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21.3.4 Postprocessing the FW-H Acoustics Model Data

At this point, you will have either the source data saved to files or the sound pressure signals computed, or both. You can process these data to compute and plot various acoustic quantities using FLUENT's FFT capabilities. See Section  28.10 for more information.



Writing Acoustic Signals


If you chose to perform the acoustic calculation "on the fly'', you will need to write the sound pressure data to files. To do so, select Write Acoustic Signals under Options in the Acoustic Signals panel (Figure  21.3.6) 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 panel (e.g., receiver-1.ard).

Solve $\rightarrow$ Acoustic Signals...



Reading Unsteady Acoustic Source Data


Computing the sound pressure signals using the source data saved to files is done in the Acoustic Signals panel (Figure  21.3.6) .

Solve $\rightarrow$ Acoustic Signals...

Figure 21.3.6: The Acoustic Signals Panel
figure

To compute the sound data, use the following procedure:

1.   In the Acoustic Signals panel, select Read Unsteady Acoustic Source Data Files under Options.

2.   Click Load Index File... and select the index file for your computation in the Select File dialog box. The file will have the name you entered in the Source Data Root Filename field in the Acoustic Sources panel, followed by the .index suffix (e.g., acoustic_example.index).

3.   In the Source Data Files list, select the source data files that you want to use to compute sound. Source data files will all contain the specified root file name followed by the suffix .asd.

figure   

You can use any number of source data files. However, note that you should select only consecutive files.

4.   In the Active Source Zones list, select the source zones you want to include to compute sound. See Section  21.3.2 for details about proper source surface selection.

5.   In the Receivers list, select the receivers for which you want to compute and save sound.

Optionally, you can click the Receivers... button to open the Acoustic Receivers panel and define additional receivers.

6.   Click the Compute/Write button to compute and save the sound pressure data. One file will be saved for each receiver you previously specified in the Acoustic Receivers panel (e.g., receiver-1.ard).

figure   

If you enabled both the Export Acoustic Source Data and Compute Acoustic Signals Simultaneously options in the Acoustics Model panel, you will need to first select the Write Acoustic Signals option in the Acoustic Signals panel after the flow simulation has been completed. If you select the Read Unsteady Acoustic Source Data Files before writing out the "on-the-fly'' data in such a case, the data will be flushed out of the internal buffer memory. To avoid such a loss of data, you should save the FLUENT case and data files whenever you begin to do an acoustic computation in the Acoustic Signals panel. The sound pressure data calculated "on the fly'' will then be saved into the .dat file. Finally, after the "on-the-fly'' data is saved, make sure to change the file names of the receivers before doing a sound pressure calculation with the Read Unsteady Acoustic Source Data Files option enabled, to avoid overwriting the "on-the-fly'' signal files.

figure   

Note that you can compute and write sound pressure signals only when the FW-H acoustics model has been enabled. See Section  21.3.1 for details about exporting source data (e.g., for SYSNOISE) without enabling the FW-H model.

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 $\tau_0$ to $\tau_n$. Without auto-pruning, the receiver register begins receiving the earliest sound pressure signal at

t_0 = \tau_0 +\frac{r_{\rm min}}{a_0}

where $r_{\rm min}$ 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 ( $r_{\rm max}$) until the receiver time becomes

t_1 = \tau_0 +\frac{r_{\rm max}}{a_0}

From time $t_0$ to $t_1$, the sound accumulated on the receiver register does not include the contribution from the entire source surface area, and thus the sound pressure data received during that time is not complete. The same thing occurs during the period from

t_m = \tau_m +\frac{r_{\rm min}}{a_0}

to

t_n = \tau_n +\frac{r_{\rm max}}{a_0}

Thus, pruning means clipping the signal on the incomplete ends, from $t_0$ to $t_1$ and $t_m$ to $t_n$. Auto-pruning can be disabled using the

define $\rightarrow$ models $\rightarrow$ acoustics $\rightarrow$ auto-prune text command.

Although auto-pruning can be disabled, it is expected that you will use only the complete sound pressure data.



Reporting the Static Pressure Time Derivative


The RMS value of the static pressure time derivative ( $\partial p/\partial t$) 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 panels.



Using the FFT Capabilities


Once the sound pressure signals are computed and saved in files, the sound data is ready to be analyzed using FLUENT's FFT tools. In the Fourier Transform panel (Figure  28.10.1), click on Load Input File... and select the appropriate .ard file. If the receiver data is still in FLUENT's memory, then it can directly be processed using the Process Receiver option. See Section  28.10 for more information on FLUENT's FFT capabilities.

Plot $\rightarrow$ FFT...


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