Communication time step

Interfaced models exchange data only at specified communication time steps. The interval between two consecutive time steps is specified by the communication step size. Generally, care must be taken to not introduce a direct transfer between exchanged inputs and outputs, as algebraic loop cannot be managed easily at interface level.

Fixed step size (synonym: discrete co-simulation)

The step size is specified before the start of simulation and remains constant during simulation. Each model is seen by the other one as a discrete system.

Example: a Simcenter Amesim model compiled for a real time target exchanges data with the real time host environment at fixed communication time steps.

Scheduled step size

The step size may vary during simulation. Time steps are specified either before starting simulation, either during simulation at request by one of the interfaced model. Communication occurs at these predefined communication time steps which cannot be changed after being set. When scheduled communication time step is met, a new time step may be defined in the future, but current time step cannot be cancelled.

• Meeting point At each time step, interfaced models agree to schedule the next time step, which is mandatory: communication must take place at the agreed time step, and cannot occur before or after this time step. Example: no known example in Simcenter Amesim.

• Master/slave relationship (synonym: Hybrid Co-simulation) Interfaced models are in master/slave relation. The master model arbitrarily chooses the next time step at which communication takes place with the slave model. Slave model is seen by master as a discrete system activated at variable time step. The resulting system is said a hybrid system since it includes continuous and discrete dynamics. Example: a CFD software acting as master is interfaced with a Simcenter Amesim model acting as slave. The CFD solver chooses a convenient time step and orders the Simcenter Amesim solver to perform simulation over the scheduled time interval, holding its inputs constant, and then communicate its outputs at the end of the time step, no matter what time steps are chosen internally by the Simcenter Amesim solver.

Variable step size

Interfaced models are fully integrated, such that the step size may vary arbitrarily during simulation. The whole model behaves like a fully continuous system with its own variable step size solver. Generally, this implies a form of global convergence error and discontinuity handling supervision algorithm, which is able to adapt the step size in order to limit the convergence error of each individual model.

Example: no known example in Simcenter Amesim.