Constraint Bubbles: Adding Efficient Zero-Density Bubbles to Incompressible Free Surface Flow

TL;DR

This paper introduces a simple, efficient method to add zero-density bubbles to incompressible free surface flow simulations, enabling realistic bubble effects without significant computational overhead.

Contribution

It proposes a new constraint for free surface flows that allows simulation of bubbles without simulating both phases or reformulating the problem.

Findings

Adding bubbles increases realism in liquid simulations.

The method adds less than 10% to the computational cost.

Effective in various bubble scenarios with complex interactions.

Abstract

Liquid simulations for computer animation often avoid simulating the air phase to reduce computational costs and ensure good conditioning of the linear systems required to enforce incompressibility. However, this free surface assumption leads to an inability to realistically treat bubbles: submerged gaps in the liquid are interpreted as empty voids that immediately collapse. To address this shortcoming, we present an efficient, practical, and conceptually simple approach to augment free surface flows with negligible density bubbles. Our method adds a new constraint to each disconnected air region that guarantees zero net flux across its entire surface, and requires neither simulating both phases nor reformulating into stream function variables. Implementation of the method requires only minor modifications to the pressure solve of a standard grid-based fluid solver, and yields linear systems that remain sparse and symmetric positive definite. In our evaluations, solving the modified pressure projection system took no more than 10% longer than the corresponding free surface solve. We demonstrate the method's effectiveness and flexibility by incorporating it into commercial fluid animation software and using it to generate a variety of dynamic bubble scenarios showcasing glugging effects, viscous and inviscid bubbles, interactions with irregularly-shaped and moving solid boundaries, and surface tension effects.