Dynamic Simulation / Steady State
One of the main question in clinical medicine is how to stabilize the patient. In the fact of the oscillating heart, breathing, circadian rhythm or menstruation cycle the model could be designed as non-oscillating with variables such as period times, amplitudes, frequencies, mean values and other phase space variables. This type of model has better numerical stability for longer simulation time and even more it can be "stabilized". This stabilization we called steady state.
To be mathematically exact, we define an steady state system (SSS) as a non-differential system derived from a original differential system (DS) by using zero derivations and by adding additional steady state equations (ASSE). The number of the ASSE must be the same as the number of algebraically dependent equations in the non-differential system derived from DS by setting zero derivations. The ASSE describes the system from the top view mostly such as the equations of mass conservation laws or the boundary equation of environment sources. To define a model as an SSS the user must switch each Simulation parameter in each block to value Types.SimulationType.SteadyState and must have correctly defined all necessary ASSE. This setting caused to ignoring any start values for any state and add zero derivation equations instead. Today does not exist Modelica environment, which could automatically find and remove generated dependent equations by this way. So the correct number of states must be marked as dependent (parameter isDependent) and the same number of ASSE must be inserted. Despite the fact, that model in this steady-state setting will be not locally balanced it should be globally balanced and without any dependent equation.
Adding of one ASSE is possible by inserting and connecting of the energy or mass conservation law block from package SteadyState.Components. Other possibilities is in blocks of environment sources, where the setting of parameter isIsolatedInSteadyState add the equation of the zero mass/volume/energy flow from or to environment.
The model in steady state often changes to one big nonlinear strong component, but without solver stiff or convergence problems. Especially in quick chemical reaction kinetics is not necessary to have very rapid molar fluxes, when it always reach equilibrium. This design also approve to create steady stated parts in dynamical model without huge rebuilding. It also brings other benefits. To see these possibilities, one have to realize that conservation laws could be invariances in a dynamical simulation. This is really useful for debugging.
It is always a big challenge to nicely solve initial values of differential system. However, it should be possible to solve the SSS in initial phase. And this is the idea behind the Types.SimulationTypes.InitSteadyState option for models already extended with ASSE to support SSS.
Extends from Modelica.Icons.Package (Icon for standard packages).
|Examples||Examples that demonstrate usage of the Pressure flow components|