add butterfly valve model

[Mathadon]

We currently do not have a model for a butterfly valve. Typically these valves are used for open/closed operation only, however in some cases they are used for control too. In these cases it would be good to have a somewhat accurate valve characteristic.

Some sources for such characteristics:

• http://cgproducts.johnsoncontrols.com/MET_PDF/347VB2.PDF?x=86 p13 figure 6
• https://www.engineeringtoolbox.com/butterfly-valves-flow-coefficients-d_224.html
• CFD study, but which does not seem to provide data for constructing a valve characteristic http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1005.2457&rep=rep1&type=pdf
• https://www.sciencedirect.com/science/article/pii/0019057895000240 fig 8
• https://pdfs.semanticscholar.org/8fa6/277c7729cd5aba9631961cdd214007bdc882.pdf fig 7 (large diameter)
• http://www.dcsc.tudelft.nl/~bdeschutter/private_20100705_acc_2010/data/papers/1640.pdf figure 3: analytical model
• https://link.springer.com/chapter/10.1007/978-3-540-75995-9_149 fig 7: experiment + CFD
• http://blog.opticontrols.com/archives/727 fig 3: flow characteristic can be computed from gain
• http://www.samson-regeltechniek.nl/2_Markten/Technische_onderwerpen/Selectie_&_trim/Klepkarakteristiek_7_5_4_1.pdf last page
• https://repository.tudelft.nl/assets/uuid:727d9a2a-a852-4fd7-99b5-b7f045c7d512/318.pdf figure 5
• http://iopscience.iop.org/article/10.1088/1755-1315/12/1/012098/pdf large diameter
• http://www.valmatic.com/pdfs/Cavitation_in_Valves_7-22-08.pdf figure 3
• http://www.polna.com.pl/en/flow-characteristic.html figure 2
• http://www.valmatic.com/pdfs/SurgeControlinPumpingSystems3-17-09.pdf figure 10

Some questions before I implement this:

• I propose to implement the characteristic by extending TwoWayPolynomial.
• We could add the valve diameter as parameter that computes a default value for dp_nominal, using another polynomial e.g. based on the data in https://www.engineeringtoolbox.com/butterfly-valves-flow-coefficients-d_224.html , or http://cgproducts.johnsoncontrols.com/MET_PDF/347VB2.PDF?x=86 page 22.
• The cleanest way to do this would be to add the option Diameter to CvData, but this would be an invalid option for the other valve models so I don't think is realistic. Unless an enumeration can be extended and redeclared. So perhaps it's better to not add the diameter option for now, or to implement it as a separate function.
• Proposed name: TwoWayButterfly

@icupeiro @damienpicard this is for Infrax

[mwetter]

@Mathadon Using TwoWayPolynomial as the base class makes sense.
How would one usually parameterize the valve? Is it common to simply use the diameter, or does one really specify the Cv value for butterfly valves? If it is the first, maybe it needs a different base class that does not use ValveParameters.

[Mathadon]

I think butterfly valves are typically selected such that they match the pipe diameter, since that makes them easy to install.

It looks quite tedious to do the new implementation without using ValveParameters and without duplicating code. Perhaps it's better to use ValveParameters and to assign final default values for Kv, Cv, etc?

[Mathadon]

In 27e0682 I implemented figure 10 from http://www.valmatic.com/pdfs/SurgeControlinPumpingSystems3-17-09.pdf. I should still verify the shape against other sources.

[Mathadon]

Butterfly valve in brown:

[Mathadon]

Could compute Kv from DN value, but sources disagree quite a bit on the pressure drop at fully opened position. That makes sense since some butterfly valves may be thinner than others. However, the Kv curve should be consistent with the Kv value since at closed position all valves probably behave more similarly.

http://www.valvias.com/flow-coefficient-butterfly-valve.php
http://www.mydatabook.org/fluid-mechanics/flow-coefficient-opening-and-closure-curves-of-butterfly-valves/

[Mathadon]

Commit ceadd7b revises the implementation to use a different curve and matching DN->Kv curve based on the data in http://www.mydatabook.org/fluid-mechanics/flow-coefficient-opening-and-closure-curves-of-butterfly-valves/. The curve difference is quite large (see image below) but it's hard to find a good source and clearly these valves have different shapes in practice too, which cause different flow profiles.