Preparing the Simulations for Co-Simulation

Modify standalone Simcenter STAR-CCM+ simulations for interactive co-simulation.

  1. Develop standalone Simcenter STAR-CCM+ simulations, without any co-simulation settings. In each simulation, activate physics models as required by your analysis:
    • For conjugate heat transfer analysis, activate an energy model in both simulations.
    • For FSI applications that model the displacement of a solid in response to fluid loads:
      • In the fluid simulation, assign a morphing motion to the fluid region, so that the fluid mesh updates in response to the imported solid displacements. For instructions, see Setting Up Morphing Motion and Solver. When the effect of the solid on the fluid can be neglected, morphing motion is not required.
      • In the solid simulation, activate the Solid Stress model and set up the simulation for stress analysis, as required. Form more information on solid stress simulations, see General Workflow for Stress Analysis.
  2. In each simulation, use provisional boundary conditions that are close to the expected values from the partner simulation.
  3. Run the standalone simulations to convergence before attempting co-simulation.
    If either standalone model is improperly defined or has numerical problems, you are unlikely to obtain a solution for a co-simulation.
To enable co-simulation:
  1. In each simulation, activate the following additional models:
    Group Physics Model
    Optional Models Co-Simulation
    Co-Simulation Models Simcenter STAR-CCM+
    Co-Simulation Treatment Models Optionally, you can activate one or more of the following co-simulation treatment models:
    • For predefined CHT co-simulation, where the fluid and the solid simulations exchange solid temperature and fluid heat flux (heat transfer coefficient and reference temperature), activate the Co-Simulation Thermal Treatment model.
    • For predefined FSI co-simulation, where the fluid and the solid simulations exchange solid displacements and fluid loads (pressure and wall shear stress), activate the Co-Simulation Mechanical Treatment model.

    With these models, you can instruct Simcenter STAR-CCM+ to automatically set boundary conditions based on the imported data. See Specifying the Exchanged Fields.

Each simulation also requires an additional physics continuum that represents the physics in the partner simulation. In this continuum, you select the External Continuum model, which indicates that the physics is defined by an external application. In each simulation:
  1. Create a physics continuum with the following physics models:
    Group box Model
    Optional Models External Continuum
    Space Surface Three Dimensional
    Time Steady or Implicit Unsteady
    External Continuum External Application
    External Application Simcenter STAR-CCM+
    Coupling Models
    • In steady simulations, activate the Steady Coupling model, which allows the steady simulation to exchange data at a specified frequency that you specify in terms of the of solver iterations.
    • In transient (unsteady) simulations, activate either one of the following models:
      • Explicit Coupling— allows the simulation to exchange data once per time-step, at a specified coupling time interval. You can either instruct Simcenter STAR-CCM+ to automatically adjust the simulation time-step to meet the coupling time, or you can preserve the specified time-step and allow Simcenter STAR-CCM+ to adjust the coupling time. See Time-Step Adjustment.
      • Implicit Coupling—allows the simulation to exchange data more than once per time-step, at a frequency that you specify in terms of the solver inner iterations.

      In general, for CHT and FSI problems, the Implicit Coupling model allows you to reach a stable solution with larger time-steps than the time-steps required by the Explicit Coupling model. Using the Implicit Coupling model is also recommended for FSI problems that require multiple data exchanges per time-step to reach a stable solution.

    NoteYou can couple two steady simulations, two transient simulations, or a steady simulation and a transient simulation. In co-simulation between two transient simulations, activate the same Simcenter STAR-CCM+ coupling model in both simulations. Coupling between a simulation that uses the explicit scheme and a simulation that uses the implicit scheme is not supported.
    The following image shows an example of model selection for two simulations that are coupled in a Simcenter STAR-CCM+ to Simcenter STAR-CCM+ CHT co-simulation.

