Alexander Vasilenko's Guided Bubbles & Wet Foam Solver is an advanced whitewater solver for creating Houdini water simulations. It resolves POP Fluid's biggest drawback, which is its lack of support for variable particle radii, allowing for more creative freedom in setting up simulations.

The solver is based on Wētā's research paper, Guided Bubbles and Wet Foam for Realistic Whitewater Simulation, dedicated to studying ways to enhance water simulations by improving the look of bubbles and foam. In the article, researchers suggest treating bubbles as discrete air particles in the water that are converted into foam and simulated separately once they reach the surface.

As the original paper suggests, Vasilenko's underwater bubble system features a locally resimulated fluid and applies two-way coupling to the bubbles. "The foam is handled via an SPH simulation projected onto a 2D manifold of the water surface. The solver implements standard SPH forces such as viscosity (mu), cohesion (C for strength, beta_c for radius), and surface friction against the water," the artist explained.

Unlike Wētā's team, Vasilenko suggests that cohesion force works better with the neighbor's volume multiplier rather than the active particle volume multiplier, although both are included in the solver. According to the creator, using neighboring particles results in better foam clustering.

"The foam is constrained to the surface via manifold advection: forces are decomposed into tangential and normal components, the normal component is discarded, and particles are projected back onto the water surface using Newton iterations," Vasilenko described in short.

You can find more Houdini simulations on the artist's LinkedIn. 

In case you're interested in Wētā's research on water simulations, here's more compulsory reading for you to do: this water wave simulation by Moisè Asta, this bubble solver by Pavel Morozov, and this river simulation by Fred Bello. 

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