To enable the FW-H acoustics model, select Ffowcs-Williams & Hawkings in the Acoustics Model panel (Figure 21.3.1).
Define Models Acoustics...
When you turn on Ffowcs-Williams & Hawkings, the panel will expand to show the relevant fields for user inputs.
Setting Model Constants
Under Model Constants in the Acoustics Model panel, specify the relevant acoustic parameters and constants used by the model.
The default values are appropriate for sound propagating in air at atmospheric pressure and temperature.
Computing Sound "on the Fly''
The FW-H acoustics model in FLUENT allows you to perform simultaneous calculation of the sound pressure signals at the prescribed receivers without having to write the source data to files, which can save a significant amount of disk space on your machine. To enable this "on-the-fly'' calculation of sound, turn on the Compute Acoustic Signals Simultaneously option in the Acoustics Model panel.
| Because the noise computation takes a negligible percentage of memory and computational time compared to a transient flow calculation, this option can be used by itself or along with the process of source data file export and sound calculation. For the latter, computing signals "on the fly'' allows you to see when the signals have become statistically steady so you can know when to stop the simulation.
When the Compute Acoustic Signals Simultaneously option is enabled, the FLUENT console window will print a message at the end of each time step indicating that the sound pressure signals have been computed (e.g., Computing sound signals at x receiver locations ..., where x is the number of receivers you specified). Enabling this option instructs FLUENT to compute sound pressure signals at the end of each time step, which will slightly increase the computation time.
| Note that this option is available only when the FW-H acoustics model has been enabled. See below for details about exporting source data without enabling the FW-H model.
Writing Source Data Files
Although the "on-the-fly'' capability is a convenient feature, you will want to save the source data as well, because the acquisition of source data during a transient flow-field calculation is the most time-consuming part of acoustics computations, and you most likely will not want to discard it. By saving the source data, you can always reuse it to compute the sound pressure signals at new or additional receiver locations.
To enable saving the source data to files, turn on the Export Acoustic Source Data option in the Acoustics Model panel. Once this option is selected, the relevant source data at all face elements of the selected source surfaces will be written into the files you specify. The source data vary depending on the solver option you have chosen and whether the source surface is a wall or not. Table 21.3.1 shows the flow variables saved as the source data.
See Section 21.3.2 for details on how to specify parameters for exporting source data.
Exporting Source Data Without Enabling the FW-H Model
You can also export sound source data for use with SYSNOISE without having to enable the Ffowcs Williams and Hawkings (FW-H) model. You will still need to specify source surfaces (see Section 21.3.2), as .index and .asd files are required by SYSNOISE. In addition, you can choose fluid zones as emission sources if you want to export quadrupole sources. To enable the selection of fluid zones as sources, use the
define models acoustics export-volumetric-sources?
text command and change the selection to yes.
SYSNOISE also requires centroid data for source zones that are being exported.
For fan noise calculations, once you have specified the source zones in the Acoustic Sources panel and you have selected Export Acoustic Source Data from the Acoustic Model panel, you can export geometry in cylindrical coordinates by using the
define models acoustics cylindrical-export?
text command and changing the selection to yes. By default, FLUENT exports source zones for SYSNOISE in Cartesian coordinates.
You can then export the centroid data to a .data file using the following text command:
define models acoustics write-centroid-info
Since you will not be using the FW-H model to compute signals, you will not need to specify any acoustic model parameters or receiver locations. Also, you will not be able to turn on the Compute Acoustic Signals Simultaneously option in the Acoustics Model panel, and Acoustic Signals... will not be available in the Solve menu.