@Tamas Csabina This is because the software is ITAR restricted. From one FUN3D manual: "Release of the high-energy, real-gas capability is further restricted because of International Traffic in Arms Regulations (ITAR)". Modeling that's inherent to the code is already restricted. There's nothing NASA can do about this.
There is absolutely nothing that NASA can do about ITAR. As much as I'm sure they would love to have international participation, they are a US Entity (and a Federal Government Entity, for that matter), and must abide by the restrictions imposed at Federal level.
I don't speak for NASA, but I can assure all our international friends that NASA values international collaboration. Sadly, ITAR is completely outside their control.
@Tamas Csabina It does, and it is sad. And I'm sure NASA realizes it. But they cannot get around ITAR restrictions imposed on them at the Federal level. I hope you are not disappointed or feel that NASA does not value or recognize international collaboration.
The eligibility does extend past US citizens to include US Persons (this includes US Citizens, Permanent Residents/Green Card holders and those in the U.S. as a protected political asylee or under amnesty); currently on US soil; and age 18 or older.