7.2 Modal analysis Estimated time to complete this course: 16–23 minutes
This course discusses setting up and performing a modal analysis. When you complete this course, you should understand the basics of modal analysis and be able to analyze a model using a modal solution.
Setup information Part folder: parts_simulat parts_simulation ion
NX role: Advanced with full menus
System preparation
7.2.1 Modal analysis Modal analysis:
Calculates natural frequencies and the corresponding mode shapes.
Does not consider damping.
How stiffness and mass affect natural frequency For a single DOF system, the natural frequency is related to the mass (M) and stiffness (K) as follows:
7.2.1 Modal analysis Modal analysis:
Calculates natural frequencies and the corresponding mode shapes.
Does not consider damping.
How stiffness and mass affect natural frequency For a single DOF system, the natural frequency is related to the mass (M) and stiffness (K) as follows:
Supported modal analysis types In Advanced Simulation, you can choose from the following modal analysis types when you create a structural solution:
Solver
Solution type SOL 103 Real Eigenvalues SOL 103 Response Simulation
NX Nastran SOL 103 Superelement SOL 103 Flexible Body SOL 103 MSC Nastran SOL 103 Superelement ANSYS
Modal
ABAQUS
Frequency Perturbation substep
7.2.2 Using elements for a modal analysis Some of the elements that can be used for a modal analysis include:
3D tetrahedral or hexahedral solid elements.
2D quadrilateral or triangular thin shell elements.
1D bar, beam, rod, rigid link, and spring elements.
0D concentrated mass elements.
Gap elements.
For more information about elements, see Physical properties and element attributes Advanced Simulation online Help.
Elements and physical properties in the
7.2.3 Using materials for a modal analysis Material types that can be used in a modal analysis include:
Isotropic
Orthotropic
Anisotropic
Fluid
For more information about materials, see Materials in the Advanced Simulation online Help.
7.2.4 Defining boundary conditions for a modal analysis Boundary conditions for modal analysis include constraints and gluing, such as:
Displacement constraints.
Coupled degrees of freedom.
Surface-to-surface gluing
For more information, see Boundary Conditions in the Advanced Simulation online Help.
7.2.5 Modal pre-stress A model can be pre-stressed prior to performing a modal analysis. Pre-stress or stress stiffening is done in a linear static load case. Pre-stress can include forces, bolt pre-load and contact.
7.2.6 Setting modal solution attributes For a modal analysis, some of the NX Nastran solution attributes include:
Number of Desired Modes
System Cells
Output Requests
Eigenvalue Method . Identifies the type of solve: Lanczos or Householder. The method specifies the real eigenvalue extraction options for the solution. Eigenvalue extraction options are stored as a solver-specific object. Lanczos is the recommended method for most models; Householder is recommended for smaller models.
For more information, see Nastran environment in the Advanced Simulation online Help.
7.2.7 Rigid body modes If you select a Lanczos solve:
You can solve for the modes of a free-free body. If you are calculating modes for an unrestrained structure, it is a good practice to calculate the rigid body modes. This reveals any unintended mechanisms or grounding resulting fr om poor modeling. To avoid calculating the rigid body modes, set the frequency range above 1 Hz.
7.2.8 Reviewing modal analysis results Natural frequencies and mode shapes are the primary results for a modal solution.
The results are ordered by fr equency, with the lowest natural frequency being the first mode, the next highest being the second mode, and so on. The normal modes represent dynamic states in which the elastic and inertial forces are balanced when no external loads are applied.
The mode shapes represent relative displacement of the nodes.
The mode shapes help you determine what load locations and directions will excite the structure.
7.2.9 Animating mode shapes In Post-processing, Animation
is particularly useful for visualizing mode shapes.
When animating mode shapes, in the Animation dialog box:
Set Style to Modal.
Select Full Cycle.
Ani mati on of mo de sh ape for a sin gle mo de
7.2.10 Activity: Modal analysis Estimated time to complete: 10–15 minutes You will learn how to:
Solve a free-free no rmal mode dynamics problem. Specify number of modes calculated. Display and animate mode shapes. Modify a mesh using beams. Resolve a model and display results.
Launch the Modal analysis activity.
Open the Simulation file You will use a speaker part for this activity.
Open Files of type:
File name:
Simulation Files (*.sim)
cabinet_sim1.sim
OK
For a normal mode solution using the Lanczos solver, boundary conditions are not required to solve the model. By examining the Simulation Navigator , you can see that the model does not contain any boundary conditions.
Reset the dialog box memory The options you select in NX dialog boxes are preserved for the next time you open the same dialog box within an NX session. Restore the default settings to ensure that the dialog box es are in the expected initial state for each s tep of the activity.
