Houdini 19.0 Vellum

Vellum fluid vs FLIP fluid


Vellum fluids and FLIP fluids are completely different solvers and they're not able to interact. There are also some fundamental differences in terms of setup, simulation, and interaction with other objects:

Vellum fluids FLIP fluids

Particle-based. Particles have constraints which are solved to simulate the fluid’s behavior.

Cell-based. Cells are filled with particles carrying field information like velocity. Fields are solved for the fluid’s behavior.

More particles mean more fluid, increasing the volume.

More particle will not increase the fluid’s volume. In FLIP, particles are only used to mark the cells and carry field information.

Unbounded approach without a domain. Stray particles are possible.

FLIP works within a domain. Particles or objects outside the domain will not be considered.

Mainly used for small-scale simulations, e.g. splashes, pouring water, or object-filling.

Can be used for a range of different scenarios from small-scale to large-scale, e.g. honey, object filling, waterfalls, or shores with breaking waves.

Vellum fluids don’t export fields for the generation of secondary effects like foam or spray.

FLIP exports field and surface information similar to the Ocean solver. This information is used to create secondary effects, such as foam, spray, and mist.

Vellum fluids usually require higher substeps, typically between 5 and 10 substeps for simulations with low viscosity and surface tension. Higher substeps have lower impact on the GPU.

FLIP fluids usually work with 1 substep. For accurate collision with fast moving fluids or obstacles, higher substeps might be required.

Vellum fluids use fixed substeps.

FLIP fluids use adaptive substeps with minimum and maximum values. Higher substeps in FLIP are expensive to solve.

Highly viscous fluids generally require higher substeps to avoid instabilities.

Highly viscous or slow moving FLIP fluids usually require higher substeps. FLIP fluids with viscosity tend be more stable.

Vellum fluids support multi-material simulations. Interaction with other Vellum objects, such as cloth, softbodies, balloons, and grains is possible

FLIP fluids can only interact with collision objects.

Vellum fluids don’t support air compressibility.

Simulation of air compressibility, for example to create bubbles.

Vellum fluids are compatible with the Vellum Brush SOP tool.

Brush tools don’t exist for FLIP fluids.

Not distributable.

Distributable for simulation on multiple machines.

Multi-phase simulations are supported.

Multi-phase simulations are supported.

No ID and age attributes with particles.

FLIP particles carry ID and age attributes.

Narrow band technology is not available for Vellum fluids.

Narrow band speeds up tank-like simulations, e.g. shores and open ocean waves.