Acoustic Modal Analysis Workflow

You perform an acoustic modal analysis from a previously converged solution.

For details of model requirements and properties, see Acoustic Modal Analysis Model Reference.
注If required, you can automate the Acoustic Modes solution using the Simulation Operations feature. See Solve Accoustic Modes.
To perform an acoustic modal analysis:
  1. Run a Simcenter STAR-CCM+ simulation to convergence using the following physics models:
    Group BoxModel
    SpaceAny
    TimeSteady
    MaterialOne of: Gas, Liquid, Multi-Component Gas, or Multiphase.
    Multiphase (only when a multiphase model is selected).Any (except Two-Phase Thermodynamic Equilibrium).
    Reaction Regime (only when Multi-Component Gas is selected)Any
    FlowAny
    Equation of StateOne of: Ideal Gas, Real Gas, or User-Defined EOS. (Any equation of state model that provides Speed of Sound).
    EnergyAny
    Viscous RegimeAny
  2. Within the same simulation that has a converged solution, for the relevant physics continuum, you add the Acoustic Modal Analysis model to the previously selected models.
    1. Right-click the Continua > [physics continuum] node and select Select Models.
    2. In the [physics continuum] Model Selection dialog, in the Optional Models group box, in addition to the previously selected models, select Acoustic Modal Analysis.
  3. Select the Continua > [physics continuum] > Models > Acoustic Modal Analysis node and set values for the following properties:
    PropertySetting
    Number of Modes

    Specify the number of modes that Simcenter STAR-CCM+ computes. For [n] modes, this represents the [n] lowest frequencies.

    Shift Frequency Specify the lowest frequency above which Simcenter STAR-CCM+ computes the nearest acoustic modes.
    Flame Transfer Function

    Available only when the Reacting model is selected in the [physics continuum].

    Zero Source
    Select this option to ignore the effect of the flame.
    N-Tau
    Select this option to allow Simcenter STAR-CCM+ to account for the unsteady heat release in the flame by using the interaction indexes and time delays that you specify within the [n-tau flame transfer function] sub-nodes. You can specify multiple n-tau flame transfer functions to model the response of the flame to several upstream fluctuations, such as multiple inlets.
    User N-Tau
    Select this option to manually specify profiles for the interaction index and time lag. Simcenter STAR-CCM+ uses these to account for the unsteady heat release in the flame. You can create further user specified n-tau flame transfer functions to model the response of the flame to several upstream fluctuations, such as multiple inlets.

    For further information, see Acoustic Modal Analysis Properties.

Set the boundary conditions.
  1. For each boundary, select the Physics Conditions > Acoustic Impedance Specification node and set the Method as required.
    All methods are available for all boundary types, although some methods are better suited for specific boundary types.
    Acoustic Impedance Specification MethodUseBoundary Type
    Perfectly Reflecting

    When acoustic pressure is perfectly reflected from a boundary.

    For example, at fully rigid walls, or at reflecting inlets where an inflow velocity is imposed which zeroes any velocity fluctuations.

    		Inflow, Outflow, Wall or Symmetry
    Zero Acoustic Pressure

    When there are no pressure fluctuations at the boundary (that is perfectly reflective).

    For example, at fully reflecting outlets where a specified pressure is imposed which zeroes the pressure fluctuations.

    		Inflow or Outflow
    Specified Impedance

    To set the magnitude and phase of reflections at a boundary that are independent of the acoustic frequency.

    For example, when modeling the effect of unresolved geometry beyond the outlet—such as a turbine vane downstream of a combustor.

    		Inflow, Outflow, or Walls
    Quadratic Impedance

    To set specified impedance boundary conditions where the magnitude and phase can be specified as a quadratic function of the acoustic frequency.

    		Inflow, Outflow, or Walls
  2. Right-click the Solvers > Acoustic Modal Solver node and select Solve for Acoustic Modes.
    The acoustic modal solver computes the modes and displays a table in the output window. This table contains details of the frequency and growth rate for each acoustic mode.
    注If the solution does not converge, select the Acoustic Modal Solver node, then gradually reduce the Tolerance Scaling Factor and increase the Iteration Scaling Factor until the solution converges.
Once the acoustic modal solver has finished solving the acoustic modes, you can visualize the results in scenes, plots and tables, as appropriate.
  1. To view or export the Acoustic Frequency Table, do the following:
    1. Expand the [physics continuum] > Models > Acoustic Modal Analysis node.
    2. Right-click the Acoustic Frequency Table node and to display the table in a pop-out window, select Tabulate, or to download and save the table, select Export.
      Upon viewing the acoustic frequency table, if any of the growth rate values are positive, the associated modes are unstable. In the following example, modes 5 and 6 are unstable.

      For further information about unstable modes, see Unstable Modes
  2. To visualize the acoustic mode shapes, do the following:
    1. Create a scalar scene and set Parts to the region in which to display the modes.
    2. In the graphics window, right-click an empty space and select Apply Representation to Displayers > Acoustic Modes View.
    3. Select the [scalar] > Scalar Field node and set the Function to Acoustic Pressure Normalized.
    4. Select the Solution Views > Acoustic Modes View node and use Acoustic Mode to select the mode number to display.
    注The Acoustic Pressure Normalized field function is only available when using the Solution Views > Acoustic Modes View. Otherwise, you can create a scalar scene/displayer for each acoustic mode and use the different Acoustic Pressure Normalized - Mode [n] field functions in each scene/displayer.
    For example, the following images show a sample of acoustic mode shapes for a closed cylindrical domain:

    The first acoustic mode shown above (top left), acoustic mode 1, shows the lowest frequency mode—which corresponds to the half wavelength.