Preferences
Reset Dialog Memory
OK
User Interface
Solve the model By default, the solver calculates 10 modes. However, you can change the number of modes the solver calculates.
Simulation Navigator
Solution 1
Solve
Edit Solution Attributes
Case Control
Edit (Lanczos Data )
Number of Desired Modes
12
all dialog boxes
OK
Wait for the job to finish and for the command window to close. the Information window
Cancel
Close
Analysis Job Monitor dialog box Solution Monitor dialog box
View the results Simulation Navigator
Results
Post-Processing Navigator The modal results are listed in the Post Processing Navigator . The first 6 modes have extremely low frequencies. These are rigid body modes. Mode 7 represents the first flexible mode with a natural frequency of about 133 Hz.
Mode 7 (expand)
Displacement – Nodal (expand)
Magnitude
Animate the mode shape You can animate the mode shape plotted in the previous step as fo llows.
Animation (Post-Processing toolbar)
Style
Modal
Number of Frames
Full-cycle
Synchronized Frame Delay (mS)
Play
OK
100
15
Stop (Post-Processing toolbar)
When you finish looking at the results, return to the model.
Return to Model (Layout Manager toolbar)
Make the FEM the work part Simulation Navigator
Simulation File View
cabinet_fem1
Make Work Part
Update the display In preparation for adding bracing between the side panels, turn off the display of the mesh and polygon geometry for the front and back panels.
Simulation Navigator
cabinet_fem1.fem (expand)
3D Collectors (expand)
Solid(1) (expand)
3d_mesh(2) (deselect)
3d_mesh(3) (deselect)
Polygon Geometry (expand)
Polygon Body (2) (deselect)
Polygon Body (3) (deselect)
Add bracing to the cabinet Bracing is sometimes used in loudspeakers to raise the natural frequencies of the flexible modes by stiffening the cabinet. In this step, you use a beam element to simulate bracing the side panels together.
1D Connection (Advanced Simulation toolbar, Connections Drop-down list)
Type
Type
Node to Node
You are selecting the node at the location of the blue mesh poi nt. The mesh point is visible on the inside face o f the cabinet side panel.
Target
Node (Select Node )
Connection Element
Type
CBEAM
OK
Create a cross section Create a 0.5 in radius circular cross section that you will later assign to the beam.
1D Element Section (Advanced Simulation toolbar, Mesh Drop-down list)
Create Section
Type
Rod
DIM1
0.5
OK
Close
Beam Section dialog box
Beam Section Manager dialog box
Assign physical properties to the beam Now assign the cross section created in the previous step and the polycarbonate material property to the beam.
Simulation Navigator
1D Collectors (expand) Beam Collector(1) Edit Edit (Beam Property)
Section Type
Constant
Fore Section
ROD(1)
Choose Material (Material)
Materials
Polycarbonate
OK
all dialog boxes
Update the display Redisplay the mesh and polygon geometry for the front and back panels.
Simulation Navigator
Polygon Body (2) (select)
Polygon Body (3) (select)
3d_mesh(2) (select)
3d_mesh(3) (select)
Make the Simulation file the work part Simulation Navigator
Simulation File View
cabinet_sim1
Make Work Part
Solve the model Simulation Navigator
Solution 1
Solve
OK
Wait for the job to finish and for the command window to close. the Information window
Cancel
Close
Analysis Job Monitor dialog box
Solution Monitor dialog box
View the results Simulation Navigator
Results
Post Processing Navigator Now the first 7 modes listed in the Post Processing Navigator have extremely low frequencies and are recognized to be rigid body modes. An extra rigid body mode has arisen because the beam is free to ro tate about its own axis. This occurs b ecause:
Nodes defining beam elements have trans lational and rotational DOF.
Nodes defining tetrahedral elements have only translational DOF.
By attaching both ends of the beam to t he tetrahedral mesh, two rigid body rotations of the beam are coupled to the tetrahedral mesh. However, rigid body rotation of the beam about its ow n axis remains uncoupled, which produces the extra rigid body mode.
Mode 8 (expand)
Displacement – Nodal (expand)
Magnitude
The addition of the brace has caused the natural frequency of the fundamental flexible mode to increase from about 133 Hz to about 184 Hz.
Animate the mode shape You can animate the mode shape plotted in the previous step as fo llows.
Animation (Post-Processing toolbar)
Style
Modal
Number of Frames
Full-Cycle
Synchronized Frame Delay (mS)
Play
OK
100
15
It is apparent that adding the brace has also altered the mode shape of the fundamental flexible mode.
Stop (Post-Processing toolbar) Return to Model (Layout Manager toolbar)
File
Close
All Parts
You completed the activity.
