Acoustic Suppression Zone Model (Deprecated)

The Acoustic Suppression Zone model is considered a practical method to suppress spurious reflective disturbances in unsteady aeroacoustic simulations at the exterior boundaries. It only applies to compressible flows, but can be used in any aeroacoustic calculation.

Running transient flow simulations produces time-dependent variations in the flow. These flow disturbances convect with the flow (for vortical disturbances) or propagate at the speed of sound (for acoustic disturbances) outward to exterior boundaries.

If the domain boundaries have reflective properties, these disturbances can be spuriously reflected back into the domain. This reflection affects the flow solution, causing problems for acoustic-type calculations.

To satisfy the requirements for good quality aeroacoustic calculations, the numerical algorithm must allow these pressure waves and vortices to travel through the boundary smoothly. Or the numerical algorithm must give the boundary quasi-non-reflective properties.

For example, a pressure outlet with non-reflecting properties allows the defined constant pressure to represent an average pressure at the outlet boundary. At the same time, it allows the flow pressure at the boundary to vary and disturbances to pass out of the domain without reflecting off the boundary.

A practical approach to constructing a non-reflective boundary treatment is to complement the respective characteristics-based boundary conditions (correct imposed flow boundary conditions values) with an Acoustics Suppression Zone model.

The Acoustic Suppression Zone model adds appropriate damping term(s) to the right-hand side of the governing equations for both coupled and segregated flow, of the form:

where:

• $\sigma$ is the relaxation coefficient.

  When the boundary distance is less than or equal to the zone thickness, the specific variation is a smooth transition from 0 to the Maximum Relaxation Coefficient at the boundary:

  $\sigma =\frac{{\sigma }_{max}}{2}(1+\cos \pi \frac{d}{w})$

  where:

  • ${\sigma }_{max}$ is the maximum relaxation coefficient. It is the inverse of the relaxation time and is measured in sec^-1. The value must be greater than zero, and values of several hundred are typical. The default value is zero, meaning no attenuation.
  • $d$ is the distance from the centroid of the cell in question to the boundary using Acoustic Suppression.
  • $w$ is the width of the Acoustic Suppression Zone.
• $Q=\{\rho ,\rho v,\rho E\}$ .

  where:

  • $v$ is the flow velocity vector.
  • $E$ is the total energy, $E=C_{v}T+0.5{\left|v\right|}^2$ , where $C_v$ is the heat capacity at constant volume.

These additional terms damp flow disturbances (computed for the mean or target flow conditions), before they reach the boundary. This Acoustic Suppression Zone treatment helps characteristics-based boundary conditions to produce a quasi non-reflective response, that is, to allow disturbances to travel through the boundary without producing spurious back-reflections.