Preparing Scenes and Plots for the Acoustics Analysis

For the acoustics analysis of the HVAC duct, the starting file provides several pre-defined plots and scenes.

To watch the Lighthill Wave solution develop over time, you set up plots that record the Lighthill pressure at pre-defined point probes. These point probes represent the microphone and the pressure transducer, respectively.

To visualize the acoustic wave fronts, you set up a scene that displays the time derivative of the Lighthill pressure in the flow domain. To compute the time derivative, you define a field function that leverages field history stored results (for more details, see Field Histories Example: Pressure Time Derivatives).

Additionally, you prepare visualization of the sources of Lighthill pressure inside the duct. You calculate the root mean square (RMS) of the Lighthill source measured in decibel (dB) as:

R M S ⁢     o f ⁢     L i g h t h i l l ⁢     S o u r c e ⁢       ( d B ) = 10 ⁢   log 10 ⁡ R M S ⁢     o f ⁢     L i g h t h i l l ⁢     S o u r c e Re f e r e n c e ⁢     S o u r c e
(5272)

where:

  • R M S ⁢     o f ⁢     L i g h t h i l l ⁢     S o u r c e is calculated using a Field Root Mean Square monitor.
  • Re f e r e n c e ⁢     S o u r c e is set to 1 Pa/m^2.
To prepare scenes and plots for the acoustics analysis:
  1. Plot the Lighthill pressure at the microphone and at the pressure transducer:
    1. Expand the Reports node.
    2. Multi-select the following nodes and set Field Function to Lighthill Pressure:
      • Microphone: Lighthill Pressure
      • Pressure Transducer: Lighthill Pressure
    3. Right-click one of the selected nodes and select Create Monitor and Plot from Report.
    4. In the Create Plot From Reports dialog, click Multiple Plots (one per report).
      The plots open in the Graphics window.
To visualize the acoustic wave front, you calculate the time derivative of Lighthill pressure and display the field in a scalar scene.
  1. Create a field history of Lighthill pressure:
    1. Right-click the Monitors node and select New Field History.
    2. In the Select Field Function dialog, select Lighthill Pressure, then click OK.
    3. Select the History of Lighthill Pressure node and set the following properties:
      Property Setting
      Parts LES
      Sliding Sample Window Size 3
      Trigger Time Step
  2. Use the field history to define a scalar field function of the Lighthill pressure time derivative:
    1. Right-click the Automation > Field Functions node and select New > Scalar.
    2. Rename the User Field Function 1 to Lighthill Pressure Time Derivative.
    3. Select the Lighthill Pressure Time Derivative node and set the following properties:
      Property Setting
      Function Name LighthillPressureTimeDerivative
      Dimensions Pressure/Time
      Definition (3.0*${HistoryofLighthillPressureSample0}-4.0*${HistoryofLighthillPressureSample1}+${HistoryofLighthillPressureSample2})/(2.0*${TimeStep})
  3. Display the created field function in a scalar scene:
    1. Expand the Scenes folder and open the LES > Lighthill Pressure Time Derivative scene.
    2. Select the Lighthill Pressure Time Derivative > Scalar 1 > Scalar Field node and set the following properties:
      Property Setting
      Function Lighthill Pressure Time Derivative
      Auto Range Off
      Min -250 Pa/s
      Max 250 Pa/s
To visualize the noise sources, you set up a scalar scene that displays a logarithmic meassure of the Lighthill source in dB:
  1. To retain the Lighthill source field, select the Solvers > Acoustic Wave node and activate Temporary Storage Retained.
  2. Create a monitor that calculates the root mean square of the Lighthill source in the flow domain:
    1. Right-click the Monitors node and select New Monitor > Field Root Mean Square.
    2. Rename the monitor to Field RMS of Lighthill Source.
    3. Select the Field RMS of Lighthill Source node and set the following properties:
      Property Setting
      Parts LES
      Trigger Time Step
      Field Function Lighthill Source
  3. Define a scalar field function that provides a logarithmic meassure of the RMS Lighthill source field:
    1. Create a scalar field function and rename it RMS of Lighthill Source (dB).
    2. Select the RMS of Lighthill Source (dB) node and set the following properties:
      Property Setting
      Function Name RMSofLighthillSource
      Definition 10*log10(max(1E-10,${FieldRMSofLighthillSourceMonitor}/1))
  4. Display the created field function in a scalar scene:
    1. Open the LES > RMS of Lighthill Source (dB) scene.
    2. Select the RMS of Lighthill Source (dB) > Scalar 1 > Scalar Field node and set Function to RMS of Lighthill Source (dB).
  5. Save the simulation.