Guidelines for Using the Acoustic Wave Model

This section contains recommendations for using the Acoustic Wave model.

• When you use the Acoustic Wave model, you are advised to activate the Cell Quality Remediation model.
• Use a high-resolution turbulence model to ensure good noise source predictions.
• Use the appropriate mesh resolution in the zone of interest.

  The mesh resolution must be based on the recommended points per wavelength (PPW) to maintain the accuracy of acoustic predictions. PPW = (speed of sound/frequency) / grid spacing.

  You are advised to use PPW ≥ 30–40, and this value must be maintained throughout the zone of interest. The recommended Acoustic CFL depends on the PPW: for PPW of 40, you are advised to set Acoustic CFL ≤ 4 inside the zone of interest. However, for higher PPW values, a higher Acoustic CFL is acceptable.

• Apply damping to improve acoustic prediction. There are two damping methods that you can use to improve simulation stability:

  • Numerical damping that is applied to the Acoustic Wave solver

    The implementation of the Acoustic Wave model takes into account the solver robustness, but some instability can still arise due to mesh imperfections. Over time in a long simulation, localized hot spots of acoustic pressure can appear. These hot spots can lead to the accumulation and spreading of instability.

    To suppress simulation instability due to spurious effects, slightly decrease the Newmark Alpha Parameter in the Acoustic Wave solver.

  • Physical damping to eliminate spurious acoustics from mesh transition or boundary effects

    In order to eliminate acoustic wave reflections into the area of interest, you can apply a damping function (f_damp) to the region outside the area of interest. The reflections are attenuated and their effects are minimized.

    For more information, see Reducing Spurious Acoustic Reflections.

• Apply APE source filtering to remove possible high frequency spurious content from the noise sources. 1st-, 2nd-, or 3rd-order time filters can be used with increasing effect of numerical damping. However, higher-order filters introduce phase errors in the acoustic solution.
• When you specify field functions to define the noise source filtering (f_source) and acoustic damping (f_damp) zones, a Hanning windowing is recommended for the transition from a value of 0–1.

  The Hanning function has the general form:

  $$\text{H}\left(r\right)=0.5\left(1-\text{cos}\left(\pi \left(\frac{\mathbf{\text{r}}}{\mathbf{\text{t}}}\right)\right)\right)$$

  (272)

  where:

  • $\mathbf{\text{r}}$ is the direction vector
  • $\mathbf{\text{t}}$ is the thickness
• A hybrid approach allows you to benefit from the acoustic wave solver and still propagate acoustics to a distant far-field observer. Achieve this goal by using the acoustic wave solver in the near field and propagating the acoustics using the Ffowcs Williams-Hawkings (FW-H) model. To do so:
  1. Activate the Acoustic Wave model property Enable Total APE Fields for Post FW-H. This makes the following field functions available:
     • APE Total Density, flow density + acoustic density. See Eqn. (4725).
     • APE Total Velocity, flow velocity + acoustic velocity. See Eqn. (4726).
     • APE Total Pressure, flow pressure + acoustic pressure. See Eqn. (4727).
  2. Create a solution history node to export a selection of these functions to a .simh file. See Recording Transient Solution Data.
  3. Select receivers under Post FW-H Receivers and set the Acoustic Data Source property to APE.
  4. Run the Post FW-H Solver using the .simh file and selected receivers.