Performing Nonlinear Normal Modes Analysis

The Solid Stress Normal Modes solver also supports nonlinear setups. To see the effect of nonlinearities on the normal modes of the structure, define the turbine blade in a rotating reference frame and recalculate the normal modes.

To create a rotating reference frame:
  1. Right-click the Tools > Reference Frames and select New > Rotating.
  2. Select the Reference Frames > Rotating node and set Rotation Rate to 30.0 rpm.
To assign the reference frame to the blade region:
  1. Multi-select both the Regions > Blade > Physics Values > Motion Specification and the Spar > Physics Values > Motion Specification nodes, and set the Reference Frame to Rotating.
  2. Right-click the Continua > Structure node and select Select Models...
  3. In the Optional Models group box select the Nonlinear Geometry model.
  4. Select the Structure > Material Law Models > Material Laws > Isotropic Linear Elastic node.
  5. Deselect the Linear Strain (Small Strain) model.
  6. Select the Green-Lagrange (Small Strain) model. Click Close.
To run the nonlinear analysis and recalculate the normal modes:
  1. Select Solution > Clear Solution....
  2. Click (Run).
  3. Once the simulation is complete, right-click the Solvers > Solid Stress Normal Modes node and select Perform Normal Modes Analysis.
  4. To view the results, right-click Solvers > Solid Stress Normal Modes > Normal Modes Results and select Tabulate.

    The results show that with the introduction of a rotating reference frame and the Nonlinear Geometry model the normal modes have changed from the linear simulation. For example, the seventh normal mode has increased. This change in results for the nonlinear simulation is due to the normal modes solver accounting for the nonlinear effects of the rotation such as centrifugal force and the variations in stiffness of the geometry. In contrast, the linear simulation only considers the initial configuration and stiffness.

  5. Save the simulation.