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21.3 Using the Ffowcs Williams and Hawkings Acoustics Model

The procedure for computing sound using the FW-H acoustics model in FLUENT consists largely of two steps. In the first step, a time-accurate flow solution is generated, from which time histories of the relevant variables (e.g., pressure, velocity, and density) on the selected source surfaces are obtained. In the second step, sound pressure signals at the user-specified receiver locations are computed using the source data collected during the first step.

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Note that you can also use the FW-H model for a steady-state simulation in the case where your model has a single rotating reference frame. Here, the loading noise due to the motion of the noise sources is computed using the FW-H integrals (see Equations  21.2-5 and 21.2-6), except that the term involving the time derivative of surface pressure ( $\dot{L}_r$ in Equation  21.2-6) is set to zero.

In computing sound pressure using the FW-H integral solution, FLUENT uses a so-called "forward-time projection'' to account for the time delay between the emission time (the time at which the sound is emitted from the source) and the reception time (the time at which the sound arrives at the receiver location). The forward-time projection approach enables you to compute sound at the same time "on the fly'' as the transient flow solution progresses, without having to save the source data.

In this section, the procedure for setting up and using the FW-H acoustics model is outlined first, followed by detailed descriptions of each of the steps involved. Remember that only the steps that are pertinent to acoustics modeling are discussed here. For information about the inputs related to other models that you are using in conjunction with the FW-H acoustics model, see the appropriate sections for those models.

The general procedure for carrying out an FW-H acoustics calculation in FLUENT is as follows:

1.   Calculate a converged flow solution. For a transient case, run the transient solution until you obtain a "statistically steady-state" solution as described below.

2.   Enable the FW-H acoustics model and set the associated model parameters.

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

3.   Specify the source surface(s) and choose the options associated with acquisition and saving of the source data. For a steady-state case, specify the rotating surface zone(s) as the source surface(s).

4.   Specify the receiver location(s).

5.   Continue the transient solution for a sufficiently long period of time and save the source data (transient cases only).

Solve $\rightarrow$ Iterate...

6.   Compute and save the sound pressure signals.

Solve $\rightarrow$ Acoustic Signals...

7.   Postprocess the sound pressure signals.

Plot $\rightarrow$ FFT...

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Before you start the acoustics calculation for a transient case, a FLUENT transient solution should have been run to a point where the transient flow field has become "statistically steady''. In practice, this means that the unsteady flow field under consideration, including all the major flow variables, has become fully developed in such a way that its statistics do not change with time. Monitoring the major flow variables at selected points in the domain is helpful for determining if this condition has been met.

As discussed earlier, URANS, DES, and LES are all legitimate candidates for transient flow calculations. For stationary source surfaces, the frequency of the aerodynamically generated sound heard at the receivers is largely determined by the time scale or frequency of the underlying flow. Therefore, one way to determine the time-step size for the transient computation is to make it small enough to resolve the smallest characteristic time scale of the flow at hand that can be reproduced by the mesh and turbulence adopted in your model.

Once you have obtained a statistically stationary flow-field solution, you are ready to acquire the source data.




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