Setting up the Solvers

The solvers setup includes the definition of the time-step size, the solver under-relaxation factors, and the auto-save settings.

Additionally, you can modify the wall heat flux computation to take into account the thickening of the turbulent boundary layer due to high temperature gradients as proposed by Angelberger [146]. Alternatively, you can apply the GruMO-UniMORE wall treatment, which leads to an improvement in wall heat flux and local temperature field predictions in studies of turbocharged / highly-downsized engines [147], [148].

For fuel injection, to increase numerical stability, you can apply smoothing of the Lagrangian sources arising from the fuel parcels entering the continuous gas phase. If the Liquid Film engine model is selected, this smoothing also applies to Lagrangian sources arising from parcels impinging on fluid film. Lagrangian source smoothing spreads the effect of the entering Lagrangian particles over several cells using cell clustering. You are advised to activate source smoothing when the sizes of the fuel droplets are close to, or greater than, the smallest cell sizes in the simulation.

To setup the solvers:
  1. Right-click the Solvers node and select Edit.
  2. Set up the time-step control:
    1. In the Time Step Settings group-box, set the type of time-step control:
      • Time Step—sets a constant time-step size.
      • Stepped Table—allows you to specify the time-step size in tabular form.
      • Automatic—automatically adjusts the time-step size based on valve lift thresholds, on the start and end of fuel injection (for charge-motion simulations), on the ignition timing (for combustion simulations), and on the intersection between liner and plenums. Additionally, you can specify checkpoint times that are to be reached exactly by the automatic time-stepping.

        If you use the CHT Export model or activated Reuse Meshes, Simcenter STAR-CCM+ In-cylinder automatically activates the automatic time-step control. For CHT export, Simcenter STAR-CCM+ In-cylinder sets checkpoint times for the crank angles at which the cycle-averaged heat transfer data are mapped. For mesh reuse, Simcenter STAR-CCM+ In-cylinder sets checkpoints 0.5 deg after the start angle, after each cycle length, and at the end of each cycle.

        The default settings of the automatic time-step control are suitable for many simulations. However, you are advised to adjust the default values in the following situations:

        • If you use the RNG K-Epsilon model, to aid solution convergence, you are advised to increase the lift offset slightly below and slightly above the valve closure tolerance and to decrease the corresponding time-step sizes.

          Click Settings and, in the Auto Time-Step Settings dialog, set the following properties:

          Valves
          Lift Offset Time-Step
          Very Low -0.05 mm 2.5E-7 s
          LCT 0.0 mm (read-only) 2.5E-7 s
          Low 0.05 mm 1.0E-6 s (default)
        • For Lash values that cut off the smooth opening and closing ramps of the valve lift profile, to provide enough time for the automatic time-step control to decrease the time step size during valve opening and closing, you are advised to adjust the lift offset values.

          Click Settings and, in the Auto Time-Step Settings dialog, set the following properties:

          Valves
          Lift Offset
          Very Low -0.05 mm
          Low 0.05 mm
        • For engines with a high angular velocity, such as ≥ 5000 RPM, where the first valve opening event in the simulation is the opening of an exhaust valve, you are advised to decrease the time-step size before and after the first valve opening.

          Click Settings and, in the Auto Time-Step Settings dialog, set the following properties:

          Valves
          First Opening Event Time-Step
          Before 1.0E-8 s
          After 1.0E-8 s
        • If you use the Liquid Film model, to aid solution convergence, you are advised to increase the period of time after fuel injection and to decrease the corresponding time-step size.

          Click Settings and, in the Auto Time-Step Settings dialog, set the following properties:

          Injectors
          Period Time-Step
          After Injection This values depends on the duration for which fuel film is present on the engine walls.

          A value of 100.0 degCA is suitable for many situations. However, you might have to increase or decrease this value if fuel film is present for a longer or shorter period in time, respectively.

          		 1.0E-6 s
      You can either select a single time-step control or combine the Stepped Table with the Automatic time-step control. This combination allows you to provide the time-step size in tabular form but also allows Simcenter STAR-CCM+ In-cylinder to control the time-step in critical situations, such as valve opening or closing. Simcenter STAR-CCM+ In-cylinder applies the minimum of the requested time-step sizes.
      See Solvers Reference—Time Step Settings.
    2. In general, Simcenter STAR-CCM+ In-cylinder allows you to specify time-step sizes in s or degCA. To check the time-step size in the alternative unit, hover over your input value.


    3. To preview the time-step size before running the simulation, click Apply, then click the Time Step Preview tab.
      The Time Step Preview plot displays the time-step size and the applied valve lift curves for the duration of the simulation as a function of crank angle.
  3. In the Solver Settings group box, set the following properties:
    • Segregated Flow under-relaxation factors
    • Segregated Energy under-relaxation factors
    See Solvers Reference—Solver Settings.
To save separate copies of the simulation file automatically at regular intervals during the run:
  1. In the Auto-Save Settings group-box, activate Enabled and set the following properties:.
    • The trigger for the auto-save operation, such as Number of Time Steps
    • Number of Files to Keep
    See Solvers Reference—Auto-Save Settings.
To modify the default wall heat flux computation:
  1. In the Wall Treatment group box, set Formulation to one of the following options:
    • Angelberger Formulation
    • Gru-MO UniMORE Formulation
  2. For the Angelberger formulation, depending on the type of engine, set Angelberger Formulation to Gasoline or Diesel.
    See Solvers Reference—Wall Treatment.
To activate Lagrangian source smoothing:
  1. In the Lagrangian Cell Cluster Length Fraction of Normalized Prism Thickness group box, activate Enabled.
  2. Set Cell Cluster Length Fraction to the ratio of the desired cluster length to the normalized prism layer thickness, then click Apply.
    Cell Cluster Length updates to display the absolute value of the cluster length.
    See Solvers Reference—Lagrangian Cell Cluster Length Fraction of Mesh Size.
  3. Click Close.
The solvers settings are now defined. In the background, the selected conditions and parameters are set in the Simcenter STAR-CCM+ simulation tree.