics Analysis • Existing Interior Acoustics Capability• New Exterior Acoustic Capabilities• Some Test Examples• Conclusion Remarks

32

Concluding Remarks• Interior, exterior, and combined interior/exterior acoustic

problem can be solved together with MD Nastran. • Efficient modeling and accurate analysis

• MD Nastran powerful solver as well as ACMS/DMP and superelement can be used together

• Greatly reduces the computational time of solving very large scale interior/exterior acoustic problems

• Reduction of noise and vibration can be achieved with MD Nastran design optimization.

• Almost all responses can be employed as design response in Design optimization.

• Responses from infinite elements and field point mesh will be supported from MD Nastran R3

Thank you!

MSC Software