dynamic response in nx nastran - structural design …€¢ random response demo dynamic response in...
TRANSCRIPT
5/22/2012
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IN THIS WEBINAR:
• FEMAP Dynamics Add-on Overview
• Frequency Response Demo
• Random Response Demo
Dynamic Response
in NX Nastran
Andrew Jabola
Senior Aerospace Stress Engineer
Structural Design and Analysis
PRESENTED BY:
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Agenda
• Introduction
• Dynamic Analysis Solutions
– Solutions
– Typical Applications
• Demo
– Frequency Response/Sine Vibration Model Setup
– Random Vibration Setup
• Conclusion
• Q/A
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Dynamic Analysis Solutions
• What is a Dynamic Analysis?– Any case where the load is time varying and inertia forces may be present
• NASTRAN Dynamics adds the following solutions to FEMAP– Transient Response
– Frequency Response
– Shock Spectrum Analysis
– Random Vibration
– DDAM Analysis
• Most Solutions offer both direct and modal solution capabilities– Direct – Applies the direct equations of motion to every grid point, providing a more accurate but more costly
answer
– Modal – Uses mode shapes to determine response and reduces overall model size
Equations of Motion – Direct Analysis
Equations of Motion – Modal Analysis
Full Mass Matrix of
System
Transforms Matrix into
Modal Coordinates
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Dynamic Analysis Solutions
• Solution Types
Type Input Output
Modal • Prescribed BC’s / No Force Input
• Natural frequencies and
corresponding mode shapes
• Stress/strain profile
Frequency
Response
• Sinusoidal excitations across a
range of frequencies
(Deterministic)
• Sinusoidal response at each
frequency
• Min/max response over frequency
range
Spectrum/Shock
• Spectrum representing the response
to a specific time history
(Deterministic)
• Maximum response if the model were
subjected to the time history
Random
• Spectrum representing probability
distribution of excitation (Non-
Deterministic)
• Response within specified range of
probabilities
Transient • Time-varying loads (Deterministic) • Time-varying response
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Typical Applications
• Typical Applications Dynamic Response– Aerospace Industry
• Heavy usage of Frequency and Random Response
– Marine• DDAM
– Vehicle Dynamics
– Civil/Structural Applications
– Heavy Equipment
– …anything where load is time varying and inertia and damping also play a factor into overall response
• Focus of webinar will be on the following use cases– Frequency Response – Sine Vibe Analysis
– Random Response – Article Verification
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Frequency Response
• Frequency Response computes the structural response to a steady-state
oscillatory excitation
– Heavily used in aerospace industry for sine vibration tests
– Provides a structural response vs. frequency for a specified range
– Used as a basis for random vibration analysis
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Frequency Response Setup
• Demonstration
– Model Overview
– Checking Modes
– Setting up Loads
– Setting up Analysis Deck
– Postprocessing Data
Model courtesy of NASA
Goddard Space Flight
Center – WFIRST
Program
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Modal Content
• Modal Mass Participation
– Ensure you’re capturing at least 80% of the mass participation or covering the
frequency range of interest
– Modal Mass Participation for first 20 modes
Mode # Frequency T1 T2 T3 R1 R2 R3
1 6.49E+01 1.00% 0.05% 2.87% 0.00% 4.10% 0.14%
2 7.99E+01 1.41% 0.24% 0.51% 0.15% 3.94% 0.01%
3 1.07E+02 0.83% 0.09% 0.04% 0.06% 0.03% 5.76%
4 1.09E+02 0.20% 4.57% 1.46% 1.99% 0.51% 0.11%
5 1.29E+02 1.41% 1.22% 0.00% 0.02% 0.18% 6.62%
6 1.94E+02 0.38% 1.32% 9.82% 0.06% 0.00% 0.00%
7 2.37E+02 0.55% 0.01% 0.22% 0.00% 0.47% 0.28%
8 2.63E+02 0.36% 0.01% 0.00% 0.00% 0.06% 0.03%
9 2.88E+02 10.29% 0.47% 0.02% 0.09% 2.06% 1.37%
10 3.24E+02 0.10% 7.98% 0.12% 2.20% 0.02% 1.28%
11 3.65E+02 0.89% 0.01% 0.33% 0.00% 0.19% 0.05%
12 3.72E+02 0.04% 0.83% 0.00% 0.18% 0.01% 0.13%
13 4.30E+02 0.47% 0.01% 0.00% 0.00% 0.04% 0.03%
14 4.59E+02 0.00% 0.00% 0.49% 0.00% 0.01% 0.00%
15 4.70E+02 0.00% 0.00% 0.01% 0.00% 0.00% 0.00%
16 4.92E+02 0.00% 0.03% 1.86% 0.01% 0.01% 0.00%
17 4.99E+02 0.00% 0.14% 0.02% 0.03% 0.00% 0.01%
18 5.05E+02 0.36% 0.06% 0.02% 0.01% 0.00% 0.00%
19 5.09E+02 0.04% 0.30% 0.00% 0.10% 0.01% 0.07%
20 5.36E+02 0.02% 0.53% 5.13% 0.06% 0.01% 0.02%
Modal Mass Participation Percentage
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Damping
• Types of Damping
• For frequency response and random response, modal
damping is typically used
• It can be specified by the following methods:
– Critical Damping (2.5%)
– Structural Damping (5.0%)
– Q Damping (20)
• Relationship shown below
Q Damping
Critical Damping
Structural Damping
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Frequency Cards
• Responses are specified at certain frequencies based off a variety of criteria using the FREQ Cards– Frequency List Methods (explicitly define frequencies by a range)
• FREQ, FREQ1, FREQ2
– Modal Frequency Methods (list frequencies based off modes and generating points around each mode)
• FREQ3, FREQ4, FREQ5
