Analyzing Normal Modes

In solid mechanics, the normal modes of a solid structure describe the free vibrational motion of the structure at its natural (or resonant) frequencies. The free modes of vibrations of a structure govern its dynamic response to external forces and have an impact on its stress characteristics.

Each mode of vibration occurs at a fixed frequency and is independent from the other modes. The natural frequencies of a structure depend on its material properties and the geometrical features. For more information, see Normal Modes.

Normal Modes analysis allows you to determine an adequate time-step in solid stress simulations. In time-dependent simulations, the time-step must be sufficiently small to capture the highest vibrational mode of the structure. Normal mode analysis is essential for modeling the dynamic response of structures that are subject to vibrations, such as suspension bridges and aircraft wings.

In Simcenter STAR-CCM+, the Solid Stress Normal Modes model allows you to calculate the normal modes of solid structures in simulations with linear or nonlinear setups (including nonlinear geometry and nonlinear material laws).

Normal modes analysis does not support the following settings:
  • Periodic interfaces.
  • FSI interfaces in nonlinear simulations. Linear simulations can contain FSI interfaces; however, the fluid side of the interface does not contribute to the normal modes calculation.
  • Motions other than Stationary and Solid Displacement. To account the effect of rotations and translations in the normal modes calculation, use a moving reference frame.
  • Contacts within a linear setup.
  • Non-zero segment constraints or initial conditions within a linear setup.
Additionally, normal modes analysis does not account for contributions from the following:
  • The Rayleigh Damping model.
  • The Gravity model.
  • Segment loads in linear simulations. In nonlinear simulations, the normal modes calculation includes the contribution from segment loads.
  • The Specified Temperature, FE Solid Energy, and Thermal Expansion models in linear simulations. In nonlinear simulations, the normal modes calculation includes the contribution from thermal strain and stress.
  • The fluid side of an FSI interface in linear simulations. Nonlinear simulations do not support FSI interfaces.

You can instruct Simcenter STAR-CCM+ to stop the normal modes calculation when the simulation contains any incompatible settings, or to continue the calculation regardless. See 模型兼容性.

To calculate the normal modes of a structure:
  1. Set up the simulation for stress analysis. For instructions, see General Workflow for Stress Analysis.
Activate the Solid Stress Normal Modes model:
  1. Right-click the Continua > [Solid Physics Continuum] node and select Select Models....
  2. In the Model Selection dialog, select the Solid Stress Normal Modes model and click Close.
The Solid Stress Normal Modes model activates the Solid Stress Normal Modes solver in which you can define and perform the normal modes analysis.
  1. Define the Solid Stress Normal Modes solver properties:
    1. Select the Solvers > Solid Stress Normal Modes node and set the Model Compatibility to one of the following options:
      • Enforce—stops the simulation if the solid region or physics continuum contains any incompatible settings.
      • Ignore—allows you to run the simulation with incompatible settings. However, results may be inaccurate as the solver ignores contributions from the incompatible settings.
    2. Select the Solid Stress Normal Modes > Eigen Solver node and specify the Convergence Tolerance and Iterations.
    3. Select the Frequency Selection node and set the Method and Number of Frequencies.
    For more information, see Solid Stress Normal Modes Solver Reference.
  2. Right-click the Solid Stress Normal Modes node and select Perform Normal Modes Analysis.
  3. To tabulate the results, right-click the Normal Modes Results node and select either Export..., Extract, or Tabulate....
  4. Visualize the results.
    For more information, see Visualizing Normal Modes.