Some parameters seems to be never assigned in Buildings models

[Francesco Casella]

Please check ​Buildings.Fluid.Actuators.Valves.Examples.ThreeWayValves. The simulation fails at runtime with

Jacobian determinant is NaN.
nonlinear system 56 fails: at t=0

I tried to investigate the issue using the debugger, I found the following chain of equations and assignments
Equation 56 has one equation in the unknown valTab.res3.m_flow

(residual) valTab.res3.dp - homotopy(Buildings.Fluid.BaseClasses.FlowModels.basicFlowFunction_m_flow(valTab.res3.m_flow, valTab.res3.k, valTab.res3.m_flow_turbulent), valTab.res3.dp_nominal_pos * valTab.res3.m_flow / valTab.res3.m_flow_nominal_pos) = 0

valTab.res3.dp_nominal_pos := abs(valTab.res3.dp_nominal)

valTab.res3.dp_nominal := valTab.res3.dpValve_nominal

A similar issue pops up with valTab.res3.m_flow_nominal_pos:

(assign) valTab.res3.m_flow_nominal_pos := abs(valTab.res3.m_flow_nominal)

but then valTab.res3.m_flow_nominal is apparently not assigned in any equation.

This of course explains the NaN in the Jacobian, that would be obtained by computing the 0/0 ratio
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Unfortunately, the debugger does not report how valTab.res3.dpValve_nominal is computed. I don't know if this is a problem with the debugger, or if it turns out that this parameter is incorrectly skipped and eventually assumed to be zero.

This issue affects many models in the Buildings library, and is possibly the root cause of #6190