time nvh analysis – chassis example

Chapter 14: Time NVH Analysis – Chassis Example

14 Time NVH Analysis – Chassis Example

Summary 231

Introduction 232

Requested Solutions 232

Model Details Time NVH scheme 232

FEM Solution 233

Results 235

Modeling Tips 237

Input File(s) 238

231CHAPTER 14

Time NVH Analysis – Chassis Example

SummaryTitle Chapter 14: Time NVH Analysis – Chassis Example

Features A potentially nonlinear periodic transient dynamic response of a chassis sub-frame analysis is followed by a fast Fourier transform to extract the modes and frequencies that characterize the dynamic solution which is compared to traditional linear modal analysis.

Geometry

Material properties , ,

Analysis type TIMNVH analysis (SOL 700)

Boundary conditions Free

Applied loads Vertical impulse load applied at point

Element type 4-node shell element

FE results Transient response, FFT, mode shapes and frequencies

L4

W1 W2

L1 L2 L3

W1= 993W2= 1,182L1= 1,518L2= 865L3= 927L4= 361Size of rectangular hollow beam: 53x111 to 53x191 depending on locations.Thickness of shell: 3.5

Units: mm

A F

G L

E 2.10x105N mm2= 0.3= 7.89x10 9– ton mm

3=

A

A

CB

HD

EF

G

I J

KL

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

0.00E+00 2.00E+01 4.00E+01 6.00E+01 8.00E+01 1.00E+02 1.20E+02 1.40E+02 1.60E+02

Am

plit

ude

Frequency (Hz)

901581

901641

901697

901865

902061

902097

902580

902595

902609

902797

902996

903063

MD Demonstration Problems

CHAPTER 14232

IntroductionThis is an example of a virtual dynamic test. A chassis of a car was modeled and a vertical impulse loading was applied at one of front corner points. Time histories were obtained at select chassis locations and they were translated to frequency domain by applying Fast Fourier Transform (FFT) to extract mode shapes and frequencies for 12 sampling points.

Requested SolutionsAcceleration time histories are obtained at 12 points and they are translated to a frequency domain. Dynamic properties such as modal natural frequencies and mode shapes are then computed. The results are then compared with those of Nastran SOL 103 for validation purposes.

Model Details Time NVH scheme

Figure 14-1 Flow Chart of TIMNVH Scheme

MD Nastran bdf Model (impulse loading)

Obtain Time-history Results - Displacement - Velocity - Acceleration (default)

Time domain results -> Frequency domain results

Extract dynamic properties: Natural frequencies and Mode shapes (f06 and modes.out files)

SOL 700

FFT

Find and compare peaks

Add PARAM, S700NVH1, TIMNAT and TIMSML cards

Re-run MD Nastran SOL 700

Use primary time history or FFT results

No

Final dynamic properties Is acceptable? Yes

233CHAPTER 14


FEM SolutionThere are two models. The first model is the initial run to determine the rough dynamic properties of the structure and second model is a re-run of the first job to find the accurate and final results using the previous time history results.

Applied Load and Selected Location for Time HistoryTo compute the dynamic responses of the chassis, a vertical impulse load is applied at the front corner as shown in Figure 14-2. Using FORCE and TABLED entries as shown below, a maximum of 0.01 tons impulse point loading is applied to node 902517.

FORCE 3 902517 0 .01 0. 0. -1.TABLED1 1 -10. 0. 0. 0. .001 1. .002 0. 10. 0. ENDT

The acceleration time histories at 12 points on the chassis are computed (see Figure 14-2) to obtain the modal responses.

Figure 14-2 Applied Impulse Loading and Nodes Selected for Getting the Acceleration Responses

Primary JobThe end time in transient run is defined by using 100 time steps at 0.4e-4 sec. for each increment. The end time is the product of these two entries. Notice here, the time increment is only for the first step. The actual number of time increments and the exact value of the time steps are determined by MD Nastran solver during the analysis. The time step is a function of the smallest element dimension during the simulation.

