Doesn’t look like anyone has giving you an answer yet so I will throw in my two cents and hopefully point you in the right direction as this is something I have not tried yet.
I think you are on the right track though looking at the HDF files. That is a lot of output to store in a plain text file. By default, I don’t think RAS prints this data to HDF however. Those file are also pretty well organized and labeled so if you don’t see what you are looking for then that computational output might need to be enabled. If you head to RAS’s run unsteady flow and find “Set Output Control Options” and go to “HDF5 Write Parameters” there is a box for writing Vel data to HDF for 2D meshes. Not exactly courant but you also have all the data there to quickly calculate it using matlab or python/pandas.
The tab before it, “Computation Level Output Options”, there are a couple boxes you can check for Courant numbers but that might be for 1D and it would write to an ascii file. Depending on how big you 2D model is that could significantly increase compute times…. if it also applies to 2D.
Anyway, hope that helps. Let me know if any of that works for because now I am curious! Thanks!