Setting Up the Acoustic Modal Analysis Model

The Acoustic Modal Analysis model allows you to simulate acoustic mode shapes, frequencies, and linear growth rates (whether the waves are damped and stable, or amplified and unstable).

If the unsteady heat released in the geometry from combustion is in phase with the acoustic pressure waves, the acoustic pressure waves can amplify—leading to thermo-acoustic instability. To model the effect of the unsteady heat release rate on the acoustic pressure, Simcenter STAR-CCM+ provides the N-Tau Model flame transfer function. The N-Tau model is a time delay model where the effect of the unsteady heat release on the local pressure is modeled as proportional to the heat release (by a factor n) delayed by some time (denoted tau).

For this burner simulation, you expect that the 5th and 6th predicted modes are unstable.To capture these modes, you set the overall number of modes for this analysis to 6. For the N-Tau model, you set the interaction index (factor n) to 4, the time delay (tau) to 0.0005 s, and the reference location to z = 0.642 m, which is just upstream of the flame front.

To set up the Acoustic Modal Analysis model:

  1. Right-click the Continua > Physics 1 > Models node and choose Select Models.
  2. In addition to the already selected models, in the Optional Models group box, select Acoustic Modal Analysis, then click Close.
  3. Edit the Continua > Physics 1 > Models > Acoustic Modal Analysis node and set the following properties:
    表 1.
    NodeProperty Setting
    Acoustic Modal AnalysisNumber of Modes6
    Flame Transfer FunctionN-Tau Model
    N-Tau Model > N-Tau Flame Transfer Function 1Reference Location[0.00003, 0.00003, 0.642]
    Reference Direction[0.0, 0.0, 1.0]
    Interaction Index4.0
    Time Delay5.0E-4 s
  4. Save the simulation.