Analyzing the Results

When the simulation is complete, you analyze the results. At the point probe, you compare the pressure and sound pressure levels with the provided data from the simulation without using the Sponge Layer model.

To analyze the results:
  1. Right-click the Plots > Pressure Monitor Plot node and select Open.
  2. Add the data from the LES simulation without sponge layer to the Pressure Monitor Plot plot:
    1. Right-click the Plots > Pressure Monitor Plot > Data Series node and select Add Data.
    2. In the Add Data Providers to Plot dialog, select Tables > noSL_pressureProbe.
    3. Open the Pressure Monitor plot.


      The plots show that the pressure fluctuations are smaller when the spong layer is activated.
  3. Create a Fast Fourier Transform (FFT) for the sound pressure levels:
    1. Right-click the Tools > Data Set Functions node and select New > Point Time Fourier Transform (G[p]).
    2. Select the Data Set Functions > G(p) 1 node and set the following properties:
      Property Setting
      Start Time0.2 s
      Cut-off Time0.4 s
      Amplitude FunctionSound Pressure Level
      Analysis Blocks2
      Overlap Factor0.5
      Window FunctionHann
  4. Create a data set for the pressure fluctuations of the undamped LES simulation:
    1. Right-click the G(p) 1 > Tabular node and select New derived data from table.
    2. Rename the Tabular > Tabular node as Pressure Probe - No SL.
    3. Select the Pressure Probe - No SL node and set the following properties:
      Property Setting
      Update ActiveOn
      Input Data 1noSL_pressureProbe
      X Column 1Time
      Y Column 1No SL Pressure Probe
    4. Right-click the G(p) 1 > Monitor node and select New derived data from monitor.
    5. Select the Monitor node and set the following properties:
      PropertySetting
      Update ActiveOn
      Input Data 1Pressure Monitor
    6. Rename the Monitor node to Pressure Probe - SL.
  5. Create a monitor plot to compare the pressure fluctuation FFTs of the undamped simulation with the damped simulation at the pressure probe:
    1. Right-click the Plots node and select New Plot > Monitor Plot.
    2. Rename the Plots > Monitor Plot 1 node as FFT Plot.
    3. Select the FFT Plot node and set Title to FFT Plot.
    4. Right-click the FFT Plot > Data Series node and select Add Data.
    5. In the Add Data Providers to Plot dialog, expand Derived Data and select the following:
      • Pressure Probe - SL
      • Pressure Probe - No SL
    6. Select the Axes > Bottom Axis node and set the following properties:
      Property Setting
      Logarithmic On
      Minimum 20
      Maximum 500
    7. Select the Bottom Axis > Title node and set Title to Frequency (Hz).
    8. Select the Axes > Left Axis node and set the following properties:
      Property Setting
      Minimum 10
      Maximum 120
    9. Select the Left Axis > Title node and set Title to Sound Pressure Level (dB).


      For the analysis plot displaying the sound pressure level (dB) versus frequency (Hz) at the pressure probe, the level of noise is distinctly lower at lower frequencies when the Sponge Layer model is activated. The Sponge layer achieves non-reflective conditions at the pressure outlet boundary. Without the sponge layer, the sound pressure level (dB) is overpredicted.

  6. Open the Sponge Layer Damping Coefficient scene.


    The functional shape of the damping coefficient allows for a smooth transition of the amount of damping in the sponge layer. From the simulation domain into the sponge layer, the damping coefficient increases until it reaches its maximum value and then decreases again. At the pressure outlet boundary, the damping coefficient has a value of zero to maintain the characteristics of the boundary condition.

  7. From your working directory, enter the image subdirectory where the images of the scalar pressure fluctuation scene are stored. To examine the scenes, click through the save image files or animate them with a media program of your choice. The following animation shows the pressure distribution over time:


    You can see that spurious pressure flucuations are gradually suppressed within the sponge layer. The Sponge Layer model damps the fluctuations towards a reference solution that is based on a moving time-average. The unsteadiness of the flow reaching the outer boundary is reduced, thus minimizing any unsteady wave reflection back into the computational domain.