22.2.2 Specifying Source Surfaces

In the Acoustics Model dialog box, click the Define Sources... button to open the Acoustic Sources dialog box (Figure  22.2.4). Here you will specify the source surface(s) to be used in the acoustics calculation and the inputs associated with saving source data to files.

Figure 22.2.4: The Acoustic Sources Dialog Box

Under Source Zones, you can select multiple emission (source) surfaces and the surface Type that you can select is not limited to a wall. You can also choose interior surfaces and sliding interfaces (both stationary and rotating) as source surfaces.

The ability to choose multiple source surfaces is useful for investigating the contributions from individual source surfaces. The results based on the use of multiple source surfaces are valid as long as there are negligible acoustic interactions among the surfaces. Thus, some caution needs to be taken when selecting multiple source surfaces.

In cases where multiple source surfaces are selected, no source surface may enclose any of the other source surfaces. Otherwise, the sound pressure calculated based on the source surfaces will not be accurate, as the contribution from the enclosed (inner) source surfaces is over predicted, since the FW-H model is unable to account for the shading of the sound from the inner source surfaces by the enclosure surface.

If you specify any interior surfaces as source surfaces, the interior surface must be generated in advance (e.g., in GAMBIT) in such a way that the two cell zones adjacent to the surface have different cell zone IDs. Furthermore, you must correctly specify which of the two zones is occupied by the quadrupole sources (interior cell zone). This will allow ANSYS FLUENT to determine the direction in which the sound will propagate. When you first attempt to select a legitimate interior surface (i.e., an interior surface having two different cell zones on both sides) as a source surface, the Interior Cell Zone Selection dialog box (Figure  22.2.5) will appear. You will then need to select the interior cell zones from the two zones listed under the Interior Cell Zone. Figure  22.2.6 shows an example of an interior source surface.

Figure 22.2.5: The Interior Cell Zone Selection Dialog Box

Like general interior surfaces, if the source surfaces selected are sliding interfaces, a dialog box similar to Figure  22.2.5 will appear that will show the two adjacent cell zones and you will be asked to specify the zone which has the sound sources.

When a permeable surface (either interior or sliding interface) is chosen as the source surface, other wall surfaces inside the volume enclosed by the permeable surface that generate sound should not be chosen for the acoustics calculation. For example, when running an "on-the-fly'' calculation, if both these surfaces are selected, the sound pressure will be counted twice.

Figure 22.2.6: An Interior Source Surface

Saving Source Data

To save the source data, you have to specify the Source Data Root File Name, Write Frequency (in number of time steps), and Number of Time Steps per File in the Acoustic Sources dialog box.

The Source Data Root File Name is used to give the names of the source data files (e.g., acoustic_examplexxxx.asd, where xxxx is the global time-step index of the transient solution) and an index file (e.g., acoustic_example.index) that will store the information associated with the source data. The Write Frequency allows you to control how often the source data will be written. This will enable you to save disk space if the time-step size used in the transient flow simulation is smaller than necessary to resolve the sound frequency you are attempting to predict. In most situations, however, you will want to save the source data at every time step and use the default value of 1.

Since acoustics calculations usually generate thousands of time steps of source data, you may want to split the data into several files. Specifying the Number of Time Steps per File allows you to write the source data into separate files for different simulation intervals, the duration of which (in terms of the number of transient flow time steps) is specified by you. For example, if you specify 100 for this parameter, each file will contain source data for an interval length of 100 time steps regardless of the write frequency.

You will find this feature useful if you want to use a selected number of source data files to compute the sound pressure rather than using all the data. For example, you may want to exclude an initial portion of the source data from your acoustics calculation because you may realize later that the flow field has not fully attained a statistically steady state.

After you click Apply, ANSYS FLUENT will create the index file (e.g., acoustics_example.index), which contains information about the source data.

If you choose to save source data, keep in mind that the source data can use up a considerable amount of disk space, especially if the mesh being used has a large number of face elements on the source surfaces you selected. ANSYS FLUENT will print out the disk space requirement per time step at the time of source surface selection if the Export Acoustic Source Data in ASD Format option is enabled in the Acoustics Model dialog box.

At this point, if you have chosen to perform your acoustics calculation in two steps, (i.e., saving the source data first, and computing the sound at a later time), you can go ahead and instruct ANSYS FLUENT to perform a suitable number of time steps, and the source data will be saved to the disk. If you have chosen to perform an "on-the-fly'' acoustic calculation, then you will need to specify receiver locations (see Section  22.2.3) before you run the unsteady ANSYS FLUENT solution any further.