GPS-X Technical Reference

Miscellaneous 430
INTEGRATION CONSIDERATIONS
Robust vs. Fast Models
A trade-off exists between the speed of the simulation and the robustness and accuracy of
the model under different conditions (e.g. sudden flow discontinuities or tanks emptying
completely). GPS-X is configured such that the average user with a typical continuous
flow activated sludge plant need not worry about the accuracy and the speed of the
simulation. However, when dealing with more complex processes it may be necessary for
the user to switch to a different integration algorithm or change the values of some
numerical parameters to improve the accuracy or speed of the simulation. In general,
robustness and accuracy are more important than simulation speed.
If you suspect that the simulation results are not accurate, you should first check that the
physical dimensions of the process objects and the values of the process flows are
reasonable. A numerical value may have been entered incorrectly.
If you are confident that the model parameters are reasonable then the next step is to
determine whether a modelling simplification or numerical problem is leading to
inaccurate results. Some helpful information is listed below:
If you suspect that the simulation results are not accurate, you may wish to
try the Exact code option. (Build > Code menu item). The Quick option
(default) results in a faster simulation but is not as accurate when recycle
flows are present. When the Quick option is used the recycle flows at each
time step are simply the values taken from the previous time step. When the
Exact option is used the recycle flows are calculated exactly using an
implicit nonlinear equation solver. In most situations using the Quick option
is acceptable, because the integration time steps are small and recycle flows
usually don't change dramatically. In an application with a number of recycle
flows and sudden extreme dynamic changes this simplification may cause
some problems.
The choice of integration algorithm (IALG) can have a significant effect on
the output of the model. For a detailed description of the available integration
algorithms consult the ACSL Reference Manual (contact Hydromantis for
details). Generally, only variable step algorithms (i.e. Adams-Moulton,
Runge-Kutta-Fehlberg(1), Runge-Kutta-Fehlberg(2), Gear's Stiff, and
Differential Algebraic Solver) are acceptable for the simulation of highly
dynamic systems. Try the Gear's Stiff algorithm if you suspect that your
system is stiff (unless you are simulating an SBR or trickling filter, in which
case only the Runge-Kutta-Fehlberg algorithms are allowed).
GPS-X Technical Reference