Under Model Constants in the Acoustics Model dialog box, 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.

The FW-H acoustics model in Ansys 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, enable the Compute Acoustic Signals Simultaneously option in the Acoustics Model dialog box.

When the Compute Acoustic Signals Simultaneously option is enabled, the Ansys Fluent console window will print a message at the end of each time step indicating that the sound pressure signals have been computed (for example, Computing sound signals at x receiver locations..., where x is the number of receivers you specified). Enabling this option instructs Ansys Fluent to compute sound pressure signals at the end of each time step, which will slightly increase the computation time.

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 save the source data to files, enable either the Export Acoustic Source Data in ASD Format or the Export Acoustic Source Data in CGNS Format option, or both. After you have made your selection, 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 23.1: Source Data Saved in Source Data Files shows the flow variables saved as the source data.

Figure 23.2: The Acoustics Model Dialog Box for a 3D Steady-State Case with a Single Moving Reference Frame

Table 23.1: Source Data Saved in Source Data Files

Solver Option	Source Surface	Source Data
incompressible	walls
incompressible	permeable surfaces
compressible	walls
compressible	permeable surfaces

See Specifying Source Surfaces for details on how to specify parameters for exporting source data.

 

23.2.1.3.2. Exporting Source Data Without Enabling the FW-H Model: Using the CGNS Format

The sound source data for non-permeable surfaces can be exported in the CGNS file format (for Simcenter 3D Acoustics) without having to enable the Ffowcs Williams and Hawkings (FW-H) model. Enable the Export Acoustic Source Data in CGNS Format option in the Acoustics Model dialog box (Figure 23.3: The Acoustics Model Dialog Box for Exporting in CGNS Format). Specify the source surfaces in the Acoustics Sources dialog box (see Specifying Source Surfaces) where, by default, the Number of Time Steps per File is set to 1.

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Note:  The following limitation currently exists for exporting acoustic source data:

• Source data cannot be simultaneously exported for multiple differently rotating walls. All walls must be either non-rotating, or rotate with the same rate around the same axis. It is a responsibility of a user to select a consistent set of wall zones. Selecting differently rotating zones may cause incorrect acoustics result.

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Figure 23.3: The Acoustics Model Dialog Box for Exporting in CGNS Format

Simcenter 3D Acoustics requires a mesh data file (named <prefix>_mesh.cgns) and a solution data file (named <prefix>_<timestep>.cgns). The string <prefix> is a generic name, which you will specify in the File Name in the Acoustics Sources dialog box. There is one single solution data file (.cgns) per time level exported, which contains the static pressure at the wall-face centroid location. The .cgns files will be stored in a directory, which you specify (named <directory_name>/<prefix>) in the File Name.

In addition, you can export quadrupole sources data by choosing fluid zones as emission sources. To enable the selection of fluid zones as sources, use the text command:

define → models → acoustics → export-volumetric-sources-cgns?

When asked if you would like to Export volumetric sources? enter yes. Note that Simcenter 3D Acoustics requires volumetric mesh data file (<prefix>_Q_mesh.cgns) and quadrupole solution data files (<prefix>_Q_<timestep>.cgns). The .cgns file will be stored in a similar way to that of dipole data export, in the directory you specified in the File Name text entry box.

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

The File Name is used to give the names of the source data files (for example, acoustic_examplexxxx.asd, where xxxx is the global time-step index of the transient solution) and an index file (for example, 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 (for example, acoustics_example.index), which contains information about the source data.

At this point, if you have chosen to perform your acoustics calculation in two steps, (that is, 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 chose to perform an “on-the-fly” acoustic calculation, then you must specify receiver locations (see Specifying Acoustic Receivers) before you run the unsteady Ansys Fluent solution any further.

If you chose to perform the acoustic calculation “on the fly”, you must write the sound pressure data to files. To do so, select Write Acoustic Signals under Options in the Acoustic Signals dialog box (Figure 23.9: The Acoustic Signals Dialog Box) and then click Write. The computed acoustic pressure will be saved from internal buffer memory into a separate file for each receiver you defined in the Acoustic Receivers dialog box (for example, receiver-1.ard).

 Solution →   Run Calculation → Acoustic Signals...

Computing the sound pressure signals using the source data saved to files is done in the Acoustic Signals dialog box (Figure 23.9: The Acoustic Signals Dialog Box)

 Solution →   Run Calculation → Acoustic Signals...