TSTEPNL 1 100 .01 1 ADAPT 2 10

TIMNVH defines the Time NVH analysis as explained below.

TIMNVH, 1, , , 1.0, 1000., 3, 0.0005, 2,++, 0, 3, 1, 0.015, 0, 3, 13, .0030,+

0 2 4 6 8 100.000

0.005

0.010

Time (ms)

Load (ton)

A

CB

HD

EF

G

I J

KL


CHAPTER 14234

The range of natural frequencies to obtain is from 1.0 Hz to 1000 Hz and translational degrees of freedom for z-direction is only considered (3). The sampling rate is 0.0005 seconds. The peaking criterion is two, which means that a peak is selected if the amplitude of the number of increasing and decreasing points around a peak is equal or greater than 2.

Acceleration is selected for the response (0) and translational eigenvectors are only requested as ASCII format (3). Eigenvalues are normalized by 1.0 (1) and 0.015 is selected as CLOSE value which means if there are two modes which distance is smaller than 0.015 Hz, it is assumed to be the same mode. ACII file format of natural frequencies and eigenvalues are asked (0) and translational time histories of z-direction are requested (3). Frequency-amplitude data of z-direction are requested (13) and a peak whose amplitude is less than 0.0030 times the maximum amplitude is ignored (.0030)

+, 901581, 901641, 901697, 901865, 902061, 902097, , ,++, 902580, 902595, 902609, 902797, 902996, 903063

The grid points 901581, 901641, 901697, 901865, 902061, 902097, 902580, 902595, 902609, 902797, 902996 and 903063 are selected to obtain time history responses for Time NVH analysis.

TIMNVH,1, , , 1.0, 1000., 3,.0005, 2,++, 0, 3, 1, 0.015, 0, 3, 13, .0030,++, 901581, 901641, 901697, 901865, 902061, 902097, , ,++, 902580, 902595, 902609, 902797, 902996, 903063

Re-running JobTo find the accurate modal properties, a re-run is required using the previous time history data. Only three entries are different from the initial job;

PARAM, S700NVH, TIMNVH and TIMNAT

The value of PARAM, S700NVH is assigned to 1 for using the previous time history binary data (binout0000). In TIMNVH entry, the PEAK option (in the first line) is changed from 2 to –2, which will require defining the TIMNAT entry.

TIMNAT is used to specify the natural frequencies selected from amplitude-frequency plot from the initial run. The natural frequencies close to 35, 43, 49, 101, and 108 Hz’s are obtained as the natural frequencies.

PARAM,S700NVH1,1TIMNVH,1, , , 1.0, 1000., 3,.0005, -2,++, 0, 3, 1, 0.015, 0, 3, 13, .0030,++, 901581, 901641, 901697, 901865, 902061, 902097, , ,++, 902580, 902595, 902609, 902797, 902996, 903063 TIMNAT,1,35.,43.,49.,101.,108.

235CHAPTER 14


ResultsThere are three result files from Time Domain NVH analysis.

• mode.out: Results for the natural frequencies and eigenvalues.

• ampl-freq- 00901865-3.txt: amplitude-frequency output of degree of freedom =3 at grid point 901865.

• time-hist- 00901865-3.txt: time history output of degree of freedom =3 at grid point 901865.

From the ampl-freq-*** files, the frequency-amplitude plots are shown in Figure 14-3. Using the plot, the modal frequencies are specified in TIMNAT option to refine the dynamic property results.

Figure 14-3 Frequency-Amplitude Plots At 12 Nodes

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

0.00E+00 2.00E+01 4.00E+01 6.00E+01 8.00E+01 1.00E+02 1.20E+02 1.40E+02 1.60E+02

Am

plit

ude

Frequency (Hz)

901581

901641

901697

901865

902061

902097

902580

902595

902609

902797

902996

903063

A

CB

HD

EF

G

I J

KL

1 2

3

4 5

6 7


CHAPTER 14236

Figure 14-4 Comparison of Mode Shapes and Frequencies for SOL 103 and SOL 700

The small peaks for modes 4 and 5 are barely observable in Figure 14-3 and arise because of the selection of the type of impulse loading. These lateral modes exhibit a low participation when the impulse loading is vertical. For a certain set of impulse loads, certain modes may not be excited and the FFT only picks up the excited modes that significantly participate in the transient response.

