I helped build a small fluid visualization application in OpenGL over the summer. Our main goal was to produce something specifically for fluids, with the hope of making something more intuitive than ParaView for at least a limited set of scenarios (ParaView is amazing, but very complex).
At the moment the project supports most of the basic types of glyph and line rendering used for many fluid visualization scenarios, and we have plans to extend it with more interesting features like volume rendering and particle tracing.
It also provides a small plugin API which can be used to integrate the visualizer directly with a fluid simulator. At the moment we include a simple Lattice-Boltzmann Method simulator with the visualizer to demonstrate this, and to show off some LBM-specific rendering capabilities for displaying the distributions in each cell, which could be helpful for debugging your simulator.
The project is completely free and open source and available on github. If you find any issues or fork it and want to contribute changes, feel free to let us know!
The project dependencies are fairly minimal, with CEGUI being the hardest to set up. A manual is included in the repo which explains all of the GUI controls for the different rendering primitives.