Surface Time Fourier Transforms

Surface Time Fourier Transforms let you calculate Fourier transforms at various point within a presentation grid.

They allow the creation of auto-spectra (Eqn. (500)) over the presentation grid.

h (\underset{̲}{x}; t) \to G (\underset{̲}{x}; ω)

Choosing this type of data set function creates a G(s) node (with a numerical suffix) in the object tree.

Visualizing Bands for Surface Time Histories

You can visualize the frequency range in a choice of subdivisions, (full) octave, 1/3, 1/6, 1/12, and 1/24. Alternatively, you can customize your frequency range. To set up this post-processing:

Select the [Surface Time Fourier Transform] > Imported > [Derived Data Set] > Output Control node, and select an option for Frequency Mode.

If you choose one of the Octave Bands options, the Octave Frequency Bands node appears.

This feature displays the number of bands along with the high and low frequencies for the currently selected option.
When you have an FFT with results generated you can use this node to visualize different bands. With the scene open, select the Octave Frequency Bands node and enter a value for Current Frequency Band up to the Number of Frequency Bands limit.
If you choose the Custom Bands option, the Custom Bands node appears. Click the property customizer button to activate the Custom Bands dialog in which you can set a definition of the number of bands and the values.

Surface Time Fourier Transform Properties


Start Signal	The start time for the sampling of this object.

End Signal	The end time for the sampling for this object. Clicking the property customizer button activates a property customizer that lets you specify both the time value and the appropriate time units (hr, min, or s). If you have multiple data set functions, you can vary the settings of this property to sample multiple time intervals from the simulation run.

Analysis Blocks	Specifies the number of analysis blocks $N_{b}$ (see Eqn. (523)) for this data set function to use.

Overlap Factor	Sets the overlap factor $α$ (see Eqn. (523)) between blocks, if any, in a range from 0 to 0.9. This property allows for smooth transitions between blocks.

Window Function	Specifies the window function applied to the FT. The window function makes the signal periodic: it is guaranteed to be zero at both start and end. For most cases, apply a window function to a signal before applying an FT for best results. Available options: None: This default setting leaves the data set function without a window function until you select one of the other options in this list. Hamming: See Eqn. (543). Hann (Hanning): See Eqn. (544). Bartlett: See Eqn. (545). Blackman: See Eqn. (546).