When creating complex models, determining the appropriate causality sequence can sometimes become difficult. In particular, two problems can sometimes occur:
•GoldSim may not be able to create a valid sequence at all. This is the result of the system having circular (recursive) logic without a state variable involved. If you tried to build such a model, GoldSim would be unable to create a valid causality sequence, and would display an error, warning you that the system is recursive. GoldSim can still represent such systems, but in order to do so, it is necessary for you to explicitly add a state variable to the system. Depending on the system, there are generally two ways to address this issue:
o In some cases, the system does in fact represent a feedback loop, but due to the lack of a state variable, you are unable to “close” the loop. These systems can usually be addressed by adding a state variable in the form of a Material Delay.
o In other cases, the system does not represent a feedback loop, but instead represents a coupled process (that may not be dynamic at all). These systems can be solved by using Previous Value elements to close the recursive loop.
•In rare cases, GoldSim can sequence the system, but there may be more than one valid way to sequence the system, and these produce different results. This can occur, for example, if state variables that are impacted by an event (e.g., a discrete change) affect another element. Should the downstream element be updated before or after the state variable is modified? From the point of view of GoldSim, both sequences for the downstream element may be equally valid, but depending on the application, you may specifically require one instead of the other. In these cases, you can explicitly force a particular causality sequence to ensure that the model accurately represents your system.
Learn more about:
Using Material Delays to Close Feedback Loops and Model Recirculating Systems