Further inStream optimizations

[Francesco Casella]

The simplifyInStream module of the backend currently implements the substitutions listed in ticket:3885#comment:15

• inStream introduces positiveMax(cref, eps) and positiveMax(-cref, eps)
• simplify positiveMax in Backend after removeSimpleEquation and EvaluateParameter and before IndexReduction
• positiveMax(cref, eps) = cref where variable is constant >= 0
• positiveMax(-cref, eps) = -cref where variable is constant <= 0
• positiveMax(cref, eps) = 0 if variable(cref).max <= 0
• positiveMax(-cref, eps) = 0 if variable(cref).min >= 0

The idea of the last two substitutions is that if a fluid never flows out of the stream connector, its contribution is zero. In case of components with only one direction of flow, one can set min=0 at the inlet to activate this optimization.

In fact, when dealing with components without flow reversal, not only the fluid never flows out of the inlet, hence inlet.m_flow.min = 0, but also the fluid never flows in the outlet, hence outlet.m_flow.max = 0. This fact can be stated by setting the max attribute of the outlet port mass flow rate to zero. If this property holds, then

• positiveMax(cref, eps) = cref if variable(cref).min >= 0 test: positiveMax(1, eps) = 1
• positiveMax(-cref, eps) = -cref if variable(cref).max <= 0 test: positiveMax(-(-1), eps) = 1

These additional optimizations would allows to remove unnecessary positiveMax function calls, hence eliminating unnecessary hard nonlinearities from the model equations.

@vitalij, if you are willing to implement this, I can prepare the tests ASAP.