– It is common to use the modal frequency to fill in data around modes and using the list methods to catch any missing data
– Can be combined
• Refer to NX NASTRAN QRG for more detail of FREQ –FREQ5 Cards
FREQ4 Frequency
Spread
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Random Response
• Random Response is used to look at load environments that are non-deterministic
– Uses a statistical approach to analyzing the response to a structure where the instantaneous magnitude is not known at a given time
• Applications include acoustic environments from launch, earthquake analysis, and wind pressure fluctuations
– Analysis uses frequency response as basis for determining overall statistical response by multiplying the transfer functions of the frequency response and the input PSD to develop a response spectral density
• This is in turn used to determine an RMS response, which is used to determine the statistical response of the structure
Sine Vibe Transfer
Function
Input PSDResponse Spectral
Density
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Random Response Setup
• Demonstration
– Show Input PSD
– Show Additional Analysis Setup
– Postprocess Data
Model courtesy of NASA
Goddard Space Flight
Center – WFIRST
Program
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Random Response
• Standard Input
– Power Spectral Density (PSD) function, determines the input environment for the
structure
– Specs are determined by analyzing time history responses of various environments
– Typical spec shown below (Source: GSFC GEVS-STD-7000)
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Random Response
• Random Output– Random will generate the following data
• PSDF Data– Used for looking at overall spectral densities to determine areas of high response
• CRMS Data– This is cumulative RMS value that shows how the energy adds together as you increase in frequency
• RMS Response– The RMS response of each data point of interest
» This is equal to the square root of the area under the PSDF data curve
– This RMS values is typically multiplied by a factor (typically 3) to show the highest stress within a certain probability
» 3 Sigma ensures that the highest stress due to the environment will not exceed this value 99.7% of the time
• Positive Crossings– Determines the amount of cycles per unit of time occur
» Typically used for fatigue calculations
RMS Output Value
RMS Von Mises StressAcceleration Spectral Density
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VIEW THE WEBINAR RECORDING
• If you would like to see the
demonstrations from this
presentation, you can view the
recording of this webinar at:
https://structures.aero/webinar/
dynamic-response-nx-nastran/
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Tips and Tricks
• Several Tips and Tricks
– Run Modes First!
• Understand the natural frequency of the system to get an idea of what frequencies will play a
major role in your system. This should be performed in combination with understand modal mass
participation to determine what modes will likely be significant
– Ensure that modal analysis captures full modal content
• Ensure that you are capturing at least 80% of mass participation or full range of frequencies of
interest
– Verify input by looking at response at constraint location
• This will verify that your load is correct for the model
– Turn on RESVEC
• Captures the higher modal content that is not captured in lower modes frequency content
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Conclusion
• FEMAP Dynamics Add-on adds several dynamic solutions to the FEMAP
environment:
– Transient Response
– Frequency Response
– Random Response
– Shock
– DDAM
• Bulk of analyses focus on random and frequency response, using the modal
method for solution
• Dynamic Analyses are very use for time varying loadings and any places
where inertia and damping play a major affect on the response of a structure
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Structural Design and Analysis (Structures.Aero)
Structural Analysis
• Team of 14 engineers that help our clients
design lightweight and load efficient structures.
• We service aerospace companies and other
industries that require high level analysis.
• Specialty in composites and lightweight
structures
• Tools used include hand analysis, HyperSizer,
Femap, NX Nastran, Fibersim, NX, Solid Edge,
Simcenter 3D, LS Dyna, and LMS.
Software Sales and Support
• Value added reseller providing software, training,
and support for products we use on a daily
basis.
• Support Femap, NX Nastran, Simcenter 3D,
Fibersim, Solid Edge, and HyperSizer.
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GET DYNAMIC RESPONSE
• You can acquire the Dynamic
Response add-on for Femap with
NX Nastran for less than $3K.
• Contact Marty Sivic for a quote
For questions about pricing, or to see
a demo, please contact Marty Sivic.
Marty SivicDirector of Sales
724-382-5290
5/22/2012
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For questions on the material covered
today, please contact Andrew Jabola.
For questions about pricing, or to see a
demo, please contact Marty Sivic.
Questions?
Marty SivicDirector of Sales
724-382-5290
Andrew JabolaSenior Aerospace Stress Engineer
703-935-2827