Setting Up a Sponge Layer

Direct noise simulations that encounter spurious acoustic reflections from exterior boundaries require an efficient non-reflective boundary treatment. This non-reflective boundary treatment usually complements a non-reflective boundary condition (for pressure outlet and stagnation inlet boundaries) with a sponge layer next to the boundary.

Before you set up a sponge layer, make sure that there is enough distance next to the boundary—at least 3 to 5 wavelengths, based on the lowest frequency of interest. If there is not enough distance, extend the geometry of the flow domain. Typically, you apply the sponge layer in the far field where the amplitude of incident fluctuations is expected to be small.
To set up a non-reflective boundary treatment using a sponge layer:
  1. Edit the physics continuum of the direct noise simulation and, in the Optional Models group box, select Sponge Layer.
  2. Select the Models > Sponge Layer node and set the following properties:
    • Relative Amount Of Suppression—the percentual reduction of a disturbance in the sponge layer. The allowed range is 0 ≤ α < 100 .
    • Sponge Layer Thickness—the thickness of the sponge layer. Set a value of 3 to 5 wavelengths.
    See Sponge Layer Model Reference—Properties.
  3. For the flow boundaries for which you encounter spurious acoustic effects, select the Boundaries > [flow boundary] > Physics Conditions > Sponge Layer Option node and activate Enable Sponge Layer.
    Simcenter STAR-CCM+ calculates the sponge layer boundary distance for the boundary.
    See Sponge Layer Model—Boundary Settings.
  4. Re-run the direct noise simulation.
  5. If you still encounter spurious acoustic effects, increase the Relative Amount of Suppression or the Sponge Layer Thickness as described in Step 2.