Extending implicit density projection for multiphase fluids

To increase the realism of fluid simulations, interactions between different fluids (such as water and air, or water and oil) must be considered. This allows for the simulation of phenomena such as the ‘glugging’ effect seen when a filled bottle is turned upside-down and air must fill the space left by the liquid. A common approach to fluid animation is to use Particle-in-Cell (PIC) methods. However, such methods are known to lose fluid volume over time due to accumulating numerical errors, an issue that is exacerbated when simulating multiple interacting fluids.

Implicit Density Projection (IDP) is used to overcome volume preservation issues for PIC methods. However, it is formulated for single fluids. To address this, we present two novel extensions to IDP: generalised IDP and D1-IDP. Generalised IDP extends IDP to multiple fluids. D1-IDP then further improves on the volume preservation capabilities. We show that D1-IDP is particularly good in multiphase fluid simulations with complex fluid interfaces. We also demonstrate its applicability to multiphase simulations involving variable density fluids. D1-IDP is able to achieve a maximum volume error of <1.0% for the majority of presented scenarios, while having a negligible impact on computation performance compared to generalised IDP. The code used to generate the results presented in this paper can be found at https://github.com/robden820/Multiphase_Fluids.