Vibration Analysis and Normal Modes Analysis - FEMAP and NX Nastran Technical Seminar

Webinar on Normal Modes Analysis

Introduction and Technical Difficulties

• The webinar begins with a welcome message, indicating that the presenter is an organizer and can communicate with other panelists.

• After some technical difficulties, the presenter confirms they are live and encourages attendees to raise their hands if they can hear them.

Humor and Context Setting

• The presenter shares a joke contrasting designers and analysts, emphasizing that designers get second chances with their work.

• A pun related to CFD (Computational Fluid Dynamics) is introduced: "In CFD, theory is when you know everything but nothing works; practice is when everything works but no one knows why."

Overview of Normal Modes Analysis

• The discussion shifts to normal modes analysis, highlighting its significance as the second most common type of linear analysis after stress analysis.

• The equation of motion for linear dynamics is presented: mass times acceleration plus damping times velocity plus stiffness times displacement equals forcing function over time.

Key Concepts in Normal Modes

• An important point about normal modes analysis is that it operates without applied loads; results show relative displacements rather than actual deformations.

• Natural frequencies are discussed as how structures prefer to vibrate, derived from the relationship between stiffness and mass.

Applications of Normal Modes Analysis

• Practical applications include ensuring systems like conveyor belts operate at harmonic frequencies or avoiding natural frequencies in sensitive equipment like electron microscopes.

• Examples provided include drivetrain systems where natural frequencies play a critical role in performance stability.

Case Study: Engine Model Analysis

• A case study involving a large engine model highlights challenges faced during modal frequency analysis due to high node counts (2.5 million nodes).

Understanding Normal Modes and Their Applications

Importance of Frequency in Modeling

• The discussion highlights the significance of setting model frequencies 20% higher than engine frequency due to trust issues with the model. This is a common practice when dead-on data is available, allowing for a more reliable analysis.

Power Spectral Density Analysis

• Once normal mode analysis is complete, it can be leveraged for power spectral density analysis, particularly in aerospace applications like rocket launches. This involves analyzing white noise spectra where multiple frequencies interact.

Applications of Response Spectrum Analysis

• Response spectrum analysis is widely used in various fields including:

• Fatigue analysis for wind turbines.

• Airflow over wings and acoustic inputs from jet engines.

• Earthquake motion and transportation wheel dynamics over rough roads.

Normal Modes and Linear Algebra

• After completing normal modes, further analyses rely on linear algebra techniques. An example includes motor frequency sweeps using shaker tables to test models based on normal modes.

Nastran's Popularity in Automotive Engineering

• Nastran software is favored in automotive engineering for its robust solutions related to body noise, vibration, and harshness (NVH). Key areas include chassis dynamics, engine vibrations, interior acoustics, and brake squeal issues.

Building Simple Models for Verification

Starting with Basic Models

• For beginners in modeling analysis, starting with simple pilot models such as beams with mass helps visualize concepts effectively.

Example Model Calculation

• A basic beam model calculation demonstrates stiffness (23,000) and mass (100), leading to a calculated natural frequency of 2.41 cycles per second. This hands-on approach aids understanding without overwhelming complexity.

Availability of Resources

• The presenter mentions that all models discussed will be available for download from their website, providing resources for self-practice.

Understanding Degrees of Freedom

Element Configuration Details

• The model utilizes a cush element configured with specific stiffness values in vertical directions (Y-axis), while maintaining high rotational stiffness across six degrees of freedom.

Results Interpretation

• Upon running the solver on the defined model, results show low frequencies consistent with expectations based on provided stiffness values. Only three modes are initially outputted due to mass constraints associated only with translational degrees of freedom.

Verification vs Validation in Modeling

Distinguishing Between Verification and Validation

• Verification involves checking calculations against hand-cranked results while validation compares modeled outcomes against experimental data. Both processes are crucial for ensuring accuracy throughout modeling work.

Challenges with Compound Modes

Understanding Orthogonality Principles

• A common challenge arises during normal mode work when encountering compound modes; understanding orthogonality principles helps clarify these complexities within modal analyses.

Understanding Rigid Body Motion and Normal Modes Analysis

Rigid Body Motion in Structures

• All structures exhibit six degrees of freedom: three translational and three rotational, similar to a satellite's movement in space.

• Unconstrained mechanisms can show near-zero frequencies, which is useful for debugging models with singular matrices during static linear stress analysis.

Normal Modes Analysis Insights

• The first mode shows movement in a specific plane; additional modes can be observed at right angles, indicating complex vibrational behavior.

• In beam structures, multiple modes exist due to symmetry, but only two orthogonal modes are typically displayed for simplicity.

Importance of Strain Energy

• A symmetric model should display double modes; absence may indicate modeling errors.

• Strain energy is generated from relative displacements during normal mode analysis, highlighting areas of localized deformation within the structure.

Application of Strain Energy in Design

• Identifying high strain energy regions allows engineers to reinforce specific areas to increase the first mode frequency, crucial for seismic analysis.

• Targeted reinforcement based on strain energy helps address structural issues without excessive modifications.

