Calculating the Spectral Density

To calculate the spectral density, you create a line time history, a line time Fourier transform, and a derived data set. You can then visualize the spectral density.

To create a line time history to record data from the line probe:

  1. Right-click the Tools > Data Set Functions node and select the New > Line Time History (h[l]).
  2. Right-click the h(l) 1 > Imported node and select New derived data from probe part on surface.
  3. Select the Imported > Multi-Point Time History 2 node and set the following properties:
    Property Setting
    Field Function 1 Pressure
    Data Surfaces 1 Plane section
    Part line-probe
    Representation cylinderPressureData
    Update Active Activated
To create a line time Fourier transform:
  1. Right-click the Tools > Data Set Functions node and select New > Line Time Fourier Transform (G[h[l]]).
  2. Rename the newly created G(l) 1 node to Gxx(l).
  3. Select the Tools > Data Set Functions > Gxx(l) node and make sure Amplitude Function is set to Power Spectral Density.
To create the derived data set node into which you import the data:
  1. Right-click the Gxx(l) > Imported node and select New derived data from derived data.
  2. Rename the newly created Multi-Point FFT 2 node to Power Spectra.
  3. Select the Gxx(l) > Imported > Power Spectra node and set the following properties:
    1. Set Input time history to Multi-Point Time History.
    2. Activate Update Active. All other properties are disabled for editing and updated automatically after you activate Update Active.
To visualize the spectral density:

With the Update Active property activated, the Fourier transform is automatically calculated once you assign the last property. This process takes a few minutes, depending on the processor speed of your computer.

When the Fourier transform has been computed, you visualize the spectral density as a function of distance. You use a logarithmic scale for the spectral density axis.

  1. Create a monitor plot and rename it to Power Spectra .
  2. Right-click the Plots > Power Spectra > Data Series node and select Add Data.
  3. In the Add Data Providers to Plot dialog, expand Derived Data and select Power Spectra.
    The plot of Power Spectral Density against Distance appears in the graphics window.

The expected profile is obtained. The average correlation is maximum at zero where the separation is minimum.