FLIP (Fluid-Implicit-Particle) is a hybrid method of particle and grids improved from PIC (Particle-In-Cell) method for fluid dynamics simulation. Because of its fast simulation speed and acceptable accuracy for visual effects, FLIP fluids are widely used in the film industry.
Houdini's FLIP Fluids system is shipped with two implementations: The FLIP/PIC method (Splashy Kernel) and the APIC method (Swirly Kernel). By default, Houdini uses APIC method for high vorticity simulations and uses FLIP/PIC for other cases. Maya's Bifröst Fluids and Blender's FLIP Fluid Plugin are based on FLIP solver as well. Here we introduce FLIP fluids mainly based on Houdini's system.
FLIP Tank is the simplest way to get started with FLIP fluids in Houdini. By clicking the FLIP Tank button in the shelf tools, Houdini will create the basic node network of a FLIP tank to you. Note that starting from Houdini 16.5 the FLIP tank is initialized with Narrow Band by default. You can change it back to Particle Field in the Input Data panel of the fliptank node in the DOP network.
The FLIP solver is integrated into the DOP by default so you can integrate with physics and other dynamic systems easily in Houdini. If you try throwing an object into the tank, hopefully you wish to see a huge splash appeared. By default, the splash might be less obvious than you expect because of the low particle density. Reduce Particle Separation in fliptank node to a smaller value would help but will increase simulation time a lot.
The initialization of the fluid is in fliptank_initial node and the geometry construction is in fliptank_fluid node. The fliptank_fluidinterior node is used for volume rendering.
You can check the simulation result immediately in the Scene View. It is usually recommended to bake the simulation result into file cache before rendering.
FLIP Fluids for Ocean
FLIP fluids can be also integrated with ocean dynamics. The Guided Ocean Layer and the Wave Tank shelf tool act as a good starting point.