When you activate the External Continuum model, Simcenter STAR-CCM+ adds the External Links node to the simulation tree and creates a link node, [Link 1]. A link represents a connection to an external application. You specify all co-simulation settings under this node.
  • You can rename links as desired. However, the name of a link must match the name of the corresponding link in the partner simulation.


For the link, you specify the external continuum that represents the partner simulation:

  1. Select the [Link 1] > Values > External Continua node and set Continua to the physics continuum that represents the partner simulation.
Specify a leading and a lagging simulation. When you run the co-simulation, the leading simulation runs first, whereas the lagging simulation waits for data from the leading simulation:
  • Fluid Lead and Solid Lag—the displacement from the previous time step is used.
  • Solid Lead and Fluid Lag—the displacement resulting from the Solid Stress Solver integrating the time-step using FSI traction from the previous time-step is used.
For FSI co-simulations specifying the Concurrency Option to lead or lag influences the initialization of the Multiphysics Stabilization Solver, see Multiphysics Stabilization Solver Reference. The difference between each case only affects the starting prediction of the unknown Interface Displacement field.
  1. In one of the simulations, select the [Link 1] > Conditions > Concurrency Option node and set Method to Lead.
    For more information, see [Link 1] Conditions and Values.
  2. Using the same procedure, set the other simulation to Lag.
  3. In each simulation, set the [Link 1] > Conditions > Mapping Option to specify how the data mapping occurs. For more information, see [Link 1] Conditions and Values.
  4. For FSI co-simulations where the fluid and solid are strongly coupled, activate the Multiphysics Stabilization Solver:
    1. In the solid simulation, make sure that the external continuum contains the Implicit Coupling model.
    2. Select the Solvers > Simcenter STAR-CCM+ Co-Simulation node and activate Multiphysics Stabilization.
    3. Select the Solvers > Multiphysics Stabilization Solver node and set the relevant properties.
      For more information, see Multiphysics Stabilization Solver Reference.
To run the co-simulation, you can click Run in either simulation. Clicking Run in either simulation does not determine which simulation runs first. The leading simulation, as specified within the Concurrency Option, always runs first. The simulation where you click Run handles the connection to the partner simulation and, in the case of inconsistent settings, enforces the correct settings in the partner simulation.

Optionally, you can set the simulation where you click Run to launch the partner simulation automatically on co-simulation run.

  1. In the simulation where you click Run, select the Conditions > Launch Partner Option node and specify the method for launching the partner simulation:
    OptionStep
    Launch the partner simulation manually Set Launch Partner Option to Do not Launch
    Launch the partner simulation automatically when the co-simulation starts Set Launch Partner Option to Launch Application
    Launch the partner simulation automatically via the command line Set Launch Partner Option to Launch Application via Command Line
    Launch the partner simulation via Resource manager Set Launch Partner Option to Launch Application via Resource Manager
    Simcenter STAR-CCM+ automatically specifies consistent settings in the partner simulation.

    For more information, see Simcenter STAR-CCM+ Link Reference.

  2. Specify the method for connecting to the partner simulation:
    • If the simulations are located in the same directory and you launch the simulations separately, you do not need to specify host and port details. In each simulation, select the Conditions > Connect Option node and set Option to Connection Settings via File. Simcenter STAR-CCM+ saves the host and port details to connection files, which are automatically used for connection when you click Run in either simulation. For more information, see Connection Settings via File.
    • If you launch the simulations manually, but they are not located in the same directory, specify the host and port of the partner simulation in the simulation where you click Run:
      1. Select the Conditions > Connect Option node and set Option to Host:Port.
      2. Select the Values > Simcenter STAR-CCM+ Partner Host:Port. In the Properties window, click (Custom Editor) and select the partner simulation from the list of all currently active Simcenter STAR-CCM+ simulations.
    • If you instruct one simulation to launch the partner simulation automatically, the simulation connects to the partner using the launching details. The Values > Assigned Partner Host:Port node displays the host and port where the partner simulation is running.