Mode SOL103 SOL 700 Diff(%) Comparison

1 36.0170 35.0002 2.82% Vertical motion dominant


3 52.5065 49.0003 6.68% Lateral motion dominant

4 67.4281 Small peak - Lateral motion dominant

5 84.7220 Small peak - Lateral motion dominant



36.01735.000

43.95243.000

52.50649.000

67.428 -

84.722 -

111.016108.001

101.969 101.001

SOL 103 Frequency HzSOL 700 Frequency Hz

1 2 3

4

Mode #

5 6

7

237CHAPTER 14


Results show that even though the vertical mode shapes of modes 2 and 3 are similar, their amplitude and lateral modes are quite different. The results are compared in Figure 14-5.

Figure 14-5 Comparison of Vertical Mode Shapes Between Mode 2 and 3

Sample OutputThe final response from the FFT steps for the 12 sampling points are contained in a file called modes.out which contains the eigenvalues (frequencies) and eigenvectors (mode shapes) in the form:

Modeling TipsTo get more accurate data, options of TIMNVH and TSTEPNL entry could be changed. For example, increasing the end time (defined as 1 second in this analysis) can result in higher resolution (the frequency increment in the frequency-amplitude plot). The resolution is determined as:

Vertical mode shape of mode 2 Vertical mode shape of mode 3

MODES 1 5EI GV 1 3. 500018E+01 901581- 3. 32998498E- 02- 2. 49243337E- 04 7. 08997618E- 01 901641- 4. 29914555E- 02 7. 70991520E- 05- 1. 08571907E- 01 901697- 4. 15069142E- 02 2. 55256359E- 04- 6. 31611930E- 01 901865 4. 37855265E- 02- 1. 51550001E- 04- 4. 18557096E- 01 902061 7. 97601410E- 02 4. 34427876E- 04 5. 67705213E- 01 902097 8. 68013598E- 02 8. 02417982E- 03 1. 00000000E+00 902580- 3. 38588683E- 02 2. 97715028E- 04 7. 28400224E- 01 902595- 4. 37831381E- 02 2. 30181446E- 04- 9. 77437006E- 02 902609- 4. 24521220E- 02- 1. 61168521E- 04- 6. 35288211E- 01 902797 4. 11242103E- 02- 3. 00773060E- 04- 4. 29582120E- 01 902996 7. 69986448E- 02 7. 40153667E- 04 5. 51699503E- 01 903063 8. 41026922E- 02- 3. 47784987E- 03 9. 82653769E- 01

} Frequency

x-component y-component

eigenvector

z-component

SampleGrid IDS

1 mode { st

1sample end time - sample start time

------------------------------------------------------------------------------------------ 11 sec 0 sec–

------------------------------------- 1 Hz= =


CHAPTER 14238

To increase the maximum frequency in the frequency-amplitude plots, the sampling rate which is defined as 0.015 seconds in this example decreases. The maximum frequency of this example is computed as:

Input File(s)

File Description

nug_14a.dat Initial run to find rough dynamic properties

nug_14b.dat Re-run to compute accurate dynamic properties

nug_14c.dat SOL 103 model

112--- sampling rate ------------------------------------------ 1

12--- 0.015 sec -------------------------------- 133.33 Hz= =

http://www.mscsoftware.com/doc/nastran/mdug/input_files/ch014/nug_14a.dat

http://www.mscsoftware.com/doc/nastran/mdug/input_files/ch014/nug_14b.dat

http://www.mscsoftware.com/doc/nastran/mdug/input_files/ch014/nug_14c.dat

time nvh analysis – chassis example

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