Figure 23.9: The Acoustic Signals Dialog Box

To compute the sound data, use the following procedure:

 

23.2.5.2.1. Pruning the Signal Data Automatically

Before the computed sound pressure data at each receiver is saved, it is by default automatically pruned. Pruning of the receiver data means clipping the tails of the signal where incomplete source information is available.

The acoustic source data is tabulated from time to . Without auto-pruning, the receiver register begins receiving the earliest sound pressure signal at

(23–1)

where is the shortest distance between the source surfaces and the receiver. However, the receiver will not receive the sound pressure signal from the farthest point on the source surfaces () until the receiver time becomes

(23–2)

From time to , the sound accumulated on the receiver register does not include the contribution from the entire source surface area, and therefore the sound pressure data received during that time is not complete. The same thing occurs during the period from

(23–3)

to

(23–4)

Therefore, pruning means clipping the signal on the incomplete ends, from to and to . Auto-pruning can be disabled using the define → models → acoustics → auto-prune text command. Although auto-pruning can be disabled, it is expected that you will use only the complete sound pressure data.

The RMS value of the static pressure time derivative () is available for postprocessing only on wall surfaces, which are at the same time sources of sound, when the FW-H acoustics model is used.

You can select Surface dpdt RMS in the Acoustics... category only when you specify at least one wall surface, which is also marked as an acoustic source, in the relevant postprocessing dialog boxes.

Once the sound pressure signals are computed and saved in files, the sound data is ready to be analyzed using Ansys Fluent’s FFT tools. In the Fourier Transform dialog box (Figure 40.124: The Fourier Transform Dialog Box), click Load Input File... and select the appropriate .ard file. If the receiver data is still in Ansys Fluent’s memory, then it can directly be processed using the Process Receiver option. See Fast Fourier Transform (FFT) Postprocessing for more information on Ansys Fluent’s FFT capabilities.

 Results → Plots → FFT   Edit...

The Acoustic Sources FFT… button is available in the Run Calculation task page under the following conditions:

Clicking the Acoustic Sources FFT… button opens the Acoustic Sources FFT dialog box, which has four tabs (each of which is shown in the figures that follow):

The first two tabs are obligatory, because the actions performed there prepare the Fourier spectra for the actions that follow, including creation of the surface variables to visualize (performed in the third tab) and/or CGNS export (performed in the fourth tab).

Postprocessing begins in the Read ASD Files tab. The Active Source Zones selection list shows all of the face zones for which transient export to ASD files has been performed (see Figure 23.10: The Read ASD Files Tab of the Acoustic Source FFT Dialog Box). The Source Data Files selection list shows all of the available ASD files. Note that you can update this information by clicking the Load Index File… button and specifying the name of the index file to be read; the index file was created during the ASD files export, and is an ASCII file containing a list of the source zones and a list of the ASD file names. Select in the selection lists the desired face zones as well as the desired ASD files from which you want to read, and click the Read button. The console displays information about the data, such as the following example:

Overall 10000 timesteps have been read
from 3.0001 s to 4 s
Time step is 0.0001 s

Figure 23.10: The Read ASD Files Tab of the Acoustic Source FFT Dialog Box

Having read the pressure histories, open the Compute FFT Fields tab (Figure 23.11: The Compute FFT Fields Tab of the Acoustic Source FFT Dialog Box). The values shown in the Spectral Resolution group box provide information about the statistical properties, which depend on the sampling rate (that is, the simulation time step size) and the signal length (that is, the simulation time). These values will change if you enable the Clip Time to Range option in the Sampling Data group box, reduce the time range for the signals by changing the Min and/or Max fields, and click the Re-Estimate Spectral Resolution button. The Window Function selection list allows you to specify a window function using the same choices that are available for plotting the single signal FFT in the Plot/Modify Input Signal dialog box. The default settings use the full signals and the Hanning window function. The FFT calculation begins when you click the Compute button.

Figure 23.11: The Compute FFT Fields Tab of the Acoustic Source FFT Dialog Box

The fields of the pressure histories and the computed fields of the Fourier spectra are stored in a large memory array, referred to here as the “storage area”. This memory is allocated by Fluent dynamically when you read the pressure signals from the ASD files. If you read data for all face zones available in the ASD files, and if they are the wall zones, then the size of the allocated storage area is equal to the total size of the ASD files being read. Since the same or a slightly smaller amount of data is produced by the FFT algorithm, the same storage area can be re-used to store the Fourier spectra without allocating additional memory. Alternatively, the pressure histories can be left intact in memory, and for their spectra the storage area is correspondingly increased.

