Clarification on consistency rules for energyDynamics and massDynamics

[licornei]

In the Zone and WellMixedAir models, consistency between energyDynamics and massDynamics is only enforced through an if statement when energyDynamics = SteadyState.

What is the reasoning for not enforcing similar constraints when energyDynamics = Dynamic?
We (BFG UGent) would like to understand why consistency is only applied in the SteadyState case, and not in the other combinations, so that we can make sure that we choose these settings correctly.

[licornei]

Update: the paths to the components are: IDEAS.Buildings.Components.Zone and IDEAS.Buildings.Components.ZoneAirModels.WellMixedAir.

[LoneMeertens]

The reason you see a check only when energyDynamics == SteadyState is deliberate: an algebraic energy balance with a dynamic mass balance is potentially inconsistent because internal energy depends on mass.

Internal energy in the model is:

U = m * u + C_Sen * (T - T_ref)

and the energy balance is implemented as:

Steady-state:  0     = Hb_flow + Q_flow
Dynamic:       dU/dt = Hb_flow + Q_flow

If the energy balance is dynamic we can expand the derivative:

dU/dt = d(m*u)/dt + C_Sen * dT/dt
      = (dm/dt) * u + m * du/dt + C_Sen * dT/dt

The key term here is (dm/dt) * u. If massDynamics = Dynamic, then dm/dt != 0 in general. When energyDynamics = SteadyState, however, the energy equation is algebraic (no dU/dt term), so there is no place for the (dm/dt) * u contribution. This can make the equations inconsistent or ill-posed. For this reason the library enforces:

energyDynamics == SteadyState  =>  massDynamics == SteadyState

Conversely, the combination energy = Dynamic and mass = SteadyState is allowed and physically consistent: the energy ODE can safely use a constant mass (dm/dt = 0).

This rule is enforced in IDEAS.Fluid.Interfaces.ConservationEquation with the following assertion:

// Make sure that if energyDynamics is SteadyState, then
// massDynamics is also SteadyState.
// Otherwise, the system of ordinary differential equations may be inconsistent.
if energyDynamics == Modelica.Fluid.Types.Dynamics.SteadyState then
  assert(massDynamics == energyDynamics, "In " + getInstanceName() + ":
       If 'massDynamics == Modelica.Fluid.Types.Dynamics.SteadyState', then it is
       required that 'energyDynamics==Modelica.Fluid.Types.Dynamics.SteadyState'.
       Otherwise, the system of equations may not be consistent.
       You need to select other parameter values.");
end if;

Hopefully this clarifies the reasoning behind the check.