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Hi there
Could anyone point me to some literature to understand better what kind of systems can be simulated/do converge under what conditions when using successive substitution as the solver strategy. I tried to do some analytical convergence analysis myself for some very simple systems, like e.g. a constant power source connected to a storage via a heat exchanger. I found that to be quite challenging and don’t really know how and if my findings translate to larger systems.
The reason for going down this route for me was to find a suitable time step for simulating my system without getting convergence errors in a bit more principle way than just trial and error.
Thanks for your help,
Damian
Hello Damian,
Trial and error can be a valuable part of the TRNSYS learning experience! That said, there are a couple of guidelines you can use to find a suitable time step for your system.
- If you have any on/off controllers in your system (differential controller, thermostat, aquastat, humidistat, and so forth), make sure the time step of your simulation is less than or equal to the minimum run time of the equipment controlled. For example, if a pump must run for at least 5 minutes each time it switches on, or rest for at least 5 minutes each time it switches off, a time step of 5 minutes would be appropriate. We commonly use either a 5-minute or a 1-minute time step for this reason.
- If you have closed fluid loops in your system, make sure you have at least a time step’s worth of mass in each fluid loop. For example, in a closed fluid loop with two pipes and a storage tank, you would want the total volume x fluid density of pipe1, pipe2, and the tank, to be greater than the maximum flow rate in the loop x the time step. Every fluid loop should have at least one component that has mass or capacitance. You will likely have convergence problems if this condition is not satisfied.
You can also read David's post here about convergence problems and their most common causes and solutions.
