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21.3.1 Enabling the FW-H Acoustics Model

To enable the FW-H acoustics model, select Ffowcs-Williams & Hawkings in the Acoustics Model panel (Figure  21.3.1).

Define $\rightarrow$ Models $\rightarrow$ Acoustics...

When you turn on Ffowcs-Williams & Hawkings, the panel will expand to show the relevant fields for user inputs.

Figure 21.3.1: The Acoustics Model Panel
figure



Setting Model Constants


Under Model Constants in the Acoustics Model panel, specify the relevant acoustic parameters and constants used by the model.

Far-Field Density   (for example, $\rho_0$ in Equation  21.2-1) is the far-field fluid density.

Far-Field Sound Speed   (for example, $a_0$ in Equation  21.2-1) is the sound speed in the far field (= $\sqrt{\gamma R T_0}$).

Reference Acoustic Pressure   (for example, $p_{\rm ref}$ in Equation  28.10-11) is used to calculate the sound pressure level in dB (see Section  28.10.4). The default reference acoustic pressure is $2 \times 10^{-5}$ Pa.

Number of Time Steps Per Revolution   is available only for steady-state cases that have a single rotating reference frame. Here you will specify the number of equivalent time steps that it will take for the rotating zone to complete one revolution.

Number of Revolutions   is available only for steady-state cases that have a single rotating reference frame. Here you will specify the number of revolutions that will be simulated in the model.

Source Correlation Length   is required when sound is to be computed using a 2D flow result. The FW-H integrals will be evaluated over this length in the depth-wise direction using the identical source data.

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.

figure   

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.

figure   

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.


Table 21.3.1: Source Data Saved in Source Data Files
Solver Option Source Surface Source Data
incompressible walls $p$
incompressible permeable surfaces $p, u, v, w$
compressible walls $p$
compressible permeable surfaces $\rho, p, u, v, w$

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 $\rightarrow$ models $\rightarrow$ acoustics $\rightarrow$ 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 $\rightarrow$ models $\rightarrow$ acoustics $\rightarrow$ 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 $\rightarrow$ models $\rightarrow$ acoustics $\rightarrow$ 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.


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