Fix wrong expression in dp_volumeFlowRate_MFLOW

[casella]

Fixes #4552.

[HansOlsson]

Ok, looking good and it fixes one of the unit-issues in the model.

(The other is the somewhat sloppy assert(a+b>0, "...") - but no need to rewrite that for 4.1.0.)

[casella]

@maltelenz I need you help here. Stefan Wisschhusen gave me thumbs up by email 10 days ago

Hey Francesco,

many thanks for the issue report! Well, obviously you are right 😊. Vflow = dp/b.

but he's probably too busy to take care of this, as he resigned from MAP-Lib long time ago. @hubertus65 and @arunkumar-narasimhan are also probably busy with other stuff.

Please accept this so we can merge it in, @Esther-Devakirubai can merge it in, and we can move on with the release.

Thanks!

[maltelenz]

Approving as requested by @casella .

(FYI I'm unavailable until 17th of March, so will be unable to help with any real MSL work until then.)

[Esther-Devakirubai]

Backported by #4580

[MatthiasBSchaefer]

The case a=0 and b>0 , corrected here, is -as far as i can see - not covered by any example model in Modelica or ModelicaTest. Maybe this should be done by enhancing the example ModelicaTest.Fluid.Dissipation.Verifications.PressureLoss.General.dp_volumeFlowRate_DPMFLOW

[casella]

The case a=0 and b>0 , corrected here, is -as far as i can see - not covered

That's true. This is really a Herculean task, it's like slashing the heads of the Hydra. You cut one, three more sprout out...

by any example model in Modelica or ModelicaTest. Maybe this should be done by enhancing the example ModelicaTest.Fluid.Dissipation.Verifications.PressureLoss.General.dp_volumeFlowRate_DPMFLOW

We could do it for MSL 4.2.0, see #4585, I really want to release beta.2 tomorrow, so I'm not sure I want to go again through the whole cycle: add PR, ask for reviews, merge on master, back-port, add for review, merge on master, add reference results. Too late now.