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Transient Series 3: Fourier Transform


Description

This video covers use of the Fourier Transform function in Tecplot 360 to analyze a time series extraction from the transient simulation of a vertical axis wind turbine, and demonstrates how changing the Probe location will affect the Fourier Transform function and fundamental frequency.

In the previous video, the Probe to Create Time Series Plot tool was used. This extracted a single point through time and created a new frame.

To gather further information from the original data set using the Time Series Plot, the Fourier Transform tool will be used. We will also see how varying the Probe location will affect the function and fundamental frequency. Tecplot 360 uses the Fourier Transform method to perform a frequency analysis on the extracted line plot.

  1. First ensure that the Time Series Plot is selected by clicking inside the frame. Then select Data > Fourier Transform. From this dialog choose the Independent variable, Dependent variables, and Source zones.
  2. For now, select Pressure as our Dependent variable (the same as the Time Series Plot), choose the Time Series Plot Zone as it is our only available Zone, change Plot placement to Tile with existing frames, and keep all the other defaults.
  3. Selecting Transform will create a new Frame with the Fourier Transform Function plotted as Amplitude versus Frequency.  (In the previous Transient videos, the Probe to Create Time Series tool was used to create a new data set, with Solution Time added as an additional Variable to plot against). The Fourier Transform tool creates a new Zone in this data set and creates 3 new variables for every Dependent Variable selected: Frequency, Amplitude, and Phase.
  4. Notice that the Fourier Transform Function of Pressure at this Probe contains 2 spikes in Amplitude, one at a lower frequency (around 5 Hertz) and the other at a higher frequency (around 77 Hertz). Compare this with the Time Series Plot, which has a low frequency / high period component, as well as a high frequency / low period component.
  5. To change the probe location to the inlet at the left edge of the initial plot, the Probe to Create Time Series Tool will overwrite the previous Time Series data set, and therefore remove the previous Fourier Transform plot. However, right-clicking the context menu generates a new Fourier Transform plot using the Dependent Variable plotted in our Time Series.
  6. Notice how probing near the inlet as well as far away from the Turbine Blades results in a Fourier Transform that isolates the high frequency changes. This indicates that the variation in pressure was set as an input boundary condition. By comparing this with our initial probe location, we can infer that the lower frequency changes are due to the effect of the Turbine Blades themselves.
  7. Further analysis can be done on other variables in our data set. For example, when probing close to where the vortices are shed, we can see how changes in Turbulent Viscosity are solely due to the low frequency changes caused by movement of the Turbine Blades.

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