These two modes of memory allocation are controlled by the Overwrite Signals by Spectra option in the Storage Area Control group box. When the Overwrite Signals by Spectra option is enabled, the Fourier spectra displace in memory the original time signals, which are not kept after the FFT has been computed. Therefore, if you want to re-compute the FFT using the Clip Time to Range tool or a different Window Function (which are both in the Compute FFT Fields tab), you have to first de-allocate the storage area by clicking the Clean Up Entire Storage Area button, and then re-read the pressure histories from the ASD files. When the Overwrite Signals by Spectra option is disabled, you can re-compute the FFT without re-reading the pressure histories, as many times as you want. To do this, click the Clean Up Only FFT Results button, and proceed with computing spectra from the available pressure histories. De-allocation of the storage area (which can be very large) is recommended when you are done using the Acoustic Sources FFT dialog box and plan to either proceed to other postprocessing work or continue your transient simulation without re-starting Fluent.

When the pressure histories are read and kept in the storage area, you can create point probes within the surface zones that have been read and extract the single pressure time signals at these probes. The extracted signals and their spectra may help to analyze the transient flow properties. For example, by creating an XY plot of a signal, you can figure out the need for clipping of the sampling time; by creating an XY plot of its spectrum, you can detect the presence of the tonal noise components.

To create a point probe, open the Point Surface Dialog Box by clicking Create and selecting Point... in the Domain ribbon tab (Surface group box), and create a point surface by any available method (for example, by specifying the coordinates). The point probe will be created at the face center that is nearest to the point surface on the read surface zones. The created point probes are then listed under the point surface names in the Existing Points selection list, which is located in the Point Probes for Time Signals group box. You can print the coordinates of the point probes to the Fluent console by selecting the desired probes and clicking the Print Coordinates button. Clicking the Extract Signals button outputs the pressure histories for the selected probes in the ASCII files; these files can then be analyzed using the Fourier Transform Dialog Box (available from the Plots Task Page) by selecting Process File Data and then reading the exported ASCII file using the Load Input File... button. The exported files names are by default the point probe names with the extension .dat. The File Name field allows you to specify a prefix for these file names.

The FFT Surface Variables tab is shown in Figure 23.12: The FFT Surface Variables Tab of the Acoustic Source FFT Dialog Box for the Octave Bands and Figure 23.14: The FFT Surface Variables Tab of the Acoustic Source FFT Dialog Box for a Set of Individual Modes. This tab allows you to Create new Fluent variables using the computed spectrum fields, which reside in the storage area and correspondingly increase its size. These variables will be defined only on those wall face zones for which the Fourier transformation has been computed; on the other face zones, they will show zero values. Created variables can be used for any kind of further postprocessing in Fluent, which typically includes contour plotting (see the variables marked with the restriction asfft in Table 42.21: Acoustics Category ) or exporting to CFD-Post.

Figure 23.12: The FFT Surface Variables Tab of the Acoustic Source FFT Dialog Box for the Octave Bands

Variables may characterize either the individual Fourier modes (if you select Set of Modes from the Modes/Frequency Bands drop-down list), or frequency bands that each represent a range of frequencies (if you select the other choices in the Modes/Frequency Bands drop-down list). The three types of frequency bands that are available include:

The Spectrum Property drop-down list displays the type of variables created according to the selected choice in the Modes/Frequency Bands drop-down list. For a set of modes, the created variables are the real and the imaginary parts of the complex Fourier amplitudes, with a pair of variables per specified mode. For the frequency bands, the two types of variables can be created:

For any type of frequency band (octave, 1/3, or constant width), the area-averaged band-specific pressure fluctuations are calculated and printed out to the Fluent console in decibels when you click Create. Conversion to decibels is done after the area-averaging of the band-filtered RMS pressure fluctuation fields. Similarly, the area-averaged band-specific values of PSD of the pressure time derivative are calculated and printed out, if this variable type is selected. The area-averaged data are printed only for those frequency bands that are covered by the resulting pressure spectrum. If you enable the Write Area-Avg Data option and specify a file name in the corresponding field, then the area-averaged data will be written to an ASCII file in the XY plot file format (see XY Plot File Format). It is recommended that you use the file name extension .xy to more easily recognize this file when using the XY plotting tool.

If you enable the Plot Area-Avg Data option, then the data will be plotted in the form of a bar chart in the graphics window when you click Create, as shown in Figure 23.13: Bar Chart of Surface Pressure Level for Octave Bands.

