Simcenter Amesim Reimporting FMU into Simcenter Amesim yields different results
Summary
FMU created by Simcenter Amesim and then reimported for validation can yield different results if the initial conditions are not set appropriately as their default value is 0.
Details
When a reimporting FMU created using Simcenter Amesim back inside Simcenter Amesim for validation under certain conditions it is possible to observe a potentially contradictive behaviour: results between native Simcenter Amesim model and Simcenter Amesim model with imported FMU diverged despite the same inputs. Such issue is observed due to initial conditions that do not match. As a default, Simcenter Amesim uses '0' as an initial condition, thus if this is not true (if initial value is different) - results may diverge.
According to FMI specification it is user’s responsibility to ensure such compliance:
FMI is about solving Ordinary Differential Equations (ODEs). Hence from a mathematical point of view, initial values need to be provided. This is called a “Cauchy” problem. Moreover, only importing tool users are likely to know about the conditions in which their simulations will be executed. There is no way for an FMU to guess which will be the inputs supplied to it. FMI steering committee decided that inputs were much more intuitive (users are more likely to know what they supply to the FMU than what the FMU will output).
Therefore, special attention needs to be given to the initial conditions as when user wants to validate a simple model they may end up with completely different results. Those conditions are available in FMU's Parameters under "amesim_interface" tab.