Analyzing Strain Energy Data

• The model underwent iterations focusing on high strain energy density regions; low values indicate minimal deformation.

Understanding Modal Frequency Analysis

Introduction to Frequency Analysis

• The speaker emphasizes the importance of reaching out for assistance regarding frequency analysis, indicating a willingness to share knowledge and resources available on their website.

• Mention of ongoing updates to the Power Spectral Density (PSD) tutorial, while noting that previous versions remain useful.

Project Overview: Headlight Assemblies

• Discussion about a project involving complex headlight assemblies for Fiat, highlighting the intricacies of mesh alignment in structural analysis.

• Explanation of testing requirements where structures are subjected to shaking from 0 to 200 Hz on a shaker table, focusing on resistance to modal frequency sweeps.

Importance of Mass Participation in Modal Analysis

• The need for understanding mass participation is introduced; it helps identify which modes carry significant mass and energy during vibration.

• Clarification that normal frequency analysis does not prioritize modes based on mass or energy but rather by frequency alone.

Energy Considerations in Structural Damage

• Insight into how low-frequency modes may not always correlate with high energy due to associated mass; this can lead to misconceptions about their significance.

• Emphasis on the relationship between force (F = ma), natural frequencies, and how mass contributes significantly to potential structural damage.

Analyzing Mass Fraction Contributions

• Presentation of data showing that the second mode carries more mass than the first mode, suggesting its greater importance in modal frequency analysis.

• Reference to guidelines from ASCE regarding capturing 90% of a structure's mass in earthquake analyses through modal summation techniques.

Practical Application: Simple Beam Example

• Transition back to a simple beam model where initial requests did not include mass participation data.

Understanding Structural Dynamics and Optimization in Engineering

Key Concepts of Modes and Mass Distribution

• The speaker discusses the difficulty in interpreting structural dynamics through visual means, emphasizing the importance of understanding modes and mass distribution rather than solely relying on visual representations.

• The first mode captures 80% of the mass of a structure, indicating its dominance. This principle is crucial for analyzing how structures respond to loads.

• The concept of orthogonality is introduced, suggesting that considering multiple modes together can yield insights into mass distribution and structural behavior.

• Loads applied at right angles to a mode shape do not affect that mode, highlighting the significance of load direction in structural analysis.

• Analyzing excitation functions reveals that loads at frequencies where natural modes have little associated mass will cause minimal damage, underscoring the relationship between frequency and structural integrity.

Challenges in Structural Analysis

• The speaker addresses common frustrations regarding the time required for mission-critical analysis work, particularly from those outside the engineering field who question its duration.

• There is a growing interest in optimization as a solution to reduce analysis time; however, this often leads to misconceptions about automation capabilities in complex engineering tasks.

Case Study: Paper Mill Forming Board

• A detailed example involving a forming board used in paper production illustrates practical applications of structural dynamics. The board's design must ensure it operates above 11 Hz to avoid catastrophic failure due to resonance with wire mesh movements.

• Emphasizing operational safety, if the forming board's frequency drops below 11 Hz, it risks exciting vibrations that could damage machinery—a critical consideration for engineers working on such projects.

• The discussion highlights challenges faced during optimization efforts when trying to balance stiffness and mass—key factors influencing frequency calculations (frequency = √(stiffness/mass)).

Optimization Efforts and Insights

• Throughout an extensive project involving stainless steel components, iterative adjustments were made while exploring various configurations aimed at reinforcing structures without compromising performance metrics like frequency.

• After numerous trials with different materials and designs (e.g., increasing plate thickness), insights emerged about effective reinforcement strategies based on observed mode shapes rather than just numerical outputs from simulations.

Understanding Structural Dynamics and Optimization

Importance of Mass Participation in Structural Design

• The speaker discusses the significance of mass participation in structural dynamics, highlighting that certain design choices can lead to unexpected outcomes, such as the creation of inch diameter rods.

• Emphasizes that the first mode's dominance is crucial for understanding structural behavior, noting a mass participation close to 48%, which indicates substantial influence on the structure's response.

• Points out that traditional optimization programs may not identify unconventional design solutions, like adding a rod, due to complex design constraints inherent in structures.

Optimization Techniques and Tools

• Introduces an optimization program within FEAP TASA, capable of maximizing stiffness and vibration frequency while reducing volume; it serves as an add-on option for users.

• Mentions practical applications of optimization tools through examples like carving plates to create innovative designs while maintaining structural integrity.

Resources for Learning Linear Dynamics

• The speaker shares a white paper on linear dynamics as a basic tutorial resource aimed at simplifying complex concepts with minimal equations.

• Recommends utilizing NX Nastran documentation for foundational knowledge in dynamic analysis, emphasizing its value for beginners in the field.

Encouragement for Self-Learning

• Stresses the importance of self-directed learning alongside formal education; encourages participants to engage deeply with material rather than relying solely on lectures.

• Offers additional resources including course notes and technical libraries related to linear vibrations, suggesting these materials could serve as comprehensive study aids.

Engagement and Follow-Up

• The speaker checks for questions from attendees but notes a quiet session overall; encourages participants to utilize learned concepts actively during discussions with peers.