Figure 23.13: Bar Chart of Surface Pressure Level for Octave Bands

Figure 23.14: The FFT Surface Variables Tab of the Acoustic Source FFT Dialog Box for a Set of Individual Modes

The allowed number of variables makes it possible for you to simultaneously keep SPL and PSD of dp/dt for all octave and 1/3 octave bands, as well as SPL and PSD of dp/dt for up to 20 constant width bands and the real and imaginary amplitudes for up to 20 individual frequencies. The SPL and PSD of dp/dt variables for the octaves and the 1/3 octaves include the band central frequency in their names (for example, SPL for Octave Band at 250Hz (dB) or PSD of dp/dt for 1/3-Octave Band at 1.25kHz). As for the user-defined frequencies and constant width bands, their variables are simply numbered from 0 to 19. In order to provide you with the information about the meaning of each such variable, Fluent prints a table in the console when you click Create. The following is an example of the table corresponding to the settings in the previous figure:

Creating variables for 20 modes from 40.5983 Hz to 466.88 Hz every 22.4359 Hz.
f( 0) = 40.5983 Hz     f( 1) = 63.0342 Hz 
f( 2) = 85.4701 Hz     f( 3) = 107.906 Hz 
f( 4) = 130.342 Hz     f( 5) = 152.778 Hz 
f( 6) = 175.214 Hz     f( 7) = 197.65 Hz 
f( 8) = 220.085 Hz     f( 9) = 242.521 Hz 
f(10) = 264.957 Hz     f(11) = 287.393 Hz 
f(12) = 309.829 Hz     f(13) = 332.265 Hz 
f(14) = 354.701 Hz     f(15) = 377.137 Hz 
f(16) = 399.573 Hz     f(17) = 422.009 Hz 
f(18) = 444.444 Hz     f(19) = 466.88 Hz

Variables for frequencies above 466.88 Hz are not created, 
because the limit of 20 modes has been reached.

You can analyze more than 20 individual modes or constant width bands, if you process them by portions of 20 items and delete the processed variables by selecting them in the Existing Variables list and clicking the Remove Selected Variables button.

Clicking either the Clean Up Entire Storage Area or the Clean Up Only FFT Results button on the left side of the dialog box not only de-allocates the spectrum storage array, but also removes all created variables.

The Write CGNS Files tab is shown in Figure 23.15: The Write CGNS Files Tab of the Acoustic Source FFT Dialog Box. This tab allows you to specify the zones you want to export to CGNS, by selecting them in the Processed Source Zones list. You can reduce the exported spectrum data by enabling the Reduce Frequency Series option and specifying the frequency range (Min and Max frequencies), as well as the frequency step (the Number of Frequencies to Skip integer field). The latter must be used with care and only if really needed, because the Fourier amplitudes in the broadband noise spectrum depend on the frequency resolution. By skipping frequencies from the exported spectrum (for example, exporting every other Fourier mode), you will artificially coarsen the frequency resolution. However, the exported amplitudes will not be automatically re-scaled for the increased frequency step, but will retain their originally computed values.

Pressure spectra, which result from the scale-resolving flow simulations, may contain thousands of frequencies. Together with dense surface meshes, this can result in a very large amount of data to export. The total size of the disk space required to store fields of the complete spectra is approximately equal to the total size of the ASD files containing the wall pressure histories. Therefore, the wall pressure spectra are written to a series of CGNS files according to the Number of Frequencies per File field. It is recommended that you select an appropriate value for this parameter, to avoid extremely large output files or a large number of very small files. The total number of frequencies in the spectrum can be seen in the Number of Modes field of the Compute FFT Fields tab (see Figure 23.11: The Compute FFT Fields Tab of the Acoustic Source FFT Dialog Box).

Figure 23.15: The Write CGNS Files Tab of the Acoustic Source FFT Dialog Box

Finally, you can specify a name for the exported CGNS files in the File Name field. For example, if you enter wall_pressure_spectrum and then click the Write button, Fluent will create the following set of files in your working folder:

wall_pressure_spectrum.cgns

wall_pressure_spectrum_1.cgns

wall_pressure_spectrum_2.cgns

...

wall_pressure_spectrum.flst

If you leave the File Name field empty, the files will be named cgns, 1.cgns, 2.cgns, ..., flst. The first CGNS file (without a number in its name) contains the mesh data, as well as links to all of the other CGNS files, which in turn contain the spectrum fields with the specified number of modes per file. The CGNS links work like the file links in the UNIX file system: in order to import spectra to the Ansys Mechanical software, you only have to specify the name of the first file, and all other files will be imported automatically if they reside in the same folder. The file with the extension flst is an ASCII file, which contains the list of the exported frequencies.