id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
5770	The backend incorrectly generates a large set of nonlinear equations	Francesco Casella	Karim Adbdelhak	"@Karim, I managed to come up with a test case for the slow behaviour in the power plant simulation that I mentioned during today's webmeeting. The test case has a reduced size to be more manageable.

Please run the attached Save Total model. The backend identifies a nonlinear system with 18 unknowns and 7 iteration variables:
{{{
der(fWPH_11_1.flowSTEAM.htilde[1])
der(fWPH_11_1.flowSTEAM.htilde[2])
der(fWPH_11_1.nusseltCondenser.hl)
der(fWPH_11_1.nusseltCondenser.hv)
der(fWPH_11_1.nusseltCondenser.p)
der(fWPH_11_1.nusseltCondenser.Vv)
fWPH_11_1.nusseltCondenser.ws_in
}}}
Dymola identifies a linear system instead, which has the 6 following unknowns after tearing.
{{{
der(fWPH_11_1.flowSTEAM.h[2])
der(fWPH_11_1.flowSTEAM.h[3])
der(fWPH_11_1.nusseltCondenser.hl)
der(fWPH_11_1.nusseltCondenser.hv)
der(fWPH_11_1.nusseltCondenser.p)
der(fWPH_11_1.nusseltCondenser.Vl)
}}}
The first two and the last ones aliases of the first two and of the sixth found by OMC.

The simulation of this model is actually a steady-state, so there are only 19 variable step-size steps, and 500 communication intervals. Yet, the nonlinear system is reported by the debugger to be executed a whopping 48209 times. In the case of larger systems, this can be really become the bottleneck.

I scanned the equations of the nonlinear system in the OMC debugger, but I couldn't understand why OMC considers them nonlinear. Could you please check?
"	defect	assigned	high		Backend				Andreas Heuermann giovanni.mangola@… matthis.thorade@…