The flow addin has its own separate C++ API. It is not very well documented or supported. The Flow API is downloaded with Flow but you have to install it manually. You should be able to find it here: C:\Program Files\SOLIDWORKS Corp\SOLIDWORKS Flow Simulation\API. (or something similar)
Installing this should give you the documentation and some very simple examples here: C:\Program Files\SOLIDWORKS Corp\SOLIDWORKS Flow Simulation (2)\API SDK 2017
This includes the documentation for the Flow API and some very old examples. What we did in order get it to work with C# was to write a C++/CLI wrapper around the Flow simulation api and have our C# code base interact with that. This was quite a bit of work. Especially figuring out how to work with the C++ smart pointers in a managed context.
You can check it out, but just know that it is likely that it won't be able to do what you want, and if it can you will likely spend a LONG time figuring out how.