GetDP results display : modal basis

In order to visualize the modal basis of natural frequencies previously computed with structural FEA (GetDP), one must activate the parameter by putting Input.Simu.is_save_modal_basis to 1 (In Mechanics\Output for the GUI). This will generate a .pos file containing the modal basis obtained with GetDP. Warning, this option extends calculation time and generates an heavy file (up to more than 1Go). The .pos file is available at Mechanics/Gmsh/solutions/u.pos.

Results display: modal basis

Once the calculation is done, you can visualize the modal basis. First, you have to run Gmsh by lauching the excutable in the folder Mechanics/Gmsh/bin. Then, you have to click on File -> Open -> solutions -> u.pos. For a better display, it is recommended to also import the geometry (see How to display geometry and meshing of a machine in Gmsh ?).

Gmsh Display
Gmsh Display

When working in frequency domain, GetDP separates the real and imaginary parts of the solution in two different time steps. Therefore, results have as twice as much solutions as the specified number of eigen frequencies (that is why there are two steps for each EigenValue).

By a double-click anywhere on the display window and a click on "All general options...", you have access to all display options of Gmsh. For example, you can change the aspect of the actual view by selecting View[0] in the left column -> Aspect -> Vector display -> Displacement.

Gmsh General Options
Gmsh General Options

You can now visualize each machine’s deflection corresponding to a given eigenvalue by using the step control buttons located at the bottom left of the window.

Gmsh step control buttons
Gmsh step control buttons

Gmsh includes a plugin called "Harmonic to Time" which converts imaginary and real parts to a temporal result with the following equation:

u(t)=Real(u)*cos(\omegat)+Imaginary(u)*sin(\omegat)

This allows to visualize the time deflection of a given modal shape associated to a given eigen frequency. For example, if one wants to see the eigenvalue n°3 in the time domain:

  • Note the two steps corresponding to the chosen eigenvalue at the bottom of the screen (for example 4 and 5)
    Gmsh steps visualization
    Gmsh steps visualization
  • Type "p" -> HarmonicToTime
    • RealPart: 4
    • ImaginaryPart: 5
    • NumSteps: number of time steps that you want
    • View: 0 (-1 is for the current view)
  • Click on "Run"
Gmsh HarmonicToTime plugin
Gmsh HarmonicToTime plugin

A new view appears in the left column. If you select it, you can now animate your structure in time domain with step control buttons previously mentioned (now the "play" button becomes very useful).

Gmsh Use of HarmonicToTime plugin
Gmsh Use of HarmonicToTime plugin

Note: In view’s option -> Aspect, factor displacement could be changed. It allows to visualize deflections that occur on the structure.

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