It seems that Gambit doesn't include a 64 bit version. However, 32 bit Gambit can be installed and used on a 64 bit Ubuntu (I tried on a Ubuntu 9.10 amd64). When trying to launch it, there will be an error saying the ARCH is not compatible. The solution is exporting a variable "FLUENT_ARCH" before the go.
:/$ export FLUENT_ARCH="lnx86"
Then Gambit will not complain about the uncompatible architecture.
Different from Gambit, Fluent does include a 64 bit version. But remember the variable FLUENT_ARCH has to be switched back before trying to run a 64 bit Fluent; (similarly, if the Fluent used is also a 32 bit version, the variable value "lnx86" has to be kept).
:/$ export FLUENT_ARCH="lnamd64"
To avoid the inconvenience, launch scripts can be written to simplify the procedure.
Besides, another potential problem which could be encountered when trying to launch Fluent within Ubuntu 9.10 is the lack of libstdc++.so.5 is complained. This library was actually provided via package libstdc++5, which is however abandoned after Ubuntu 9.10. However, it can still be found from a Debian source, by which I downloaded the package libstdc++5_3.3.6-18_amd64.deb and fixed the error.
A helpful tutorial on Gambit and Fluent can be found from here, although it is using Windows.
UDF (User Defined Functions) is important for extending the flexibility of Fluent. A programming guide of UDF can be found from this pdf file, and this is the complete UDF manual.
An interesting wave tank example was found from a useful post. It is for a solution of the 2D laminar fluid flow in a tank with oscillating motion of a wall by using multi-phase models. More instructions on this example can be found from the Fluent website, from which related mesh files can also be downloaded.
A detailed set up procedure of the example was described in this pdf file as well.