Ben Watts from BW Design, has a look at creating custom forces inside of Houdini. The tutorial walks through how to build custom forces, apply them to simulations, and generate interesting behaviors.
The video shows you how to create custom forces using VDB Volumes and VEX. Ben uses a tornado as an example and inspiration for the custom force goal.
Houdini has a variety of DOP Forces that you can bust out at any time. Those force types include the standard fare such as gravity and drag and wind, plus others. Best part of Houdini, is that you can create any tool you like. You can create any custom force and use it to help guide your simulation.
Three are other methods that can help shape simulations in Houdini. A while back, Stefan Sietzen showed how to use a geometry based vector field to guide a pyro simulation.
