Houdini has apprentice version which is free to use and able to use the exporter to generate the fga file. it’s also possible for you to write a program to generate the fga file. The file format for fga is pretty straightforward. The first 3 field is the resolution for the whole volume, then followed by min and max bounding box and finally a bunch of velocities for each grid. The link (https://dl.dropboxusercontent.com/u/12880761/ForShare/spiral.fga) is the data I used for the spiral effect test as shown above. Hope it helps. 